The Unifying Mystery of Black Hole Jets: Stellar and Supermassive Connections

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Anna: Take a black hole 10 times the mass of the

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Sun. Now take one 10 million times

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heavier, the kind that sits in the middle of

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a galaxy and anchors a hundred billion

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stars.

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Avery: Those two objects have almost nothing in

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common. Different sizes, different

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neighbourhoods, different life storeys.

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One of them you could fit inside a city. The

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other one you could pour the entire inner

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solar system into and still have room.

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Anna: And this week, a team led out of Perth has

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shown that when it comes to the single most

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dramatic thing a black hole does, firing a

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jet of material out into space at close to

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the speed of light, both of them wait for

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exactly the same moment,

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Avery: same trigger, same threshold,

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seven orders of magnitude apart.

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Anna: Hello and welcome to Astronomy AstroDailyPod.

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I'm Anna.

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Avery: And I'm avery. It's Friday the 18th

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of September, 2026, and this is

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episode 197.

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Anna: So let's start with the thing that makes this

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hard. Black holes don't emit anything

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themselves. What we actually see is the

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material falling in. It piles up into a

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disc. It heats up through friction until it's

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glowing across the electromagnetic spectrum.

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And then sometimes, not always, but

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sometimes, some of it gets flung back out in

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a narrow, tightly collimated jet.

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Avery: And the question of when you get a jet and

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when you don't has been open for decades.

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Anna: It m has. But for one class of black hole,

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we actually have a very good answer. Stellar

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mass. Black holes, the ones left behind when

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a massive Star collapses maybe five to 20

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times the Sun's mass, often sit in binary

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systems, pulling material off a companion

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star. And those systems cycle. They

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brighten, they fade, they go through what

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astronomers call state transitions, and they

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do it over weeks and months, which

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Avery: means we've watched the whole process start

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to finish many times over

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many, many times.

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Anna: And what we've learned is that the jets

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switch on and off at particular points in

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that cycle. And those points are tied to how

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fast the black hole is feeding, measured

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against something called the Eddington limit.

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Avery: And this is worth spelling out, because

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everything else today rests on

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really does.

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Anna: So material falling toward a black hole gets

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extremely hot, and hot material radiates.

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That radiation pushes outward, gravity

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pulls inward. The Eddington limit is the

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point where those two balance, where the

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radiation coming off the infalling material

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is pushing outward just as hard as the black

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hole's gravity is pulling in.

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Avery: So if you try to feed a black hole faster

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than that, the radiation starts blowing the

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meal away.

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Anna: Anna. Roughly, yes.

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Though nature has ways around it, and we'll

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come back to that. The useful thing about the

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Eddington limit is that it scales with mass.

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A, uh, black hole a million times heavier has

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a limit a million times higher. So instead of

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talking about absolute brightness,

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astronomers talk in fractions of Eddington,

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10% of Eddington, 1% of

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Eddington. And that gives you a way to

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compare black holes of wildly different sizes

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on the same scale.

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Avery: Which brings us to the supermassive black

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holes. And the reason this has been so

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stubborn.

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Anna: The reason is simply time. Everything about

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an accretion flow scales with the mass of the

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black hole, and that includes how fast it

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evolves. A stellar mass system cycles in

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months. A supermassive black hole in an

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ordinary active galaxy takes something on the

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order of thousands of years to do the

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equivalent.

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Avery: So you can't watch one. You get a single

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frame of a film that runs for millennia,

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exactly that.

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Anna: You can look at thousands of active galaxies

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and take a census. Here's one with a jet,

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here's one without. But you can never watch a

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single object cross a threshold and see what

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happens. Which means the critical accretion

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rates that we know so precisely for stellar

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mass black holes have simply been

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unmeasurable for the big ones.

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Avery: Unless something gives a supermassive black

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hole a sudden, well defined meal.

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Anna: And that's the move. Tidal disruption

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events. A, uh, star wanders too close to the

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supermassive black hole at the centre of its

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galaxy. The difference in gravitational pull

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across the star exceeds what's holding the

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star together and it comes apart.

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Roughly half the debris gets flung away and

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the other half falls back and builds a brand

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new accretion disc from nothing.

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Avery: And that one plays out fast.

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Anna: Years, not millennia. You get to watch

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an accretion flow around a supermassive black

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hole be born, rise,

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peak and decline. And you can do it

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inside a single research career. That is the

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entire reason this result was possible.

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Avery: So Dr. Adele Goodwin at, uh, Curtin

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University's International Centre for Radio

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Astronomy Research in Perth, who's also

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a Forest Research foundation fellow,

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working with Dr. Andrew Mummery at the

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Institute for Advanced Study in Princeton,

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published in Nature astronomy yesterday,

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Thursday 17th September.

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Anna: And what they show is that tidal disruption

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events actually launch outflows twice

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in two physically distinct episodes. The

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first comes early, while the black hole is

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feeding above its Eddington limit. That brief

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ferocious phase right after the star comes

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apart, when there's far more material trying

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to get in than the black hole can comfortably

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swallow.

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Avery: The prompt outflow, the prompt one.

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Anna: And then later, sometimes months or years

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later, there's a second separate

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outflow. And that One arrives as the

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accretion rate falls through a specific

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value, about 2% of the Eddington

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limit.

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Avery: 2%. And that number is

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familiar.

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Anna: That number is the same critical accretion

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rate at which stellar mass black holes in

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binaries make their state transitions.

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The same value we've measured over and over

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in objects millions of times smaller.

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Goodwin and Mummery are arguing that the

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coupling between how a black hole feeds and

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how it throws material back out is scale

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invariant, that it doesn't care

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Avery: about mass at all across roughly seven

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orders of magnitude, which, if it

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holds,

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Anna: is a genuinely unifying statement about a

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class of object we usually treat as two

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separate populations.

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Avery: And it also clears up a mess, doesn't it?

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Because radio astronomers have had an awkward

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problem with these events for a while,

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Anna: a very awkward one. Some tidal disruption

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events produce a radio flare almost

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immediately. Some produce one much later,

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out of nowhere, sometimes years after

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everyone has stopped paying attention. Some

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appear to produce nothing at all that looked

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like three different phenomena or worse, like

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noise.

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Avery: And the two outflow picture makes it

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Anna: one phenomenon, one phenomenon with two

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stages, where whether you see either of them

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depends on when you happen to be looking and

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how the accretion rate was falling. The

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prompt and delayed outflows stop being a

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puzzle and start being a prediction.

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Avery: And Goodwin makes a very practical point

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about that.

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Anna: She does, and it's my favourite part of the

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release. Her line is quote,

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radio telescopes are incredibly powerful, but

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knowing when to look is just as important as

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knowing where to look. And she goes on,

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if we can anticipate when a black hole is

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more likely to launch a jet, we can run

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better targeted campaigns, waste fewer

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observations and improve our chances of

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catching these rare events. At the moment,

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they matter most, which

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Avery: is not a small thing when you're competing

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for time on the world's big dishes.

