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00:00:00 --> 00:00:02 Anna: Take a black hole 10 times the mass of the
00:00:02 --> 00:00:05 Sun. Now take one 10 million times
00:00:05 --> 00:00:08 heavier, the kind that sits in the middle of
00:00:08 --> 00:00:10 a galaxy and anchors a hundred billion
00:00:10 --> 00:00:11 stars.
00:00:11 --> 00:00:14 Avery: Those two objects have almost nothing in
00:00:14 --> 00:00:17 common. Different sizes, different
00:00:17 --> 00:00:19 neighbourhoods, different life storeys.
00:00:20 --> 00:00:23 One of them you could fit inside a city. The
00:00:23 --> 00:00:25 other one you could pour the entire inner
00:00:25 --> 00:00:27 solar system into and still have room.
00:00:28 --> 00:00:31 Anna: And this week, a team led out of Perth has
00:00:31 --> 00:00:33 shown that when it comes to the single most
00:00:33 --> 00:00:36 dramatic thing a black hole does, firing a
00:00:36 --> 00:00:38 jet of material out into space at close to
00:00:38 --> 00:00:40 the speed of light, both of them wait for
00:00:40 --> 00:00:42 exactly the same moment,
00:00:43 --> 00:00:45 Avery: same trigger, same threshold,
00:00:46 --> 00:00:48 seven orders of magnitude apart.
00:00:48 --> 00:00:51 Anna: Hello and welcome to Astronomy AstroDailyPod.
00:00:51 --> 00:00:52 I'm Anna.
00:00:52 --> 00:00:55 Avery: And I'm avery. It's Friday the 18th
00:00:55 --> 00:00:58 of September, 2026, and this is
00:00:58 --> 00:01:00 episode 197.
00:01:01 --> 00:01:02 Anna: So let's start with the thing that makes this
00:01:02 --> 00:01:05 hard. Black holes don't emit anything
00:01:05 --> 00:01:08 themselves. What we actually see is the
00:01:08 --> 00:01:11 material falling in. It piles up into a
00:01:11 --> 00:01:14 disc. It heats up through friction until it's
00:01:14 --> 00:01:16 glowing across the electromagnetic spectrum.
00:01:16 --> 00:01:18 And then sometimes, not always, but
00:01:18 --> 00:01:21 sometimes, some of it gets flung back out in
00:01:21 --> 00:01:23 a narrow, tightly collimated jet.
00:01:24 --> 00:01:26 Avery: And the question of when you get a jet and
00:01:26 --> 00:01:28 when you don't has been open for decades.
00:01:29 --> 00:01:32 Anna: It m has. But for one class of black hole,
00:01:32 --> 00:01:35 we actually have a very good answer. Stellar
00:01:35 --> 00:01:37 mass. Black holes, the ones left behind when
00:01:37 --> 00:01:40 a massive Star collapses maybe five to 20
00:01:40 --> 00:01:43 times the Sun's mass, often sit in binary
00:01:43 --> 00:01:46 systems, pulling material off a companion
00:01:46 --> 00:01:48 star. And those systems cycle. They
00:01:48 --> 00:01:51 brighten, they fade, they go through what
00:01:51 --> 00:01:54 astronomers call state transitions, and they
00:01:54 --> 00:01:56 do it over weeks and months, which
00:01:56 --> 00:01:59 Avery: means we've watched the whole process start
00:01:59 --> 00:02:01 to finish many times over
00:02:02 --> 00:02:03 many, many times.
00:02:04 --> 00:02:06 Anna: And what we've learned is that the jets
00:02:06 --> 00:02:08 switch on and off at particular points in
00:02:08 --> 00:02:11 that cycle. And those points are tied to how
00:02:11 --> 00:02:13 fast the black hole is feeding, measured
00:02:13 --> 00:02:16 against something called the Eddington limit.
00:02:16 --> 00:02:18 Avery: And this is worth spelling out, because
00:02:18 --> 00:02:20 everything else today rests on
00:02:21 --> 00:02:22 really does.
00:02:23 --> 00:02:25 Anna: So material falling toward a black hole gets
00:02:25 --> 00:02:28 extremely hot, and hot material radiates.
00:02:29 --> 00:02:31 That radiation pushes outward, gravity
00:02:31 --> 00:02:34 pulls inward. The Eddington limit is the
00:02:34 --> 00:02:36 point where those two balance, where the
00:02:36 --> 00:02:38 radiation coming off the infalling material
00:02:38 --> 00:02:41 is pushing outward just as hard as the black
00:02:41 --> 00:02:43 hole's gravity is pulling in.
00:02:44 --> 00:02:46 Avery: So if you try to feed a black hole faster
00:02:46 --> 00:02:49 than that, the radiation starts blowing the
00:02:49 --> 00:02:50 meal away.
00:02:50 --> 00:02:52 Anna: Anna. Roughly, yes.
00:02:53 --> 00:02:55 Though nature has ways around it, and we'll
00:02:55 --> 00:02:57 come back to that. The useful thing about the
00:02:57 --> 00:03:00 Eddington limit is that it scales with mass.
00:03:00 --> 00:03:02 A, uh, black hole a million times heavier has
00:03:02 --> 00:03:05 a limit a million times higher. So instead of
00:03:05 --> 00:03:07 talking about absolute brightness,
00:03:07 --> 00:03:09 astronomers talk in fractions of Eddington,
00:03:10 --> 00:03:12 10% of Eddington, 1% of
00:03:12 --> 00:03:15 Eddington. And that gives you a way to
00:03:15 --> 00:03:17 compare black holes of wildly different sizes
00:03:17 --> 00:03:18 on the same scale.
00:03:19 --> 00:03:21 Avery: Which brings us to the supermassive black
00:03:21 --> 00:03:24 holes. And the reason this has been so
00:03:24 --> 00:03:25 stubborn.
00:03:25 --> 00:03:28 Anna: The reason is simply time. Everything about
00:03:28 --> 00:03:30 an accretion flow scales with the mass of the
00:03:30 --> 00:03:33 black hole, and that includes how fast it
00:03:33 --> 00:03:36 evolves. A stellar mass system cycles in
00:03:36 --> 00:03:39 months. A supermassive black hole in an
00:03:39 --> 00:03:41 ordinary active galaxy takes something on the
00:03:41 --> 00:03:43 order of thousands of years to do the
00:03:43 --> 00:03:44 equivalent.
00:03:45 --> 00:03:47 Avery: So you can't watch one. You get a single
00:03:47 --> 00:03:50 frame of a film that runs for millennia,
00:03:51 --> 00:03:52 exactly that.
00:03:52 --> 00:03:54 Anna: You can look at thousands of active galaxies
00:03:54 --> 00:03:57 and take a census. Here's one with a jet,
00:03:57 --> 00:04:00 here's one without. But you can never watch a
00:04:00 --> 00:04:02 single object cross a threshold and see what
00:04:02 --> 00:04:05 happens. Which means the critical accretion
00:04:05 --> 00:04:07 rates that we know so precisely for stellar
00:04:07 --> 00:04:09 mass black holes have simply been
00:04:09 --> 00:04:11 unmeasurable for the big ones.
