SHOW NOTES Black hole jets follow one universal rule Adelle Goodwin (Curtin University / ICRAR, Forrest Research Foundation Fellow) and Andrew Mummery (Institute for Advanced Study, Princeton) show that supermassive black holes launch their outflows at the same critical accretion rate as stellar-mass black holes — about 2% of the Eddington limit — making accretion-outflow coupling scale-invariant across seven orders of magnitude in mass. Nature Astronomy, 17 September 2026. DOI: 10.1038/s41550-026-02951-1 JWST's Little Red Dots are black hole seeds caught mid-growth Sunmyon Chon, Shingo Hirano, Tomoaki Ishiyama, Seok-Jun Chang and Volker Springel ran fully cosmological radiation-hydrodynamic simulations on Japan's ATERUI III supercomputer and produced heavy black hole seeds of about a million solar masses, whose optically thick discs generate exactly the broad hydrogen emission seen in Little Red Dots. Nature 657, 621-625, 16 September 2026. DOI: 10.1038/s41586-026-10985-8 Earth's centre of mass moves about half as far as we thought Donald Argus, Felix Landerer and colleagues at NASA's Jet Propulsion Laboratory, with the University of Nevada, the University of Montana and the Helmholtz Centre for Geosciences, revise the annual oscillation between Earth's centre of mass and its centre of figure down from roughly 11 millimetres to about 4 to 5.5. Geophysical Journal International, ggag314. DOI: 10.1093/gji/ggag314 Two numbers explain the weather on a world 20 light-years away Merle Schrader and colleagues at Trinity College Dublin applied principal component analysis to one rotation of JWST spectroscopy of the brown dwarf SIMP 0136 and found two components — temperature and vertical cloud structure — account for all the detectable variability. Astronomy & Astrophysics, open access. DOI: 10.1051/0004-6361/202660109 The Sun goes blank With only active region AR4528 left on the Earth-facing disc and rotating out of view, the Sun was on the verge of its first spotless day since 24 February 2026 — which itself ended a 1,335-day run of continuous sunspot activity stretching back to June 2022. Skywatch: Venus at greatest brilliancy, first quarter Moon, Observe the Moon Night Venus reaches greatest brilliancy on 18 September at magnitude -4.8; the Moon reaches exact first quarter at 20:43 UTC the same day, setting up International Observe the Moon Night on Saturday 19 September. All times and altitudes in this segment were computed for Sydney, Los Angeles, New York and London. SKYWATCH — COMPUTED REFERENCE Computed in-session with PyEphem 4.2.1. Local times. Sydney on AEST (UTC+10); Los Angeles PDT; New York EDT; London BST. City Sunset Venus alt Venus sets Mercury Mars pre-dawn Sydney 17:45 39.2 deg 20:58 (3h13m) 16.4 deg (sets 1h20m) 20.4 deg Los Angeles 18:52 14.3 deg 20:10 (1h19m) 8.8 deg (sets 0h43m) 48.4 deg New York 18:56 10.3 deg 19:59 (1h03m) 7.0 deg (sets 0h38m) 46.1 deg London 19:04 3.6 deg 19:32 (0h28m) 3.9 deg (sets 0h26m) 40.6 deg · Venus greatest brilliancy: 18 September 2026, magnitude -4.8. Disc 39.6 arcsec, 26% illuminated. · By 22 September the disc has grown to 42.3 arcsec but the lit fraction has fallen to 22.7% — the two changes nearly cancel, which is why listings disagree on the date. The peak is genuinely flat. · Moon reaches exact first quarter 18 September at 20:43 UTC = 06:43 AEST Sat 19 Sep / 16:43 EDT Fri 18 / 13:43 PDT Fri 18 / 21:43 BST Fri 18. · Moon altitude at sunset: Sydney 81.6 deg (near overhead), Los Angeles 27.1 deg, New York 20.4 deg, London 9.4 deg. · International Observe the Moon Night: Saturday 19 September 2026. NASA event at the U.S. Space & Rocket Center, Huntsville, 5:30-8:00pm CST. · Pre-dawn Sat 19 Sep at nautical dawn — Jupiter: Los Angeles 26.4 deg, New York 25.1 deg, London 21.9 deg, Sydney 8.9 deg. · Mars-Jupiter separation: 22.9 deg on 18 Sep, 12.0 deg mid-October, 1.2 deg mid-November 2026. · Saturn is up essentially all night; sets 06:47 Sydney, 07:55 Los Angeles. Opposition 4 October 2026, disc 19.6 arcsec, rings approx 7 deg open. · September equinox: a single instant, 00:05:09 UTC on 23 September 2026 — evening of the 22nd in the Americas, mid-morning of the 23rd in Australia. · All figures computed in-session with PyEphem 4.2.1 for the four reference cities. Sydney is on AEST (UTC+10); daylight saving begins 4 October.