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Anna: It's the difference between a survey and a

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stakeout. And it matters enormously for

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what's coming, because the Vera Rubin

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Observatory is about to start finding tidal

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disruption events in numbers we've never had

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before. If you can look at the optical light

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curve and predict roughly when the radio

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outflow should arrive, you can have the radio

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telescopes ready.

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Avery: Without that, you're guessing, and this

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one is properly ours.

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Anna: It is led from Curtin University in

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Perth through icrar, and the underlying data

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set is the product of years of multi

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wavelength campaigns, drawing on telescopes

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in Australia, the United States, India,

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South Africa and in space. This is

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not a single instrument result. It's a

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decade of patient radio follow up on rare

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events assembled into One

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Avery: picture, and it lands in the same week that

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we're talking about the Square Kilometre

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Array's southern half taking shape out in

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Western Australia, which is precisely the

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Anna: instrument you'd want pointed at the next

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one.

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Avery: Two things to keep straight, though.

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Anna: Yes. The first is that this is a

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threshold, not a switch. Saying

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black holes launch outflows at around

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2% of Eddington is a statement

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about where the transition sits, not a

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promise that every black hole crossing that

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line will produce a jet you can detect.

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Whether you actually see one depends on the

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environment, the geometry, the distance

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and the luck of having a telescope pointed in

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the right direction.

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Avery: And the second?

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Anna: The second is about timing. And we want to be

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straight with you. This was published in

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Nature Astronomy yesterday. That's real. And

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the peer review is done. But the preprint

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went up on the archive back in February. So

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while this is newly published, it is not

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newly written. And if it feels vaguely

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familiar to anyone who follows the preprint

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servers closely, that's why.

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Avery: Which is a pattern we flagged before on this

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show.

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Anna: It is, and we'll keep flagging it. The

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science is no less good for having waited

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seven months in review. But published

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this week and discovered this week are

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different claims, and we try very hard not to

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blur them.

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Avery: Longtime listeners will hear some threads

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here, too. We covered a wandering black hole

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revealed by a tidal disruption event back in

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episode 153. And a, uh, quasar

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driving turbulence across 300,000 light

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years in 154.

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Anna: And the partial disruptions that fade and

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come back in episode 185.

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This result is the framework those individual

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objects have been waiting for.

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Avery: Now on to storey two. That phrase.

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Super Eddington came up in the lead, and

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it's about to come up again. Because the

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second storey this week is also about a black

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hole eating faster than it should be able to

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just 11 billion years earlier.

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Anna: The little red dots.

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Avery: The little red dots. And if you've been with

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us a while, you'll know these have been one

268
00:10:56.970 --> 00:10:59.650
of the most irritating things the James Webb

269
00:10:59.650 --> 00:11:02.490
Space Telescope has found. Extremely

270
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compact, extremely red,

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extremely numerous in the early universe.

272
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They show the broad hydrogen emission lines

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you'd expect from gas whipping around a black

274
00:11:13.170 --> 00:11:15.810
hole. But there are far too many of them.

275
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And the black holes implied are far too heavy

276
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for the galaxies they sit in.

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Anna: Which has produced a small industry of

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explanations.

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Avery: Some of them fairly exotic, some very

280
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exotic. So here's what's new.

281
00:11:29.930 --> 00:11:32.490
A team led by Sun Myun Chan, with

282
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Shingo Hirano Tomoaki ishiyama

283
00:11:35.530 --> 00:11:38.090
Sukjoon Chang and Volker Springle,

284
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published in Nature on 16 September,

285
00:11:41.620 --> 00:11:43.860
ran fully cosmological radiation

286
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hydrodynamic simulations of a dense

287
00:11:46.660 --> 00:11:49.220
protocluster region in the early universe

288
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using Japan's Atarui UH3

289
00:11:51.780 --> 00:11:52.740
supercomputer.

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Anna: And crucially, without putting the answer in

291
00:11:55.380 --> 00:11:55.940
by hand.

292
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Avery: That's the point. They didn't seed the

293
00:11:58.780 --> 00:12:01.140
simulation with the black holes they wanted.

294
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They let the physics run. And what happens

295
00:12:04.140 --> 00:12:07.140
is in that intensely over dense environment,

296
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the the giant gas clouds are bathed in so

297
00:12:10.380 --> 00:12:13.300
much ultraviolet radiation that they can't

298
00:12:13.300 --> 00:12:16.140
fragment into ordinary stars. Normally

299
00:12:16.140 --> 00:12:18.780
a big cloud breaks up into lots of little

300
00:12:18.780 --> 00:12:21.380
stellar nurseries. Here it can't.

301
00:12:21.860 --> 00:12:24.780
So instead the whole thing collapses as one

302
00:12:24.780 --> 00:12:27.780
object, a primordial supermassive star,

303
00:12:28.100 --> 00:12:30.740
which then promptly collapses into a black

304
00:12:30.820 --> 00:12:33.640
hole, a heavy seed. A

305
00:12:33.640 --> 00:12:36.120
heavy seed of around a million solar

306
00:12:36.120 --> 00:12:38.920
masses, which is roughly 10 times heavier

307
00:12:38.920 --> 00:12:41.880
than theory typically expects. And then those

308
00:12:41.880 --> 00:12:44.840
seeds develop dense optically thick discs,

309
00:12:45.320 --> 00:12:47.720
so thick that light bounces around inside

310
00:12:47.720 --> 00:12:50.400
them, scattering off free electrons on the

311
00:12:50.400 --> 00:12:53.040
way out. And that electron scattering

312
00:12:53.040 --> 00:12:56.040
smears the hydrogen emission into exactly the

313
00:12:56.040 --> 00:12:58.920
broad lines we see in little red dots.

314
00:12:59.700 --> 00:13:01.740
Anna: So the broad lines aren't necessarily telling

315
00:13:01.740 --> 00:13:03.740
you the gas is moving as fast as you'd

316
00:13:03.740 --> 00:13:04.260
assumed.

317
00:13:04.580 --> 00:13:07.460
Avery: That's the elegant bit. Part of the width is

318
00:13:07.460 --> 00:13:10.340
scattering, not velocity. And then

319
00:13:10.420 --> 00:13:13.060
sustained super Eddington accretion drives

320
00:13:13.060 --> 00:13:15.700
the thing up to around 30 million solar

321
00:13:15.700 --> 00:13:18.580
masses by a redshift of 8, which puts

322
00:13:18.580 --> 00:13:21.100
it right in the population of overmassive

323
00:13:21.100 --> 00:13:23.140
quasars. Webb keeps finding.