00:04:11 --> 00:04:14 Avery: Unless something gives a supermassive black
00:04:14 --> 00:04:16 hole a sudden, well defined meal.
00:04:17 --> 00:04:20 Anna: And that's the move. Tidal disruption
00:04:20 --> 00:04:23 events. A, uh, star wanders too close to the
00:04:23 --> 00:04:25 supermassive black hole at the centre of its
00:04:25 --> 00:04:28 galaxy. The difference in gravitational pull
00:04:28 --> 00:04:30 across the star exceeds what's holding the
00:04:30 --> 00:04:32 star together and it comes apart.
00:04:32 --> 00:04:35 Roughly half the debris gets flung away and
00:04:35 --> 00:04:37 the other half falls back and builds a brand
00:04:37 --> 00:04:39 new accretion disc from nothing.
00:04:40 --> 00:04:42 Avery: And that one plays out fast.
00:04:42 --> 00:04:45 Anna: Years, not millennia. You get to watch
00:04:45 --> 00:04:48 an accretion flow around a supermassive black
00:04:48 --> 00:04:50 hole be born, rise,
00:04:50 --> 00:04:53 peak and decline. And you can do it
00:04:53 --> 00:04:56 inside a single research career. That is the
00:04:56 --> 00:04:58 entire reason this result was possible.
00:04:59 --> 00:05:02 Avery: So Dr. Adele Goodwin at, uh, Curtin
00:05:02 --> 00:05:05 University's International Centre for Radio
00:05:05 --> 00:05:08 Astronomy Research in Perth, who's also
00:05:08 --> 00:05:10 a Forest Research foundation fellow,
00:05:10 --> 00:05:13 working with Dr. Andrew Mummery at the
00:05:13 --> 00:05:15 Institute for Advanced Study in Princeton,
00:05:16 --> 00:05:18 published in Nature astronomy yesterday,
00:05:19 --> 00:05:21 Thursday 17th September.
00:05:21 --> 00:05:24 Anna: And what they show is that tidal disruption
00:05:24 --> 00:05:26 events actually launch outflows twice
00:05:27 --> 00:05:29 in two physically distinct episodes. The
00:05:29 --> 00:05:32 first comes early, while the black hole is
00:05:32 --> 00:05:34 feeding above its Eddington limit. That brief
00:05:34 --> 00:05:37 ferocious phase right after the star comes
00:05:37 --> 00:05:39 apart, when there's far more material trying
00:05:39 --> 00:05:41 to get in than the black hole can comfortably
00:05:41 --> 00:05:42 swallow.
00:05:42 --> 00:05:45 Avery: The prompt outflow, the prompt one.
00:05:46 --> 00:05:48 Anna: And then later, sometimes months or years
00:05:48 --> 00:05:51 later, there's a second separate
00:05:51 --> 00:05:53 outflow. And that One arrives as the
00:05:53 --> 00:05:55 accretion rate falls through a specific
00:05:55 --> 00:05:58 value, about 2% of the Eddington
00:05:58 --> 00:05:58 limit.
00:05:58 --> 00:06:01 Avery: 2%. And that number is
00:06:01 --> 00:06:02 familiar.
00:06:02 --> 00:06:05 Anna: That number is the same critical accretion
00:06:05 --> 00:06:07 rate at which stellar mass black holes in
00:06:07 --> 00:06:09 binaries make their state transitions.
00:06:10 --> 00:06:12 The same value we've measured over and over
00:06:12 --> 00:06:15 in objects millions of times smaller.
00:06:15 --> 00:06:17 Goodwin and Mummery are arguing that the
00:06:17 --> 00:06:20 coupling between how a black hole feeds and
00:06:20 --> 00:06:23 how it throws material back out is scale
00:06:23 --> 00:06:25 invariant, that it doesn't care
00:06:25 --> 00:06:27 Avery: about mass at all across roughly seven
00:06:28 --> 00:06:30 orders of magnitude, which, if it
00:06:30 --> 00:06:31 holds,
00:06:31 --> 00:06:34 Anna: is a genuinely unifying statement about a
00:06:34 --> 00:06:36 class of object we usually treat as two
00:06:36 --> 00:06:37 separate populations.
00:06:38 --> 00:06:40 Avery: And it also clears up a mess, doesn't it?
00:06:40 --> 00:06:43 Because radio astronomers have had an awkward
00:06:43 --> 00:06:45 problem with these events for a while,
00:06:45 --> 00:06:48 Anna: a very awkward one. Some tidal disruption
00:06:48 --> 00:06:50 events produce a radio flare almost
00:06:50 --> 00:06:53 immediately. Some produce one much later,
00:06:53 --> 00:06:56 out of nowhere, sometimes years after
00:06:56 --> 00:06:58 everyone has stopped paying attention. Some
00:06:58 --> 00:07:01 appear to produce nothing at all that looked
00:07:01 --> 00:07:04 like three different phenomena or worse, like
00:07:04 --> 00:07:04 noise.
00:07:05 --> 00:07:07 Avery: And the two outflow picture makes it
00:07:07 --> 00:07:10 Anna: one phenomenon, one phenomenon with two
00:07:10 --> 00:07:12 stages, where whether you see either of them
00:07:12 --> 00:07:15 depends on when you happen to be looking and
00:07:15 --> 00:07:18 how the accretion rate was falling. The
00:07:18 --> 00:07:20 prompt and delayed outflows stop being a
00:07:20 --> 00:07:22 puzzle and start being a prediction.
00:07:23 --> 00:07:25 Avery: And Goodwin makes a very practical point
00:07:25 --> 00:07:26 about that.
00:07:26 --> 00:07:28 Anna: She does, and it's my favourite part of the
00:07:28 --> 00:07:30 release. Her line is quote,
00:07:31 --> 00:07:34 radio telescopes are incredibly powerful, but
00:07:34 --> 00:07:36 knowing when to look is just as important as
00:07:36 --> 00:07:39 knowing where to look. And she goes on,
00:07:39 --> 00:07:41 if we can anticipate when a black hole is
00:07:41 --> 00:07:43 more likely to launch a jet, we can run
00:07:43 --> 00:07:46 better targeted campaigns, waste fewer
00:07:46 --> 00:07:48 observations and improve our chances of
00:07:48 --> 00:07:50 catching these rare events. At the moment,
00:07:50 --> 00:07:52 they matter most, which
00:07:52 --> 00:07:54 Avery: is not a small thing when you're competing
00:07:54 --> 00:07:56 for time on the world's big dishes.
00:07:57 --> 00:07:59 Anna: It's the difference between a survey and a
00:07:59 --> 00:08:02 stakeout. And it matters enormously for
00:08:02 --> 00:08:04 what's coming, because the Vera Rubin
00:08:04 --> 00:08:06 Observatory is about to start finding tidal
00:08:06 --> 00:08:09 disruption events in numbers we've never had
00:08:09 --> 00:08:12 before. If you can look at the optical light
00:08:12 --> 00:08:14 curve and predict roughly when the radio
00:08:14 --> 00:08:16 outflow should arrive, you can have the radio
00:08:16 --> 00:08:18 telescopes ready.