324
00:13:23.700 --> 00:13:26.360
Anna: So it's one continuous storey seed

325
00:13:26.360 --> 00:13:29.240
dot quasar and eventually the

326
00:13:29.240 --> 00:13:31.680
supermassive black holes sitting in galaxies

327
00:13:31.680 --> 00:13:32.040
today.

328
00:13:32.600 --> 00:13:35.360
Avery: One storey, with the little red dot phase

329
00:13:35.360 --> 00:13:38.280
being short and dust shrouded. A, uh, stage

330
00:13:38.280 --> 00:13:40.920
rather than a species. And the timing is

331
00:13:40.920 --> 00:13:43.560
rather lovely because Nature Astronomy

332
00:13:43.560 --> 00:13:46.080
published a whole focus issue on little red

333
00:13:46.080 --> 00:13:48.800
dots the same week, including a review

334
00:13:48.800 --> 00:13:51.560
by Hannah Hubler on massive black holes

335
00:13:51.560 --> 00:13:54.540
in the first billion years. M the field is

336
00:13:54.540 --> 00:13:57.100
consolidating and this happens to be the week

337
00:13:57.100 --> 00:13:57.860
it did it.

338
00:13:58.260 --> 00:14:00.620
Anna: One caution, though, and it's the same one we

339
00:14:00.620 --> 00:14:02.660
gave for the black hole feedback work last

340
00:14:02.660 --> 00:14:02.980
month.

341
00:14:03.380 --> 00:14:04.020
Avery: Go on.

342
00:14:04.420 --> 00:14:06.700
Anna: This is a simulation that reproduces the

343
00:14:06.700 --> 00:14:09.380
observations. That's a demonstration that the

344
00:14:09.380 --> 00:14:11.500
physics is sufficient. You don't need

345
00:14:11.500 --> 00:14:14.500
anything exotic to get little red dots. It is

346
00:14:14.500 --> 00:14:16.980
not, on its own proof that this is what

347
00:14:16.980 --> 00:14:19.580
actually happened. Other routes may also

348
00:14:19.580 --> 00:14:22.350
produce them. What would settle it is a

349
00:14:22.350 --> 00:14:24.390
direct measurement of one of these objects

350
00:14:24.550 --> 00:14:27.110
that distinguishes scattering broadened lines

351
00:14:27.110 --> 00:14:28.350
from genuinely fast

352
00:14:28.350 --> 00:14:30.950
Avery: gas, which is exactly the Kind of thing

353
00:14:30.950 --> 00:14:31.910
Webb could do.

354
00:14:32.390 --> 00:14:35.270
Anna: Right, next up, let's come all the way back,

355
00:14:35.750 --> 00:14:38.230
about 6,000 kilometres, in fact, to the

356
00:14:38.230 --> 00:14:39.630
middle of the Earth, which

357
00:14:39.630 --> 00:14:41.110
Avery: is not where you think it is.

358
00:14:41.510 --> 00:14:43.550
Anna: It really isn't. And this is one of those

359
00:14:43.550 --> 00:14:45.630
facts that sounds like a trick until you sit

360
00:14:45.630 --> 00:14:48.050
with it. There are two different centres of

361
00:14:48.050 --> 00:14:50.770
the Earth. There's the centre of figure, the

362
00:14:50.770 --> 00:14:53.290
geometric middle of the solid planet surface,

363
00:14:53.450 --> 00:14:56.210
which is essentially fixed. And there's the

364
00:14:56.210 --> 00:14:58.450
centre of mass, the balance point of

365
00:14:58.450 --> 00:15:00.010
everything the planet is made of.

366
00:15:00.330 --> 00:15:02.770
Avery: And the planet is partly made of water and

367
00:15:02.770 --> 00:15:04.970
air, which move constantly.

368
00:15:05.210 --> 00:15:07.770
Anna: Snow falls across Siberia and Canada.

369
00:15:08.330 --> 00:15:11.290
The Apple basin floods. The monsoon

370
00:15:11.290 --> 00:15:14.180
arrives over Southeast Asia. Every one of

371
00:15:14.180 --> 00:15:16.820
those shifts an enormous amount of mass from

372
00:15:16.820 --> 00:15:19.060
one part of the planet to another, and the

373
00:15:19.060 --> 00:15:21.700
balance point moves to follow it. So the

374
00:15:21.700 --> 00:15:23.980
centre of mass and the centre of figure drift

375
00:15:23.980 --> 00:15:26.100
apart and back together over the course of a

376
00:15:26.100 --> 00:15:26.380
year.

377
00:15:26.620 --> 00:15:27.980
Avery: And we care because.

378
00:15:28.380 --> 00:15:30.700
Anna: Because the centre of mass is what satellites

379
00:15:30.700 --> 00:15:33.580
orbit. It's the origin of the reference frame

380
00:15:33.580 --> 00:15:36.220
that underpins satellite navigation and

381
00:15:36.300 --> 00:15:38.660
every precise elevation measurement on the

382
00:15:38.660 --> 00:15:41.100
planet. If you're wrong about where it is,

383
00:15:41.260 --> 00:15:43.180
you're wrong about where everything else is.

384
00:15:44.060 --> 00:15:45.900
Avery: So how big is the wobble?

385
00:15:45.900 --> 00:15:48.740
Anna: That's the news. The traditional figure has

386
00:15:48.740 --> 00:15:51.700
been around 11 millimetres. A team led

387
00:15:51.700 --> 00:15:54.100
by Donald Argus at NASA's Jet Propulsion

388
00:15:54.100 --> 00:15:56.940
Laboratory now puts it at roughly four to

389
00:15:56.940 --> 00:15:59.500
five and a half, about half what we thought.

390
00:16:00.140 --> 00:16:01.820
Published in Geophysical Journal

391
00:16:01.820 --> 00:16:04.180
International with the JPL release on

392
00:16:04.180 --> 00:16:04.460
Tuesday.

393
00:16:04.700 --> 00:16:06.800
Avery: Half is a big, big correction for something

394
00:16:06.800 --> 00:16:08.880
people have been measuring for decades.

395
00:16:08.960 --> 00:16:11.920
Anna: It is. And here's how uncertain it's been.

396
00:16:12.480 --> 00:16:14.960
The last two international estimates, one

397
00:16:14.960 --> 00:16:17.520
from 2017 and one from 2023,

398
00:16:18.000 --> 00:16:20.280
disagree with each other by about 7

399
00:16:20.280 --> 00:16:22.840
millimetres, which is almost as large as the

400
00:16:22.840 --> 00:16:24.560
entire motion they're trying to measure.

401
00:16:24.640 --> 00:16:27.160
Avery: So the error bar was the same size as the

402
00:16:27.160 --> 00:16:27.600
signal.