00:08:18 --> 00:08:21 Avery: Without that, you're guessing, and this
00:08:21 --> 00:08:23 one is properly ours.
00:08:23 --> 00:08:26 Anna: It is led from Curtin University in
00:08:26 --> 00:08:29 Perth through icrar, and the underlying data
00:08:29 --> 00:08:31 set is the product of years of multi
00:08:31 --> 00:08:34 wavelength campaigns, drawing on telescopes
00:08:34 --> 00:08:36 in Australia, the United States, India,
00:08:37 --> 00:08:40 South Africa and in space. This is
00:08:40 --> 00:08:42 not a single instrument result. It's a
00:08:42 --> 00:08:45 decade of patient radio follow up on rare
00:08:45 --> 00:08:47 events assembled into One
00:08:47 --> 00:08:50 Avery: picture, and it lands in the same week that
00:08:50 --> 00:08:52 we're talking about the Square Kilometre
00:08:52 --> 00:08:55 Array's southern half taking shape out in
00:08:55 --> 00:08:58 Western Australia, which is precisely the
00:08:58 --> 00:08:59 Anna: instrument you'd want pointed at the next
00:08:59 --> 00:09:00 one.
00:09:00 --> 00:09:02 Avery: Two things to keep straight, though.
00:09:02 --> 00:09:04 Anna: Yes. The first is that this is a
00:09:04 --> 00:09:07 threshold, not a switch. Saying
00:09:08 --> 00:09:10 black holes launch outflows at around
00:09:10 --> 00:09:13 2% of Eddington is a statement
00:09:13 --> 00:09:16 about where the transition sits, not a
00:09:16 --> 00:09:18 promise that every black hole crossing that
00:09:18 --> 00:09:20 line will produce a jet you can detect.
00:09:21 --> 00:09:23 Whether you actually see one depends on the
00:09:23 --> 00:09:26 environment, the geometry, the distance
00:09:26 --> 00:09:28 and the luck of having a telescope pointed in
00:09:28 --> 00:09:29 the right direction.
00:09:30 --> 00:09:30 Avery: And the second?
00:09:31 --> 00:09:34 Anna: The second is about timing. And we want to be
00:09:34 --> 00:09:36 straight with you. This was published in
00:09:36 --> 00:09:38 Nature Astronomy yesterday. That's real. And
00:09:38 --> 00:09:41 the peer review is done. But the preprint
00:09:41 --> 00:09:44 went up on the archive back in February. So
00:09:44 --> 00:09:46 while this is newly published, it is not
00:09:46 --> 00:09:48 newly written. And if it feels vaguely
00:09:48 --> 00:09:50 familiar to anyone who follows the preprint
00:09:50 --> 00:09:52 servers closely, that's why.
00:09:53 --> 00:09:55 Avery: Which is a pattern we flagged before on this
00:09:55 --> 00:09:55 show.
00:09:56 --> 00:09:58 Anna: It is, and we'll keep flagging it. The
00:09:58 --> 00:10:00 science is no less good for having waited
00:10:00 --> 00:10:03 seven months in review. But published
00:10:03 --> 00:10:06 this week and discovered this week are
00:10:06 --> 00:10:09 different claims, and we try very hard not to
00:10:09 --> 00:10:09 blur them.
00:10:09 --> 00:10:11 Avery: Longtime listeners will hear some threads
00:10:11 --> 00:10:14 here, too. We covered a wandering black hole
00:10:14 --> 00:10:17 revealed by a tidal disruption event back in
00:10:17 --> 00:10:19 episode 153. And a, uh, quasar
00:10:19 --> 00:10:22 driving turbulence across 300 light
00:10:22 --> 00:10:24 years in 154.
00:10:25 --> 00:10:27 Anna: And the partial disruptions that fade and
00:10:27 --> 00:10:29 come back in episode 185.
00:10:29 --> 00:10:32 This result is the framework those individual
00:10:32 --> 00:10:33 objects have been waiting for.
00:10:34 --> 00:10:36 Avery: Now on to storey two. That phrase.
00:10:37 --> 00:10:39 Super Eddington came up in the lead, and
00:10:39 --> 00:10:42 it's about to come up again. Because the
00:10:42 --> 00:10:44 second storey this week is also about a black
00:10:44 --> 00:10:47 hole eating faster than it should be able to
00:10:47 --> 00:10:49 just 11 billion years earlier.
00:10:50 --> 00:10:51 Anna: The little red dots.
00:10:52 --> 00:10:54 Avery: The little red dots. And if you've been with
00:10:54 --> 00:10:56 us a while, you'll know these have been one
00:10:56 --> 00:10:59 of the most irritating things the James Webb
00:10:59 --> 00:11:02 Space Telescope has found. Extremely
00:11:02 --> 00:11:04 compact, extremely red,
00:11:05 --> 00:11:07 extremely numerous in the early universe.
00:11:08 --> 00:11:10 They show the broad hydrogen emission lines
00:11:10 --> 00:11:13 you'd expect from gas whipping around a black
00:11:13 --> 00:11:15 hole. But there are far too many of them.
00:11:16 --> 00:11:18 And the black holes implied are far too heavy
00:11:18 --> 00:11:20 for the galaxies they sit in.
00:11:20 --> 00:11:22 Anna: Which has produced a small industry of
00:11:22 --> 00:11:23 explanations.
00:11:24 --> 00:11:26 Avery: Some of them fairly exotic, some very
00:11:26 --> 00:11:29 exotic. So here's what's new.
00:11:29 --> 00:11:32 A team led by Sun Myun Chan, with
00:11:32 --> 00:11:35 Shingo Hirano Tomoaki ishiyama
00:11:35 --> 00:11:38 Sukjoon Chang and Volker Springle,
00:11:38 --> 00:11:41 published in Nature on 16 September,
00:11:41 --> 00:11:43 ran fully cosmological radiation
00:11:43 --> 00:11:46 hydrodynamic simulations of a dense
00:11:46 --> 00:11:49 protocluster region in the early universe
00:11:49 --> 00:11:51 using Japan's Atarui UH3
00:11:51 --> 00:11:52 supercomputer.
00:11:53 --> 00:11:55 Anna: And crucially, without putting the answer in
00:11:55 --> 00:11:55 by hand.
00:11:56 --> 00:11:58 Avery: That's the point. They didn't seed the
00:11:58 --> 00:12:01 simulation with the black holes they wanted.
00:12:01 --> 00:12:04 They let the physics run. And what happens
00:12:04 --> 00:12:07 is in that intensely over dense environment,
00:12:07 --> 00:12:10 the the giant gas clouds are bathed in so
00:12:10 --> 00:12:13 much ultraviolet radiation that they can't
00:12:13 --> 00:12:16 fragment into ordinary stars. Normally
00:12:16 --> 00:12:18 a big cloud breaks up into lots of little
00:12:18 --> 00:12:21 stellar nurseries. Here it can't.
00:12:21 --> 00:12:24 So instead the whole thing collapses as one
00:12:24 --> 00:12:27 object, a primordial supermassive star,
00:12:28 --> 00:12:30 which then promptly collapses into a black
00:12:30 --> 00:12:33 hole, a heavy seed. A
00:12:33 --> 00:12:36 heavy seed of around a million solar
00:12:36 --> 00:12:38 masses, which is roughly 10 times heavier
00:12:38 --> 00:12:41 than theory typically expects. And then those
00:12:41 --> 00:12:44 seeds develop dense optically thick discs,
00:12:45 --> 00:12:47 so thick that light bounces around inside
00:12:47 --> 00:12:50 them, scattering off free electrons on the
00:12:50 --> 00:12:53 way out. And that electron scattering
00:12:53 --> 00:12:56 smears the hydrogen emission into exactly the
00:12:56 --> 00:12:58 broad lines we see in little red dots.
00:12:59 --> 00:13:01 Anna: So the broad lines aren't necessarily telling
00:13:01 --> 00:13:03 you the gas is moving as fast as you'd
00:13:03 --> 00:13:04 assumed.
00:13:04 --> 00:13:07 Avery: That's the elegant bit. Part of the width is
00:13:07 --> 00:13:10 scattering, not velocity. And then
00:13:10 --> 00:13:13 sustained super Eddington accretion drives
00:13:13 --> 00:13:15 the thing up to around 30 million solar
00:13:15 --> 00:13:18 masses by a redshift of 8, which puts
00:13:18 --> 00:13:21 it right in the population of overmassive
00:13:21 --> 00:13:23 quasars. Webb keeps finding.
00:13:23 --> 00:13:26 Anna: So it's one continuous storey seed
00:13:26 --> 00:13:29 dot quasar and eventually the
00:13:29 --> 00:13:31 supermassive black holes sitting in galaxies
00:13:31 --> 00:13:32 today.
00:13:32 --> 00:13:35 Avery: One storey, with the little red dot phase
00:13:35 --> 00:13:38 being short and dust shrouded. A, uh, stage
00:13:38 --> 00:13:40 rather than a species. And the timing is
00:13:40 --> 00:13:43 rather lovely because Nature Astronomy
00:13:43 --> 00:13:46 published a whole focus issue on little red
00:13:46 --> 00:13:48 dots the same week, including a review
00:13:48 --> 00:13:51 by Hannah Hubler on massive black holes
00:13:51 --> 00:13:54 in the first billion years. M the field is
00:13:54 --> 00:13:57 consolidating and this happens to be the week
00:13:57 --> 00:13:57 it did it.
00:13:58 --> 00:14:00 Anna: One caution, though, and it's the same one we
00:14:00 --> 00:14:02 gave for the black hole feedback work last
00:14:02 --> 00:14:02 month.
00:14:03 --> 00:14:04 Avery: Go on.
00:14:04 --> 00:14:06 Anna: This is a simulation that reproduces the
00:14:06 --> 00:14:09 observations. That's a demonstration that the
00:14:09 --> 00:14:11 physics is sufficient. You don't need
00:14:11 --> 00:14:14 anything exotic to get little red dots. It is
00:14:14 --> 00:14:16 not, on its own proof that this is what
00:14:16 --> 00:14:19 actually happened. Other routes may also
00:14:19 --> 00:14:22 produce them. What would settle it is a
00:14:22 --> 00:14:24 direct measurement of one of these objects
00:14:24 --> 00:14:27 that distinguishes scattering broadened lines
00:14:27 --> 00:14:28 from genuinely fast
00:14:28 --> 00:14:30 Avery: gas, which is exactly the Kind of thing
00:14:30 --> 00:14:31 Webb could do.
00:14:32 --> 00:14:35 Anna: Right, next up, let's come all the way back,
00:14:35 --> 00:14:38 about 6 kilometres, in fact, to the
00:14:38 --> 00:14:39 middle of the Earth, which
00:14:39 --> 00:14:41 Avery: is not where you think it is.
00:14:41 --> 00:14:43 Anna: It really isn't. And this is one of those
00:14:43 --> 00:14:45 facts that sounds like a trick until you sit
00:14:45 --> 00:14:48 with it. There are two different centres of
00:14:48 --> 00:14:50 the Earth. There's the centre of figure, the
00:14:50 --> 00:14:53 geometric middle of the solid planet surface,
00:14:53 --> 00:14:56 which is essentially fixed. And there's the
00:14:56 --> 00:14:58 centre of mass, the balance point of
00:14:58 --> 00:15:00 everything the planet is made of.
00:15:00 --> 00:15:02 Avery: And the planet is partly made of water and
00:15:02 --> 00:15:04 air, which move constantly.
00:15:05 --> 00:15:07 Anna: Snow falls across Siberia and Canada.
00:15:08 --> 00:15:11 The Apple basin floods. The monsoon
00:15:11 --> 00:15:14 arrives over Southeast Asia. Every one of
00:15:14 --> 00:15:16 those shifts an enormous amount of mass from
00:15:16 --> 00:15:19 one part of the planet to another, and the
00:15:19 --> 00:15:21 balance point moves to follow it. So the
00:15:21 --> 00:15:23 centre of mass and the centre of figure drift
00:15:23 --> 00:15:26 apart and back together over the course of a
00:15:26 --> 00:15:26 year.
00:15:26 --> 00:15:27 Avery: And we care because.
00:15:28 --> 00:15:30 Anna: Because the centre of mass is what satellites
00:15:30 --> 00:15:33 orbit. It's the origin of the reference frame
00:15:33 --> 00:15:36 that underpins satellite navigation and
00:15:36 --> 00:15:38 every precise elevation measurement on the
00:15:38 --> 00:15:41 planet. If you're wrong about where it is,
00:15:41 --> 00:15:43 you're wrong about where everything else is.
00:15:44 --> 00:15:45 Avery: So how big is the wobble?
00:15:45 --> 00:15:48 Anna: That's the news. The traditional figure has
00:15:48 --> 00:15:51 been around 11 millimetres. A team led
00:15:51 --> 00:15:54 by Donald Argus at NASA's Jet Propulsion
00:15:54 --> 00:15:56 Laboratory now puts it at roughly four to
00:15:56 --> 00:15:59 five and a half, about half what we thought.
00:16:00 --> 00:16:01 Published in Geophysical Journal
00:16:01 --> 00:16:04 International with the JPL release on
00:16:04 --> 00:16:04 Tuesday.
00:16:04 --> 00:16:06 Avery: Half is a big, big correction for something
00:16:06 --> 00:16:08 people have been measuring for decades.
00:16:08 --> 00:16:11 Anna: It is. And here's how uncertain it's been.
00:16:12 --> 00:16:14 The last two international estimates, one
00:16:14 --> 00:16:17 from 2017 and one from 2023,
00:16:18 --> 00:16:20 disagree with each other by about 7
00:16:20 --> 00:16:22 millimetres, which is almost as large as the
00:16:22 --> 00:16:24 entire motion they're trying to measure.
00:16:24 --> 00:16:27 Avery: So the error bar was the same size as the
00:16:27 --> 00:16:27 signal.
00:16:27 --> 00:16:30 Anna: Essentially, Argus's own line
00:16:30 --> 00:16:32 is that the movement now looks like about
00:16:32 --> 00:16:35 half of of what we believed eight years ago,
00:16:36 --> 00:16:38 and that the mass of water and air sloshing
00:16:38 --> 00:16:41 between the hemispheres is smaller than we
00:16:41 --> 00:16:42 thought.