403
00:16:27.680 --> 00:16:30.560
Anna: Essentially, Argus's own line

404
00:16:30.560 --> 00:16:32.880
is that the movement now looks like about

405
00:16:32.960 --> 00:16:35.360
half of of what we believed eight years ago,

406
00:16:36.000 --> 00:16:38.680
and that the mass of water and air sloshing

407
00:16:38.680 --> 00:16:41.640
between the hemispheres is smaller than we

408
00:16:41.640 --> 00:16:42.000
thought.

409
00:16:42.400 --> 00:16:44.440
Avery: And where does the movement actually come

410
00:16:44.440 --> 00:16:44.720
from?

411
00:16:44.880 --> 00:16:47.280
Anna: Three main ocean,

412
00:16:47.520 --> 00:16:49.600
atmosphere and land water.

413
00:16:50.400 --> 00:16:53.360
In March, snow sitting across Eurasia and

414
00:16:53.360 --> 00:16:56.240
North America pulls the centre of mass about

415
00:16:56.240 --> 00:16:58.960
3 millimetres toward the northern Pole.

416
00:16:59.530 --> 00:17:02.250
Then, in April, the Apple hits its annual

417
00:17:02.250 --> 00:17:04.890
water maximum, around 2,400

418
00:17:04.890 --> 00:17:07.210
gigatons, and tugs it roughly

419
00:17:07.210 --> 00:17:09.690
2.2 millimetres towards south America.

420
00:17:10.090 --> 00:17:11.970
Later in the year, monsoon water across

421
00:17:11.970 --> 00:17:14.810
Southeast Asia peaks at around 600 gigatons

422
00:17:14.890 --> 00:17:16.570
and pulls in its own direction.

423
00:17:16.650 --> 00:17:17.850
Avery: None of which wins.

424
00:17:17.850 --> 00:17:20.850
Anna: None of which wins they combine into an

425
00:17:20.850 --> 00:17:23.730
annual oscillation that never settles. And

426
00:17:23.730 --> 00:17:25.730
my favourite complication in the whole study

427
00:17:25.730 --> 00:17:28.699
is this. When you pile trillions of tonnes of

428
00:17:28.699 --> 00:17:31.059
water onto a continent, the crust

429
00:17:31.059 --> 00:17:33.979
flexes under the load, which means the ground

430
00:17:33.979 --> 00:17:36.019
stations you're using to measure the movement

431
00:17:36.019 --> 00:17:37.219
are themselves moving.

432
00:17:37.379 --> 00:17:39.739
Avery: You have to subtract the observatory from the

433
00:17:39.739 --> 00:17:41.539
observation you do.

434
00:17:41.939 --> 00:17:44.379
Anna: They handled it by combining laser ranging to

435
00:17:44.379 --> 00:17:47.059
satellites, and that's the LAGEOS technique,

436
00:17:47.139 --> 00:17:49.259
which Australia contributes to from the Matt

437
00:17:49.259 --> 00:17:51.899
Stromlo station outside Canberra with

438
00:17:51.899 --> 00:17:54.520
GPS and with the Grace Follow On Gravity

439
00:17:54.520 --> 00:17:56.640
mission, then modelling the crustal

440
00:17:56.640 --> 00:17:58.400
deformation out and

441
00:17:58.400 --> 00:17:59.520
Avery: the practical end, um, of it.

442
00:17:59.520 --> 00:18:02.200
Anna: Felix Landerer, one of the CO authors, put it

443
00:18:02.200 --> 00:18:04.280
well. He said that while these movements

444
00:18:04.280 --> 00:18:07.200
might appear tiny, our modern world relies on

445
00:18:07.200 --> 00:18:10.080
extremely accurate positioning and that by

446
00:18:10.080 --> 00:18:11.880
understanding what changes the reference

447
00:18:11.880 --> 00:18:14.040
system, we can build better reference

448
00:18:14.040 --> 00:18:16.400
systems. He lists the beneficiaries as

449
00:18:16.400 --> 00:18:18.640
everything from global shipping logistics to

450
00:18:18.640 --> 00:18:20.580
precision agriculture, which

451
00:18:20.580 --> 00:18:23.540
Avery: is a long way from black holes. And I rather

452
00:18:23.540 --> 00:18:24.900
like that about this job.

453
00:18:25.460 --> 00:18:26.100
Anna: Me too.

454
00:18:26.740 --> 00:18:29.380
Avery: Last storey, and it's a small, clever one.

455
00:18:30.020 --> 00:18:32.820
20 light years away, there's an object called

456
00:18:32.820 --> 00:18:35.540
Simp M0136.

457
00:18:36.100 --> 00:18:38.940
It's a brown dwarf, too heavy to be a, uh,

458
00:18:39.060 --> 00:18:42.060
planet in the ordinary sense, too light to

459
00:18:42.060 --> 00:18:44.860
have ever ignited hydrogen fusion and become

460
00:18:44.860 --> 00:18:47.490
a star. It sits right on the

461
00:18:47.490 --> 00:18:50.410
boundary and it has no host star at

462
00:18:50.410 --> 00:18:53.290
all. It's just drifting, which makes

463
00:18:53.290 --> 00:18:55.170
it unusually easy to study,

464
00:18:55.810 --> 00:18:58.690
enormously easier. Normally, if

465
00:18:58.690 --> 00:19:00.810
you want to look at a giant planet's

466
00:19:00.810 --> 00:19:03.410
atmosphere, you're fighting the glare of the

467
00:19:03.410 --> 00:19:06.290
star next to it. Here, there's no star.

468
00:19:06.610 --> 00:19:09.250
You just point and look. Which is why

469
00:19:09.250 --> 00:19:11.890
Simp0136

470
00:19:12.130 --> 00:19:14.330
has become the reference object for what

471
00:19:14.330 --> 00:19:17.130
directly imaged giant planets are probably

472
00:19:17.130 --> 00:19:17.490
like.

473
00:19:18.030 --> 00:19:18.830
Anna: And it varies.

474
00:19:19.390 --> 00:19:21.870
Avery: It spins once every 2 hours and

475
00:19:21.870 --> 00:19:24.830
25 minutes, and as it spins, its

476
00:19:24.830 --> 00:19:27.710
brightness changes by a few percent different

477
00:19:27.710 --> 00:19:30.470
amounts at different wavelengths. Which tells

478
00:19:30.470 --> 00:19:33.390
you there's weather, clouds, hotspots,

479
00:19:33.470 --> 00:19:36.030
chemistry, all of it changing as different

480
00:19:36.030 --> 00:19:39.030
faces rotate into view. The trouble has

481
00:19:39.030 --> 00:19:40.670
been that it's looked fantastically

482
00:19:40.670 --> 00:19:43.070
complicated. Multiple mechanisms,

483
00:19:43.150 --> 00:19:45.390
multiple layers, all tangled together.

484
00:19:46.180 --> 00:19:47.380
Anna: So what did they do differently?