00:16:42 --> 00:16:44 Avery: And where does the movement actually come
00:16:44 --> 00:16:44 from?
00:16:44 --> 00:16:47 Anna: Three main ocean,
00:16:47 --> 00:16:49 atmosphere and land water.
00:16:50 --> 00:16:53 In March, snow sitting across Eurasia and
00:16:53 --> 00:16:56 North America pulls the centre of mass about
00:16:56 --> 00:16:58 3 millimetres toward the northern Pole.
00:16:59 --> 00:17:02 Then, in April, the Apple hits its annual
00:17:02 --> 00:17:04 water maximum, around 2
00:17:04 --> 00:17:07 gigatons, and tugs it roughly
00:17:07 --> 00:17:09 2.2 millimetres towards south America.
00:17:10 --> 00:17:11 Later in the year, monsoon water across
00:17:11 --> 00:17:14 Southeast Asia peaks at around 600 gigatons
00:17:14 --> 00:17:16 and pulls in its own direction.
00:17:16 --> 00:17:17 Avery: None of which wins.
00:17:17 --> 00:17:20 Anna: None of which wins they combine into an
00:17:20 --> 00:17:23 annual oscillation that never settles. And
00:17:23 --> 00:17:25 my favourite complication in the whole study
00:17:25 --> 00:17:28 is this. When you pile trillions of tonnes of
00:17:28 --> 00:17:31 water onto a continent, the crust
00:17:31 --> 00:17:33 flexes under the load, which means the ground
00:17:33 --> 00:17:36 stations you're using to measure the movement
00:17:36 --> 00:17:37 are themselves moving.
00:17:37 --> 00:17:39 Avery: You have to subtract the observatory from the
00:17:39 --> 00:17:41 observation you do.
00:17:41 --> 00:17:44 Anna: They handled it by combining laser ranging to
00:17:44 --> 00:17:47 satellites, and that's the LAGEOS technique,
00:17:47 --> 00:17:49 which Australia contributes to from the Matt
00:17:49 --> 00:17:51 Stromlo station outside Canberra with
00:17:51 --> 00:17:54 GPS and with the Grace Follow On Gravity
00:17:54 --> 00:17:56 mission, then modelling the crustal
00:17:56 --> 00:17:58 deformation out and
00:17:58 --> 00:17:59 Avery: the practical end, um, of it.
00:17:59 --> 00:18:02 Anna: Felix Landerer, one of the CO authors, put it
00:18:02 --> 00:18:04 well. He said that while these movements
00:18:04 --> 00:18:07 might appear tiny, our modern world relies on
00:18:07 --> 00:18:10 extremely accurate positioning and that by
00:18:10 --> 00:18:11 understanding what changes the reference
00:18:11 --> 00:18:14 system, we can build better reference
00:18:14 --> 00:18:16 systems. He lists the beneficiaries as
00:18:16 --> 00:18:18 everything from global shipping logistics to
00:18:18 --> 00:18:20 precision agriculture, which
00:18:20 --> 00:18:23 Avery: is a long way from black holes. And I rather
00:18:23 --> 00:18:24 like that about this job.
00:18:25 --> 00:18:26 Anna: Me too.
00:18:26 --> 00:18:29 Avery: Last storey, and it's a small, clever one.
00:18:30 --> 00:18:32 20 light years away, there's an object called
00:18:32 --> 00:18:35 Simp M0136.
00:18:36 --> 00:18:38 It's a brown dwarf, too heavy to be a, uh,
00:18:39 --> 00:18:42 planet in the ordinary sense, too light to
00:18:42 --> 00:18:44 have ever ignited hydrogen fusion and become
00:18:44 --> 00:18:47 a star. It sits right on the
00:18:47 --> 00:18:50 boundary and it has no host star at
00:18:50 --> 00:18:53 all. It's just drifting, which makes
00:18:53 --> 00:18:55 it unusually easy to study,
00:18:55 --> 00:18:58 enormously easier. Normally, if
00:18:58 --> 00:19:00 you want to look at a giant planet's
00:19:00 --> 00:19:03 atmosphere, you're fighting the glare of the
00:19:03 --> 00:19:06 star next to it. Here, there's no star.
00:19:06 --> 00:19:09 You just point and look. Which is why
00:19:09 --> 00:19:11 Simp0136
00:19:12 --> 00:19:14 has become the reference object for what
00:19:14 --> 00:19:17 directly imaged giant planets are probably
00:19:17 --> 00:19:17 like.
00:19:18 --> 00:19:18 Anna: And it varies.
00:19:19 --> 00:19:21 Avery: It spins once every 2 hours and
00:19:21 --> 00:19:24 25 minutes, and as it spins, its
00:19:24 --> 00:19:27 brightness changes by a few percent different
00:19:27 --> 00:19:30 amounts at different wavelengths. Which tells
00:19:30 --> 00:19:33 you there's weather, clouds, hotspots,
00:19:33 --> 00:19:36 chemistry, all of it changing as different
00:19:36 --> 00:19:39 faces rotate into view. The trouble has
00:19:39 --> 00:19:40 been that it's looked fantastically
00:19:40 --> 00:19:43 complicated. Multiple mechanisms,
00:19:43 --> 00:19:45 multiple layers, all tangled together.
00:19:46 --> 00:19:47 Anna: So what did they do differently?
00:19:47 --> 00:19:50 Avery: They stopped assuming. Merle Schrader,
00:19:50 --> 00:19:53 a PhD candidate at Trinity College
00:19:53 --> 00:19:55 Dublin with colleagues there and elsewhere,
00:19:56 --> 00:19:58 took one full rotation of Webb
00:19:58 --> 00:20:01 spectroscopy and ran principal Component
00:20:01 --> 00:20:03 Analysis on it, which is a technique for
00:20:03 --> 00:20:06 asking a data set the blunt question, how
00:20:06 --> 00:20:09 many independent things are actually changing
00:20:09 --> 00:20:11 here? Not, does my model fit,
00:20:11 --> 00:20:14 Just how many knobs are being turned?
00:20:14 --> 00:20:17 Anna: And the answer was two.
00:20:17 --> 00:20:20 Avery: Two components are enough to push what's left
00:20:20 --> 00:20:22 over down to the Noise floor of the
00:20:22 --> 00:20:25 instrument. Which means that within what Webb
00:20:25 --> 00:20:27 can detect, there is nothing else going on.
00:20:28 --> 00:20:29 Anna: And what are the two?
00:20:29 --> 00:20:32 Avery: The first is broadband. It moves the whole
00:20:32 --> 00:20:35 spectrum together and that's temperature. The
00:20:35 --> 00:20:38 second is chromatic wavelength dependent
00:20:38 --> 00:20:40 and that traces the vertical structure of the
00:20:40 --> 00:20:43 clouds, how high and how thick they are.