485
00:19:47.780 --> 00:19:50.500
Avery: They stopped assuming. Merle Schrader,

486
00:19:50.660 --> 00:19:53.220
a PhD candidate at Trinity College

487
00:19:53.220 --> 00:19:55.940
Dublin with colleagues there and elsewhere,

488
00:19:56.180 --> 00:19:58.300
took one full rotation of Webb

489
00:19:58.300 --> 00:20:01.100
spectroscopy and ran principal Component

490
00:20:01.100 --> 00:20:03.860
Analysis on it, which is a technique for

491
00:20:03.860 --> 00:20:06.660
asking a data set the blunt question, how

492
00:20:06.660 --> 00:20:09.100
many independent things are actually changing

493
00:20:09.100 --> 00:20:11.380
here? Not, does my model fit,

494
00:20:11.930 --> 00:20:14.330
Just how many knobs are being turned?

495
00:20:14.730 --> 00:20:17.290
Anna: And the answer was two.

496
00:20:17.850 --> 00:20:20.330
Avery: Two components are enough to push what's left

497
00:20:20.330 --> 00:20:22.250
over down to the Noise floor of the

498
00:20:22.250 --> 00:20:25.010
instrument. Which means that within what Webb

499
00:20:25.010 --> 00:20:27.530
can detect, there is nothing else going on.

500
00:20:28.010 --> 00:20:29.050
Anna: And what are the two?

501
00:20:29.530 --> 00:20:32.530
Avery: The first is broadband. It moves the whole

502
00:20:32.530 --> 00:20:35.530
spectrum together and that's temperature. The

503
00:20:35.530 --> 00:20:38.380
second is chromatic wavelength dependent

504
00:20:38.380 --> 00:20:40.900
and that traces the vertical structure of the

505
00:20:40.900 --> 00:20:43.860
clouds, how high and how thick they are.

506
00:20:44.180 --> 00:20:46.940
And between them, those two resolve into

507
00:20:46.940 --> 00:20:49.820
three recurring patches that are

508
00:20:49.820 --> 00:20:52.380
hotter with thinner cloud, patches that are

509
00:20:52.380 --> 00:20:54.980
cooler with thick vertically extended cloud

510
00:20:55.300 --> 00:20:57.700
and transitional regions between the two.

511
00:20:58.100 --> 00:20:59.940
Anna: So it looks chaotic and it's actually

512
00:20:59.940 --> 00:21:00.660
organised.

513
00:21:00.900 --> 00:21:03.320
Avery: Low dimensional is the term. Uh, a

514
00:21:03.320 --> 00:21:05.880
famously messy atmosphere turns out to be

515
00:21:05.880 --> 00:21:08.640
running on two dials. And the team then

516
00:21:08.640 --> 00:21:11.280
went further and projected model atmospheres

517
00:21:11.280 --> 00:21:14.040
into the same mathematical space and

518
00:21:14.040 --> 00:21:16.600
found the models largely occupy the same

519
00:21:16.600 --> 00:21:19.520
territory, which is a decent sign the models

520
00:21:19.520 --> 00:21:21.880
are capturing the right physics rather than

521
00:21:21.880 --> 00:21:23.280
accidentally agreeing.

522
00:21:23.920 --> 00:21:26.240
Anna: Two notes of care on this one, please.

523
00:21:26.800 --> 00:21:28.960
The first is that some of the coverage has

524
00:21:28.960 --> 00:21:31.240
described these patterns as persisting over

525
00:21:31.240 --> 00:21:34.150
more than a dozen rotations. The paper's

526
00:21:34.150 --> 00:21:37.110
core analysis is one rotation. That's a

527
00:21:37.110 --> 00:21:39.390
real and interesting result. But a second

528
00:21:39.390 --> 00:21:42.070
high quality rotation is precisely the test

529
00:21:42.070 --> 00:21:44.590
that would confirm the pattern's hold. So

530
00:21:44.590 --> 00:21:46.670
we'll describe it as the test rather than the

531
00:21:46.670 --> 00:21:47.150
finding.

532
00:21:47.630 --> 00:21:49.870
Avery: And the second is our usual one.

533
00:21:50.190 --> 00:21:52.830
Anna: The preprint went up in late July, so there's

534
00:21:52.830 --> 00:21:54.670
about a seven week gap before the journal

535
00:21:54.670 --> 00:21:56.670
version and the Trinity release this week.

536
00:21:57.230 --> 00:21:59.310
Shorter than yesterday's, but worth saying.

537
00:22:00.040 --> 00:22:02.680
Avery: And one lovely human detail to finish.

538
00:22:03.320 --> 00:22:06.240
Simp 0136 is

539
00:22:06.240 --> 00:22:09.040
20 light years away. The web data

540
00:22:09.040 --> 00:22:11.160
Schrader analysed was gathered in

541
00:22:11.160 --> 00:22:13.880
2023, so the light she was working

542
00:22:13.880 --> 00:22:16.360
with left that object in the year she was

543
00:22:16.360 --> 00:22:16.760
born.

544
00:22:17.400 --> 00:22:18.920
Anna: That's a very good reason to go into

545
00:22:18.920 --> 00:22:19.640
astronomy.

546
00:22:20.200 --> 00:22:22.680
Avery: Now we have one more quick one before

547
00:22:22.680 --> 00:22:23.480
Skywatch.

548
00:22:24.040 --> 00:22:27.040
Anna: We do indeed. The sun has gone very quiet

549
00:22:27.040 --> 00:22:27.640
indeed.

550
00:22:28.040 --> 00:22:28.840
Avery: How quiet?

551
00:22:29.320 --> 00:22:32.320
Anna: Very nearly blank. As of yesterday, there was

552
00:22:32.320 --> 00:22:34.800
exactly one numbered active region left on

553
00:22:34.800 --> 00:22:35.960
the Earth facing side,

554
00:22:36.280 --> 00:22:39.040
AR4528. And it's

555
00:22:39.040 --> 00:22:42.040
rotating out of view as we speak. If nothing

556
00:22:42.040 --> 00:22:44.480
new emerges behind it, we're about to get the

557
00:22:44.480 --> 00:22:47.280
Sun's first spotless day since the 24th of

558
00:22:47.280 --> 00:22:48.360
February this year.

559
00:22:48.760 --> 00:22:50.920
Avery: And February was itself notable.

560
00:22:51.480 --> 00:22:53.080
Anna: February ended a streak of

561
00:22:53.080 --> 00:22:56.040
1,335 consecutive

562
00:22:56.040 --> 00:22:59.040
days, with at least one sunspot going all

563
00:22:59.040 --> 00:23:01.080
the way back to June 2022.

564
00:23:01.800 --> 00:23:04.354
Solar cycle 25 peaked in October

565
00:23:04.486 --> 00:23:07.279
2024. And this is what the downhill side

566
00:23:07.279 --> 00:23:09.920
looks like for scale. The last

567
00:23:09.920 --> 00:23:12.360
solar minimum between 2018 and

568
00:23:12.360 --> 00:23:15.160
2020 delivered something like 700

569
00:23:15.240 --> 00:23:16.200
spotless days.

570
00:23:16.850 --> 00:23:19.410
Avery: And minimum itself is still Some way

571
00:23:19.410 --> 00:23:22.290
Anna: off, not expected before about 2030.

572
00:23:22.850 --> 00:23:25.290
But here's the part worth holding onto. And

573
00:23:25.290 --> 00:23:28.250
it connects two storeys we've run recently. A

574
00:23:28.250 --> 00:23:31.010
quiet sun is not a harmless sun. It's a

575
00:23:31.010 --> 00:23:33.450
differently hazardous one. We Talked in

576
00:23:33.450 --> 00:23:36.050
episode 192 about the energy a, uh, big

577
00:23:36.050 --> 00:23:38.730
active region can store for a superflare, and

578
00:23:38.730 --> 00:23:41.330
in 193 about cosmic radiation at

579
00:23:41.330 --> 00:23:44.180
aviation altitudes. And that second one

580
00:23:44.180 --> 00:23:46.980
runs the opposite way. When the sun is quiet,

581
00:23:47.300 --> 00:23:49.700
its magnetic field does less to shield the

582
00:23:49.700 --> 00:23:52.700
inner solar system, so galactic cosmic rays

583
00:23:52.700 --> 00:23:55.540
get through more easily. Radiation dose

584
00:23:55.540 --> 00:23:58.499
at cruising altitude runs 40 to 60% higher

585
00:23:58.499 --> 00:24:00.580
at solar minimum than at maximum.

586
00:24:01.140 --> 00:24:03.060
Avery: Same dial, opposite end.

587
00:24:03.540 --> 00:24:05.940
Anna: Fewer auroras, more cosmic rays.

588
00:24:06.100 --> 00:24:08.580
Avery: And that's a very good excuse to talk about

589
00:24:08.580 --> 00:24:11.510
the sky, because tonight there is something

590
00:24:11.590 --> 00:24:14.430
genuinely worth walking outside for. And

591
00:24:14.430 --> 00:24:16.070
it works from everywhere.

592
00:24:16.790 --> 00:24:17.430
Anna: Venus.

593
00:24:18.390 --> 00:24:20.630
Avery: Venus at, uh, greatest brilliancy.

594
00:24:20.870 --> 00:24:23.110
Tonight, the 18th of September,

595
00:24:23.430 --> 00:24:25.830
Venus reaches its peak brightness for this

596
00:24:25.830 --> 00:24:28.710
entire evening. Apparition magnitude M

597
00:24:29.644 --> 00:24:32.430
4.8. There is nothing else in the night

598
00:24:32.430 --> 00:24:34.710
sky that comes close except the Moon.

599
00:24:35.200 --> 00:24:37.080
Anna: And the reason it peaks tonight rather than

600
00:24:37.080 --> 00:24:39.440
when Venus is full is genuinely

601
00:24:39.440 --> 00:24:40.400
counterintuitive.

602
00:24:40.960 --> 00:24:43.400
Avery: It's the best bit of physics in the whole

603
00:24:43.400 --> 00:24:46.240
segment. Venus is not full tonight.

604
00:24:46.240 --> 00:24:49.200
It's a Crescent, only about 26%

605
00:24:49.200 --> 00:24:52.120
lit. But because it's swinging in closer

606
00:24:52.120 --> 00:24:54.720
to us, that crescent has swollen to

607
00:24:54.720 --> 00:24:56.720
nearly 40 arcseconds across.

608
00:24:57.440 --> 00:25:00.080
Brightness is lit fraction multiplied by

609
00:25:00.080 --> 00:25:02.870
apparent size. And right now, the disc is

610
00:25:02.870 --> 00:25:05.310
growing faster than the illuminated fraction

611
00:25:05.310 --> 00:25:06.190
is shrinking.

612
00:25:06.590 --> 00:25:08.910
Anna: A big, thin crescent beats a small

613
00:25:09.070 --> 00:25:11.310
Avery: full disc every time.

614
00:25:11.790 --> 00:25:14.550
And if you have binoculars, hold them steady

615
00:25:14.550 --> 00:25:16.990
and you'll actually see the crescent shape.

616
00:25:17.550 --> 00:25:20.030
At 40 arcseconds, it's within reach,

617
00:25:20.510 --> 00:25:22.710
which surprises people, because we don't

618
00:25:22.710 --> 00:25:25.070
think of Venus as something you can resolve.

619
00:25:25.470 --> 00:25:27.190
Anna: And if you see a different date quoted

620
00:25:27.190 --> 00:25:29.560
somewhere, some listings say the 22nd.

621
00:25:30.040 --> 00:25:32.800
Both are defensible. The peak is very

622
00:25:32.800 --> 00:25:35.480
flat. Between tonight and the 22nd,

623
00:25:35.480 --> 00:25:37.480
the lit fraction drops from about

624
00:25:37.560 --> 00:25:40.480
26% to 23, while

625
00:25:40.480 --> 00:25:43.240
the disc grows from roughly 40 arc seconds

626
00:25:43.240 --> 00:25:46.000
to 42. And those two changes

627
00:25:46.000 --> 00:25:48.440
very nearly cancel. So you're not going to

628
00:25:48.440 --> 00:25:50.120
miss it by going out on the wrong night.

629
00:25:50.760 --> 00:25:53.440
Avery: Now, how well you do tonight depends

630
00:25:53.440 --> 00:25:55.800
enormously on where you're standing and. And

631
00:25:55.800 --> 00:25:57.320
the gap is dramatic.

632
00:25:57.880 --> 00:26:00.680
Anna: From Sydney, this is a spectacle. Sunset is

633
00:26:00.680 --> 00:26:03.480
at quarter to six, and at that moment, Venus

634
00:26:03.480 --> 00:26:06.120
is 39 degrees above the western horizon.

635
00:26:06.440 --> 00:26:08.400
That's more than a third of the way up the

636
00:26:08.400 --> 00:26:11.200
sky. It doesn't set until just before 9 o',

637
00:26:11.200 --> 00:26:14.000
clock, which gives you 3 hours and 13 minutes

638
00:26:14.000 --> 00:26:16.920
of Venus after sunset. You do not need

639
00:26:16.920 --> 00:26:19.480
a clear horizon. You do not need to hurry.

640
00:26:19.960 --> 00:26:22.320
Avery: And from the Northern Hemisphere, it's a

641
00:26:22.320 --> 00:26:25.210
different evening entirely. From Los

642
00:26:25.210 --> 00:26:27.930
Angeles, sunset is at 10 to 7

643
00:26:27.930 --> 00:26:30.410
and Venus is 14 degrees up.