00:20:44 --> 00:20:46 And between them, those two resolve into
00:20:46 --> 00:20:49 three recurring patches that are
00:20:49 --> 00:20:52 hotter with thinner cloud, patches that are
00:20:52 --> 00:20:54 cooler with thick vertically extended cloud
00:20:55 --> 00:20:57 and transitional regions between the two.
00:20:58 --> 00:20:59 Anna: So it looks chaotic and it's actually
00:20:59 --> 00:21:00 organised.
00:21:00 --> 00:21:03 Avery: Low dimensional is the term. Uh, a
00:21:03 --> 00:21:05 famously messy atmosphere turns out to be
00:21:05 --> 00:21:08 running on two dials. And the team then
00:21:08 --> 00:21:11 went further and projected model atmospheres
00:21:11 --> 00:21:14 into the same mathematical space and
00:21:14 --> 00:21:16 found the models largely occupy the same
00:21:16 --> 00:21:19 territory, which is a decent sign the models
00:21:19 --> 00:21:21 are capturing the right physics rather than
00:21:21 --> 00:21:23 accidentally agreeing.
00:21:23 --> 00:21:26 Anna: Two notes of care on this one, please.
00:21:26 --> 00:21:28 The first is that some of the coverage has
00:21:28 --> 00:21:31 described these patterns as persisting over
00:21:31 --> 00:21:34 more than a dozen rotations. The paper's
00:21:34 --> 00:21:37 core analysis is one rotation. That's a
00:21:37 --> 00:21:39 real and interesting result. But a second
00:21:39 --> 00:21:42 high quality rotation is precisely the test
00:21:42 --> 00:21:44 that would confirm the pattern's hold. So
00:21:44 --> 00:21:46 we'll describe it as the test rather than the
00:21:46 --> 00:21:47 finding.
00:21:47 --> 00:21:49 Avery: And the second is our usual one.
00:21:50 --> 00:21:52 Anna: The preprint went up in late July, so there's
00:21:52 --> 00:21:54 about a seven week gap before the journal
00:21:54 --> 00:21:56 version and the Trinity release this week.
00:21:57 --> 00:21:59 Shorter than yesterday's, but worth saying.
00:22:00 --> 00:22:02 Avery: And one lovely human detail to finish.
00:22:03 --> 00:22:06 Simp 0136 is
00:22:06 --> 00:22:09 20 light years away. The web data
00:22:09 --> 00:22:11 Schrader analysed was gathered in
00:22:11 --> 00:22:13 2023, so the light she was working
00:22:13 --> 00:22:16 with left that object in the year she was
00:22:16 --> 00:22:16 born.
00:22:17 --> 00:22:18 Anna: That's a very good reason to go into
00:22:18 --> 00:22:19 astronomy.
00:22:20 --> 00:22:22 Avery: Now we have one more quick one before
00:22:22 --> 00:22:23 Skywatch.
00:22:24 --> 00:22:27 Anna: We do indeed. The sun has gone very quiet
00:22:27 --> 00:22:27 indeed.
00:22:28 --> 00:22:28 Avery: How quiet?
00:22:29 --> 00:22:32 Anna: Very nearly blank. As of yesterday, there was
00:22:32 --> 00:22:34 exactly one numbered active region left on
00:22:34 --> 00:22:35 the Earth facing side,
00:22:36 --> 00:22:39 AR4528. And it's
00:22:39 --> 00:22:42 rotating out of view as we speak. If nothing
00:22:42 --> 00:22:44 new emerges behind it, we're about to get the
00:22:44 --> 00:22:47 Sun's first spotless day since the 24th of
00:22:47 --> 00:22:48 February this year.
00:22:48 --> 00:22:50 Avery: And February was itself notable.
00:22:51 --> 00:22:53 Anna: February ended a streak of
00:22:53 --> 00:22:56 1 consecutive
00:22:56 --> 00:22:59 days, with at least one sunspot going all
00:22:59 --> 00:23:01 the way back to June 2022.
00:23:01 --> 00:23:04 Solar cycle 25 peaked in October
00:23:04 --> 00:23:07 2024. And this is what the downhill side
00:23:07 --> 00:23:09 looks like for scale. The last
00:23:09 --> 00:23:12 solar minimum between 2018 and
00:23:12 --> 00:23:15 2020 delivered something like 700
00:23:15 --> 00:23:16 spotless days.
00:23:16 --> 00:23:19 Avery: And minimum itself is still Some way
00:23:19 --> 00:23:22 Anna: off, not expected before about 2030.
00:23:22 --> 00:23:25 But here's the part worth holding onto. And
00:23:25 --> 00:23:28 it connects two storeys we've run recently. A
00:23:28 --> 00:23:31 quiet sun is not a harmless sun. It's a
00:23:31 --> 00:23:33 differently hazardous one. We Talked in
00:23:33 --> 00:23:36 episode 192 about the energy a, uh, big
00:23:36 --> 00:23:38 active region can store for a superflare, and
00:23:38 --> 00:23:41 in 193 about cosmic radiation at
00:23:41 --> 00:23:44 aviation altitudes. And that second one
00:23:44 --> 00:23:46 runs the opposite way. When the sun is quiet,
00:23:47 --> 00:23:49 its magnetic field does less to shield the
00:23:49 --> 00:23:52 inner solar system, so galactic cosmic rays
00:23:52 --> 00:23:55 get through more easily. Radiation dose
00:23:55 --> 00:23:58 at cruising altitude runs 40 to 60% higher
00:23:58 --> 00:24:00 at solar minimum than at maximum.
00:24:01 --> 00:24:03 Avery: Same dial, opposite end.
00:24:03 --> 00:24:05 Anna: Fewer auroras, more cosmic rays.
00:24:06 --> 00:24:08 Avery: And that's a very good excuse to talk about
00:24:08 --> 00:24:11 the sky, because tonight there is something
00:24:11 --> 00:24:14 genuinely worth walking outside for. And
00:24:14 --> 00:24:16 it works from everywhere.
00:24:16 --> 00:24:17 Anna: Venus.
00:24:18 --> 00:24:20 Avery: Venus at, uh, greatest brilliancy.
00:24:20 --> 00:24:23 Tonight, the 18th of September,
00:24:23 --> 00:24:25 Venus reaches its peak brightness for this
00:24:25 --> 00:24:28 entire evening. Apparition magnitude M
00:24:29 --> 00:24:32 4.8. There is nothing else in the night
00:24:32 --> 00:24:34 sky that comes close except the Moon.
00:24:35 --> 00:24:37 Anna: And the reason it peaks tonight rather than
00:24:37 --> 00:24:39 when Venus is full is genuinely
00:24:39 --> 00:24:40 counterintuitive.
00:24:40 --> 00:24:43 Avery: It's the best bit of physics in the whole
00:24:43 --> 00:24:46 segment. Venus is not full tonight.
00:24:46 --> 00:24:49 It's a Crescent, only about 26%
00:24:49 --> 00:24:52 lit. But because it's swinging in closer
00:24:52 --> 00:24:54 to us, that crescent has swollen to
00:24:54 --> 00:24:56 nearly 40 arcseconds across.
00:24:57 --> 00:25:00 Brightness is lit fraction multiplied by
00:25:00 --> 00:25:02 apparent size. And right now, the disc is
00:25:02 --> 00:25:05 growing faster than the illuminated fraction
00:25:05 --> 00:25:06 is shrinking.