644
00:26:30.650 --> 00:26:33.610
It sets an hour and 19 minutes after the

645
00:26:33.610 --> 00:26:36.490
sun. From New York, 10 degrees up

646
00:26:36.730 --> 00:26:39.290
one hour and three minutes. From

647
00:26:39.290 --> 00:26:42.050
London, three and a half degrees at sunset

648
00:26:42.050 --> 00:26:44.170
and gone 28 minutes later.

649
00:26:44.570 --> 00:26:47.050
Anna: Which is not Venus being fainter. It's

650
00:26:47.050 --> 00:26:49.170
exactly as bright everywhere. It's the

651
00:26:49.170 --> 00:26:49.770
geometry.

652
00:26:50.390 --> 00:26:53.190
Avery: It's the tilt of the ecliptic. At this

653
00:26:53.190 --> 00:26:55.270
time of year, the plane of the solar system

654
00:26:55.350 --> 00:26:57.950
stands almost vertically out of the western

655
00:26:57.950 --> 00:27:00.470
horizon at dusk from the Southern Hemisphere.

656
00:27:00.950 --> 00:27:03.630
So anything on that plane climbs steeply and

657
00:27:03.630 --> 00:27:06.630
takes a long time to set. From the Northern

658
00:27:06.630 --> 00:27:09.390
Hemisphere in September, that same plane

659
00:27:09.390 --> 00:27:12.230
lies down almost flat against the horizon,

660
00:27:12.310 --> 00:27:15.070
and everything on it skims sideways and

661
00:27:15.070 --> 00:27:15.910
sets quickly.

662
00:27:15.910 --> 00:27:18.600
Anna: So northern listeners, here's the practical

663
00:27:18.600 --> 00:27:21.400
version. Find a spot with a genuinely clear,

664
00:27:21.720 --> 00:27:24.520
flat western horizon. No trees,

665
00:27:24.760 --> 00:27:27.560
no buildings. Start looking 20 minutes after

666
00:27:27.560 --> 00:27:30.120
sunset and don't leave it much past 45.

667
00:27:30.440 --> 00:27:32.280
It'll be the brightest thing in that part of

668
00:27:32.280 --> 00:27:34.760
the sky by an enormous margin. So you won't

669
00:27:34.760 --> 00:27:36.360
be in any doubt once you've got it.

670
00:27:36.600 --> 00:27:39.440
Avery: And a bonus for the South, Mercury is

671
00:27:39.440 --> 00:27:42.160
up there, too. From Sydney, Mercury

672
00:27:42.160 --> 00:27:44.720
is 16 degrees above the horizon at

673
00:27:44.720 --> 00:27:47.570
sunset and and doesn't set for an hour and

674
00:27:47.570 --> 00:27:50.170
20 minutes, which for Mercury is a

675
00:27:50.170 --> 00:27:52.930
comfortable viewing window. From Los Angeles,

676
00:27:52.930 --> 00:27:55.730
it's 9 degrees and 43 minutes.

677
00:27:56.050 --> 00:27:58.970
From London, 4 degrees and 26

678
00:27:58.970 --> 00:28:01.970
minutes. That one really is a southern target

679
00:28:01.970 --> 00:28:02.610
this week.

680
00:28:03.010 --> 00:28:05.170
Anna: And then there's the Moon, which is doing

681
00:28:05.170 --> 00:28:06.770
something rather precise tonight.

682
00:28:07.170 --> 00:28:10.050
Avery: First quarter, and it's exact. The Moon

683
00:28:10.050 --> 00:28:12.610
reaches first quarter at 2043

684
00:28:12.850 --> 00:28:15.670
Universal Time tonight. That's quarter to

685
00:28:15.670 --> 00:28:18.510
5 on Friday afternoon in New York, quarter

686
00:28:18.510 --> 00:28:21.390
to 2 in Los Angeles, quarter to 10

687
00:28:21.390 --> 00:28:23.990
on Friday evening in London, and for

688
00:28:23.990 --> 00:28:26.550
Australia, 20 to 7 on Saturday

689
00:28:26.710 --> 00:28:27.110
morning.

690
00:28:27.510 --> 00:28:29.790
Anna: And from Sydney tonight, the Moon is very

691
00:28:29.790 --> 00:28:32.670
nearly overhead, 82 degrees up at

692
00:28:32.670 --> 00:28:34.670
sunset, which is close enough to straight up

693
00:28:34.670 --> 00:28:36.310
that you'll find yourself leaning back to

694
00:28:36.310 --> 00:28:36.870
look at it.

695
00:28:37.110 --> 00:28:39.190
Avery: Which matters, because tomorrow night,

696
00:28:39.430 --> 00:28:41.670
Saturday the 19th, is international.

697
00:28:42.070 --> 00:28:43.510
Observe the Moon night.

698
00:28:43.980 --> 00:28:46.140
Anna: And the timing of that is not an accident.

699
00:28:46.460 --> 00:28:48.380
It's deliberately scheduled near first

700
00:28:48.380 --> 00:28:50.660
quarter, because first quarter is when the

701
00:28:50.660 --> 00:28:53.060
Moon is at its most interesting. Through any

702
00:28:53.060 --> 00:28:55.820
optical aid along the terminator,

703
00:28:55.900 --> 00:28:58.700
the line dividing day from night, the sun is

704
00:28:58.700 --> 00:29:01.340
striking the surface at a grazing angle. So

705
00:29:01.340 --> 00:29:03.540
every crater rim and mountain throws a long

706
00:29:03.540 --> 00:29:05.420
shadow across the ground behind it.

707
00:29:05.740 --> 00:29:08.700
Avery: A full moon looks Flat. A half moon

708
00:29:08.700 --> 00:29:09.900
looks three dimensional.

709
00:29:10.570 --> 00:29:12.810
Anna: Completely three dimensional. Run the

710
00:29:12.810 --> 00:29:14.530
terminator with even a small pair of

711
00:29:14.530 --> 00:29:17.170
binoculars and the whole landscape stands up

712
00:29:17.170 --> 00:29:19.650
out of the surface. If you've only ever

713
00:29:19.650 --> 00:29:21.090
looked at a full moon and found it

714
00:29:21.090 --> 00:29:23.370
disappointing, this is the night to try

715
00:29:23.370 --> 00:29:26.169
again. NASA's own event is at the US

716
00:29:26.169 --> 00:29:28.050
Space and Rocket Centre in Huntsville

717
00:29:28.050 --> 00:29:30.290
Alabama. But the whole point is that it's

718
00:29:30.290 --> 00:29:33.290
global. You just need to go outside and

719
00:29:33.290 --> 00:29:35.730
Avery: to balance the ledger. Because the south has

720
00:29:35.730 --> 00:29:37.960
had the better of the evening, the morning

721
00:29:37.960 --> 00:29:40.360
sky belongs decisively to the north.