00:25:06 --> 00:25:08 Anna: A big, thin crescent beats a small
00:25:09 --> 00:25:11 Avery: full disc every time.
00:25:11 --> 00:25:14 And if you have binoculars, hold them steady
00:25:14 --> 00:25:16 and you'll actually see the crescent shape.
00:25:17 --> 00:25:20 At 40 arcseconds, it's within reach,
00:25:20 --> 00:25:22 which surprises people, because we don't
00:25:22 --> 00:25:25 think of Venus as something you can resolve.
00:25:25 --> 00:25:27 Anna: And if you see a different date quoted
00:25:27 --> 00:25:29 somewhere, some listings say the 22nd.
00:25:30 --> 00:25:32 Both are defensible. The peak is very
00:25:32 --> 00:25:35 flat. Between tonight and the 22nd,
00:25:35 --> 00:25:37 the lit fraction drops from about
00:25:37 --> 00:25:40 26% to 23, while
00:25:40 --> 00:25:43 the disc grows from roughly 40 arc seconds
00:25:43 --> 00:25:46 to 42. And those two changes
00:25:46 --> 00:25:48 very nearly cancel. So you're not going to
00:25:48 --> 00:25:50 miss it by going out on the wrong night.
00:25:50 --> 00:25:53 Avery: Now, how well you do tonight depends
00:25:53 --> 00:25:55 enormously on where you're standing and. And
00:25:55 --> 00:25:57 the gap is dramatic.
00:25:57 --> 00:26:00 Anna: From Sydney, this is a spectacle. Sunset is
00:26:00 --> 00:26:03 at quarter to six, and at that moment, Venus
00:26:03 --> 00:26:06 is 39 degrees above the western horizon.
00:26:06 --> 00:26:08 That's more than a third of the way up the
00:26:08 --> 00:26:11 sky. It doesn't set until just before 9 o',
00:26:11 --> 00:26:14 clock, which gives you 3 hours and 13 minutes
00:26:14 --> 00:26:16 of Venus after sunset. You do not need
00:26:16 --> 00:26:19 a clear horizon. You do not need to hurry.
00:26:19 --> 00:26:22 Avery: And from the Northern Hemisphere, it's a
00:26:22 --> 00:26:25 different evening entirely. From Los
00:26:25 --> 00:26:27 Angeles, sunset is at 10 to 7
00:26:27 --> 00:26:30 and Venus is 14 degrees up.
00:26:30 --> 00:26:33 It sets an hour and 19 minutes after the
00:26:33 --> 00:26:36 sun. From New York, 10 degrees up
00:26:36 --> 00:26:39 one hour and three minutes. From
00:26:39 --> 00:26:42 London, three and a half degrees at sunset
00:26:42 --> 00:26:44 and gone 28 minutes later.
00:26:44 --> 00:26:47 Anna: Which is not Venus being fainter. It's
00:26:47 --> 00:26:49 exactly as bright everywhere. It's the
00:26:49 --> 00:26:49 geometry.
00:26:50 --> 00:26:53 Avery: It's the tilt of the ecliptic. At this
00:26:53 --> 00:26:55 time of year, the plane of the solar system
00:26:55 --> 00:26:57 stands almost vertically out of the western
00:26:57 --> 00:27:00 horizon at dusk from the Southern Hemisphere.
00:27:00 --> 00:27:03 So anything on that plane climbs steeply and
00:27:03 --> 00:27:06 takes a long time to set. From the Northern
00:27:06 --> 00:27:09 Hemisphere in September, that same plane
00:27:09 --> 00:27:12 lies down almost flat against the horizon,
00:27:12 --> 00:27:15 and everything on it skims sideways and
00:27:15 --> 00:27:15 sets quickly.
00:27:15 --> 00:27:18 Anna: So northern listeners, here's the practical
00:27:18 --> 00:27:21 version. Find a spot with a genuinely clear,
00:27:21 --> 00:27:24 flat western horizon. No trees,
00:27:24 --> 00:27:27 no buildings. Start looking 20 minutes after
00:27:27 --> 00:27:30 sunset and don't leave it much past 45.
00:27:30 --> 00:27:32 It'll be the brightest thing in that part of
00:27:32 --> 00:27:34 the sky by an enormous margin. So you won't
00:27:34 --> 00:27:36 be in any doubt once you've got it.
00:27:36 --> 00:27:39 Avery: And a bonus for the South, Mercury is
00:27:39 --> 00:27:42 up there, too. From Sydney, Mercury
00:27:42 --> 00:27:44 is 16 degrees above the horizon at
00:27:44 --> 00:27:47 sunset and and doesn't set for an hour and
00:27:47 --> 00:27:50 20 minutes, which for Mercury is a
00:27:50 --> 00:27:52 comfortable viewing window. From Los Angeles,
00:27:52 --> 00:27:55 it's 9 degrees and 43 minutes.
00:27:56 --> 00:27:58 From London, 4 degrees and 26
00:27:58 --> 00:28:01 minutes. That one really is a southern target
00:28:01 --> 00:28:02 this week.
00:28:03 --> 00:28:05 Anna: And then there's the Moon, which is doing
00:28:05 --> 00:28:06 something rather precise tonight.
00:28:07 --> 00:28:10 Avery: First quarter, and it's exact. The Moon
00:28:10 --> 00:28:12 reaches first quarter at 2043
00:28:12 --> 00:28:15 Universal Time tonight. That's quarter to
00:28:15 --> 00:28:18 5 on Friday afternoon in New York, quarter
00:28:18 --> 00:28:21 to 2 in Los Angeles, quarter to 10
00:28:21 --> 00:28:23 on Friday evening in London, and for
00:28:23 --> 00:28:26 Australia, 20 to 7 on Saturday
00:28:26 --> 00:28:27 morning.
00:28:27 --> 00:28:29 Anna: And from Sydney tonight, the Moon is very
00:28:29 --> 00:28:32 nearly overhead, 82 degrees up at
00:28:32 --> 00:28:34 sunset, which is close enough to straight up
00:28:34 --> 00:28:36 that you'll find yourself leaning back to
00:28:36 --> 00:28:36 look at it.
00:28:37 --> 00:28:39 Avery: Which matters, because tomorrow night,
00:28:39 --> 00:28:41 Saturday the 19th, is international.
00:28:42 --> 00:28:43 Observe the Moon night.
00:28:43 --> 00:28:46 Anna: And the timing of that is not an accident.
00:28:46 --> 00:28:48 It's deliberately scheduled near first
00:28:48 --> 00:28:50 quarter, because first quarter is when the
00:28:50 --> 00:28:53 Moon is at its most interesting. Through any
00:28:53 --> 00:28:55 optical aid along the terminator,
00:28:55 --> 00:28:58 the line dividing day from night, the sun is
00:28:58 --> 00:29:01 striking the surface at a grazing angle. So
00:29:01 --> 00:29:03 every crater rim and mountain throws a long
00:29:03 --> 00:29:05 shadow across the ground behind it.