722
00:29:40.760 --> 00:29:43.760
Anna: It does. Mars and Jupiter are both climbing

723
00:29:43.760 --> 00:29:46.440
in the pre dawn east. And the same ecliptic

724
00:29:46.440 --> 00:29:48.560
geometry that flattened Venus for northern

725
00:29:48.560 --> 00:29:50.800
viewers works the other way around. Before

726
00:29:50.800 --> 00:29:53.720
sunrise at nautical dawn tomorrow morning,

727
00:29:54.120 --> 00:29:57.120
Mars is 48 degrees up from Los Angeles and

728
00:29:57.120 --> 00:30:00.040
46 from New York and 41 from London.

729
00:30:00.360 --> 00:30:02.960
Avery: Avery and from Sydney, 20

730
00:30:02.960 --> 00:30:03.400
degree.

731
00:30:04.090 --> 00:30:06.170
Anna: So that's very much your sky, not ours.

732
00:30:06.650 --> 00:30:09.650
Jupiter is 26 degrees up from Los Angeles and

733
00:30:09.650 --> 00:30:11.250
nine from Sydney and those

734
00:30:11.250 --> 00:30:14.090
Avery: two are closing on each other. They're about

735
00:30:14.090 --> 00:30:16.930
23 degrees apart this morning. By mid

736
00:30:16.930 --> 00:30:19.850
October that's down to 12 and

737
00:30:19.850 --> 00:30:22.450
by the middle of November they'll be a little

738
00:30:22.450 --> 00:30:25.250
over a degree apart, close enough to cover

739
00:30:25.250 --> 00:30:28.130
with a fingertip at arm's length. That's the

740
00:30:28.130 --> 00:30:29.450
one to diarize.

741
00:30:31.040 --> 00:30:33.200
Anna: Saturn meanwhile is up uh, essentially all

742
00:30:33.200 --> 00:30:35.560
night from everywhere. It doesn't set until

743
00:30:35.560 --> 00:30:38.400
nearly 7 in the morning from Sydney and just

744
00:30:38.400 --> 00:30:41.160
before 8 from Los Angeles. And it's building

745
00:30:41.160 --> 00:30:44.040
toward opposition on the 4th of October when

746
00:30:44.040 --> 00:30:46.520
the disc will be about 19 and a half arc

747
00:30:46.520 --> 00:30:49.240
seconds across with the rings roughly 7

748
00:30:49.240 --> 00:30:49.920
degrees open.

749
00:30:51.280 --> 00:30:54.080
Avery: And the equinox next week, which

750
00:30:54.080 --> 00:30:54.360
we'll

751
00:30:54.360 --> 00:30:56.440
Anna: flag carefully because we got this slightly

752
00:30:56.440 --> 00:30:58.800
wrong in an earlier episode and corrected it.

753
00:30:59.490 --> 00:31:01.890
The September equinox is a single instant,

754
00:31:02.050 --> 00:31:04.730
not a day. Five minutes past midnight

755
00:31:04.730 --> 00:31:07.450
Universal Time on the 23rd, that's the

756
00:31:07.450 --> 00:31:10.450
evening of the 22nd across the Americas and

757
00:31:10.450 --> 00:31:13.450
mid morning on the 23rd in Australia. So the

758
00:31:13.450 --> 00:31:15.290
date depends entirely on where you're

759
00:31:15.290 --> 00:31:15.730
standing.

760
00:31:16.450 --> 00:31:18.850
Avery: And one last thing which follows directly

761
00:31:18.850 --> 00:31:21.010
from that quick hit about the blank sun,

762
00:31:21.330 --> 00:31:23.810
because every time we mention sunspots

763
00:31:23.970 --> 00:31:26.370
people quite reasonably want to go and

764
00:31:26.370 --> 00:31:29.120
Anna: look and you can, but

765
00:31:29.120 --> 00:31:32.120
never, ever with unprotected eyes and never

766
00:31:32.120 --> 00:31:34.480
through binoculars or a telescope that isn't

767
00:31:34.480 --> 00:31:37.240
purpose built for it. The only safe way to

768
00:31:37.240 --> 00:31:39.960
look directly at the sun is through filters

769
00:31:39.960 --> 00:31:42.959
certified to the international standard ISO

770
00:31:42.959 --> 00:31:45.880
123122. That's

771
00:31:45.880 --> 00:31:48.480
eclipse glasses or a proper solar filter

772
00:31:48.480 --> 00:31:50.240
fitted over the front of the instrument,

773
00:31:50.640 --> 00:31:53.290
never on the eyepiece end and

774
00:31:53.290 --> 00:31:56.250
Avery: ordinary sunglasses are not solar filters.

775
00:31:56.570 --> 00:31:59.210
Neither is smoked glass, exposed film,

776
00:31:59.370 --> 00:32:02.130
a cd, or stacking several pairs of

777
00:32:02.130 --> 00:32:03.210
sunglasses together.

778
00:32:04.010 --> 00:32:06.370
Anna: None of those are safe. Cheque your eclipse

779
00:32:06.370 --> 00:32:07.650
glasses for the ISO

780
00:32:07.650 --> 00:32:10.330
123122 marking,

781
00:32:10.410 --> 00:32:12.530
and if they're scratched, punctured or you

782
00:32:12.530 --> 00:32:15.330
can't find the marking, don't use them. The

783
00:32:15.330 --> 00:32:17.770
safest option of all, and honestly the best

784
00:32:17.770 --> 00:32:20.080
one for a group, is projection.

785
00:32:20.560 --> 00:32:22.920
Put the sun's image onto a white card and

786
00:32:22.920 --> 00:32:24.800
everybody can look at once without anyone

787
00:32:24.800 --> 00:32:25.440
looking up.

788
00:32:26.160 --> 00:32:28.600
Avery: Although this week there may be nothing to

789
00:32:28.600 --> 00:32:30.400
see, which is rather the point.

790
00:32:31.040 --> 00:32:33.310
Anna: That's Astronomy daily for Friday 18th

791
00:32:33.450 --> 00:32:35.600
September. Everything we've talked about

792
00:32:35.600 --> 00:32:38.200
today, the papers, the DoIs, the full

793
00:32:38.200 --> 00:32:41.120
Skywatch figures for all four cities is in

794
00:32:41.120 --> 00:32:43.400
the show notes and on the website at

795
00:32:43.400 --> 00:32:44.320
astronomydaily

796
00:32:44.560 --> 00:32:47.450
Avery: IO, where you'll also find the full

797
00:32:47.450 --> 00:32:50.290
back catalogue, the daily newsletter and

798
00:32:50.290 --> 00:32:53.290
the contact form. And do use that contact

799
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form. Some of our favourite segments this

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year have come from listener questions, and

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we read every one of them.

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Anna: Go out tonight and look west. Venus will not

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be this bright again

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Avery: this year, and tomorrow night, look

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at the Moon.

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Anna: We'll be back on the weekend with this week's

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weekend wrap. Until then, clear skies.