00:29:05 --> 00:29:08 Avery: A full moon looks Flat. A half moon
00:29:08 --> 00:29:09 looks three dimensional.
00:29:10 --> 00:29:12 Anna: Completely three dimensional. Run the
00:29:12 --> 00:29:14 terminator with even a small pair of
00:29:14 --> 00:29:17 binoculars and the whole landscape stands up
00:29:17 --> 00:29:19 out of the surface. If you've only ever
00:29:19 --> 00:29:21 looked at a full moon and found it
00:29:21 --> 00:29:23 disappointing, this is the night to try
00:29:23 --> 00:29:26 again. NASA's own event is at the US
00:29:26 --> 00:29:28 Space and Rocket Centre in Huntsville
00:29:28 --> 00:29:30 Alabama. But the whole point is that it's
00:29:30 --> 00:29:33 global. You just need to go outside and
00:29:33 --> 00:29:35 Avery: to balance the ledger. Because the south has
00:29:35 --> 00:29:37 had the better of the evening, the morning
00:29:37 --> 00:29:40 sky belongs decisively to the north.
00:29:40 --> 00:29:43 Anna: It does. Mars and Jupiter are both climbing
00:29:43 --> 00:29:46 in the pre dawn east. And the same ecliptic
00:29:46 --> 00:29:48 geometry that flattened Venus for northern
00:29:48 --> 00:29:50 viewers works the other way around. Before
00:29:50 --> 00:29:53 sunrise at nautical dawn tomorrow morning,
00:29:54 --> 00:29:57 Mars is 48 degrees up from Los Angeles and
00:29:57 --> 00:30:00 46 from New York and 41 from London.
00:30:00 --> 00:30:02 Avery: Avery and from Sydney, 20
00:30:02 --> 00:30:03 degree.
00:30:04 --> 00:30:06 Anna: So that's very much your sky, not ours.
00:30:06 --> 00:30:09 Jupiter is 26 degrees up from Los Angeles and
00:30:09 --> 00:30:11 nine from Sydney and those
00:30:11 --> 00:30:14 Avery: two are closing on each other. They're about
00:30:14 --> 00:30:16 23 degrees apart this morning. By mid
00:30:16 --> 00:30:19 October that's down to 12 and
00:30:19 --> 00:30:22 by the middle of November they'll be a little
00:30:22 --> 00:30:25 over a degree apart, close enough to cover
00:30:25 --> 00:30:28 with a fingertip at arm's length. That's the
00:30:28 --> 00:30:29 one to diarize.
00:30:31 --> 00:30:33 Anna: Saturn meanwhile is up uh, essentially all
00:30:33 --> 00:30:35 night from everywhere. It doesn't set until
00:30:35 --> 00:30:38 nearly 7 in the morning from Sydney and just
00:30:38 --> 00:30:41 before 8 from Los Angeles. And it's building
00:30:41 --> 00:30:44 toward opposition on the 4th of October when
00:30:44 --> 00:30:46 the disc will be about 19 and a half arc
00:30:46 --> 00:30:49 seconds across with the rings roughly 7
00:30:49 --> 00:30:49 degrees open.
00:30:51 --> 00:30:54 Avery: And the equinox next week, which
00:30:54 --> 00:30:54 we'll
00:30:54 --> 00:30:56 Anna: flag carefully because we got this slightly
00:30:56 --> 00:30:58 wrong in an earlier episode and corrected it.
00:30:59 --> 00:31:01 The September equinox is a single instant,
00:31:02 --> 00:31:04 not a day. Five minutes past midnight
00:31:04 --> 00:31:07 Universal Time on the 23rd, that's the
00:31:07 --> 00:31:10 evening of the 22nd across the Americas and
00:31:10 --> 00:31:13 mid morning on the 23rd in Australia. So the
00:31:13 --> 00:31:15 date depends entirely on where you're
00:31:15 --> 00:31:15 standing.
00:31:16 --> 00:31:18 Avery: And one last thing which follows directly
00:31:18 --> 00:31:21 from that quick hit about the blank sun,
00:31:21 --> 00:31:23 because every time we mention sunspots
00:31:23 --> 00:31:26 people quite reasonably want to go and
00:31:26 --> 00:31:29 Anna: look and you can, but
00:31:29 --> 00:31:32 never, ever with unprotected eyes and never
00:31:32 --> 00:31:34 through binoculars or a telescope that isn't
00:31:34 --> 00:31:37 purpose built for it. The only safe way to
00:31:37 --> 00:31:39 look directly at the sun is through filters
00:31:39 --> 00:31:42 certified to the international standard ISO
00:31:42 --> 00:31:45 123122. That's
00:31:45 --> 00:31:48 eclipse glasses or a proper solar filter
00:31:48 --> 00:31:50 fitted over the front of the instrument,
00:31:50 --> 00:31:53 never on the eyepiece end and
00:31:53 --> 00:31:56 Avery: ordinary sunglasses are not solar filters.
00:31:56 --> 00:31:59 Neither is smoked glass, exposed film,
00:31:59 --> 00:32:02 a cd, or stacking several pairs of
00:32:02 --> 00:32:03 sunglasses together.
00:32:04 --> 00:32:06 Anna: None of those are safe. Cheque your eclipse
00:32:06 --> 00:32:07 glasses for the ISO
00:32:07 --> 00:32:10 123122 marking,
00:32:10 --> 00:32:12 and if they're scratched, punctured or you
00:32:12 --> 00:32:15 can't find the marking, don't use them. The
00:32:15 --> 00:32:17 safest option of all, and honestly the best
00:32:17 --> 00:32:20 one for a group, is projection.
00:32:20 --> 00:32:22 Put the sun's image onto a white card and
00:32:22 --> 00:32:24 everybody can look at once without anyone
00:32:24 --> 00:32:25 looking up.
00:32:26 --> 00:32:28 Avery: Although this week there may be nothing to
00:32:28 --> 00:32:30 see, which is rather the point.
00:32:31 --> 00:32:33 Anna: That's Astronomy daily for Friday 18th
00:32:33 --> 00:32:35 September. Everything we've talked about
00:32:35 --> 00:32:38 today, the papers, the DoIs, the full
00:32:38 --> 00:32:41 Skywatch figures for all four cities is in
00:32:41 --> 00:32:43 the show notes and on the website at
00:32:43 --> 00:32:44 astronomydaily
00:32:44 --> 00:32:47 Avery: IO, where you'll also find the full
00:32:47 --> 00:32:50 back catalogue, the daily newsletter and
00:32:50 --> 00:32:53 the contact form. And do use that contact
00:32:53 --> 00:32:55 form. Some of our favourite segments this
00:32:55 --> 00:32:58 year have come from listener questions, and
00:32:58 --> 00:32:59 we read every one of them.
00:33:00 --> 00:33:03 Anna: Go out tonight and look west. Venus will not
00:33:03 --> 00:33:04 be this bright again
00:33:04 --> 00:33:06 Avery: this year, and tomorrow night, look
00:33:06 --> 00:33:07 at the Moon.
00:33:08 --> 00:33:10 Anna: We'll be back on the weekend with this week's
00:33:10 --> 00:33:12 weekend wrap. Until then, clear skies.