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00:00:00 --> 00:00:02 Take a black hole 10 times the mass of
00:00:02 --> 00:00:05 the sun. Now take one 10 million times
00:00:05 --> 00:00:07 heavier. The kind that sits in the
00:00:07 --> 00:00:10 middle of a galaxy and anchors a 100
00:00:10 --> 00:00:13 billion stars. Those two objects have
00:00:13 --> 00:00:16 almost nothing in common. Different
00:00:16 --> 00:00:18 sizes, different neighborhoods,
00:00:18 --> 00:00:20 different life stories. One of them you
00:00:20 --> 00:00:23 could fit inside a city. The other one
00:00:23 --> 00:00:25 you could pour the entire inner solar
00:00:25 --> 00:00:28 system into and still have room. And
00:00:28 --> 00:00:31 this week, a team led out of Perth has
00:00:31 --> 00:00:33 shown that when it comes to the single
00:00:33 --> 00:00:35 most dramatic thing a black hole does,
00:00:35 --> 00:00:38 firing a jet of material out into space
00:00:38 --> 00:00:40 at close to the speed of light, both of
00:00:40 --> 00:00:43 them wait for exactly the same moment.
00:00:43 --> 00:00:46 >> Same trigger, same threshold, seven
00:00:46 --> 00:00:49 orders of magnitude apart.
00:00:49 --> 00:00:51 >> Hello and welcome to Astronomy Daily.
00:00:51 --> 00:00:52 I'm Anna.
00:00:52 --> 00:00:55 >> And I'm Avery. It's Friday, the 18th of
00:00:55 --> 00:00:59 September, 2026, and this is episode
00:00:59 --> 00:01:01 197.
00:01:01 --> 00:01:02 >> So, let's start with the thing that
00:01:02 --> 00:01:05 makes this hard. Black holes don't emit
00:01:05 --> 00:01:07 anything themselves. What we actually
00:01:07 --> 00:01:10 see is the material falling in. It piles
00:01:10 --> 00:01:13 up into a disc. It heats up through
00:01:13 --> 00:01:15 friction until it's glowing across the
00:01:15 --> 00:01:17 electromagnetic spectrum. And then
00:01:17 --> 00:01:19 sometimes, not always, but sometimes,
00:01:19 --> 00:01:21 some of it gets flung back out in a
00:01:21 --> 00:01:24 narrow, tightly columnated jet.
00:01:24 --> 00:01:26 >> And the question of when you get a jet
00:01:26 --> 00:01:28 and when you don't, has been open for
00:01:28 --> 00:01:29 decades.
00:01:29 --> 00:01:32 >> It has. But for one class of black hole,
00:01:32 --> 00:01:34 we actually have a very good answer.
00:01:34 --> 00:01:37 Stellar mass black holes, the ones left
00:01:37 --> 00:01:39 behind when a massive star collapses,
00:01:39 --> 00:01:42 maybe 5 to 20 times the sun's mass,
00:01:42 --> 00:01:44 often sit in binary systems, pulling
00:01:44 --> 00:01:47 material off a companion star. And those
00:01:47 --> 00:01:50 systems cycle. They brighten, they fade,
00:01:50 --> 00:01:52 they go through what astronomers call
00:01:52 --> 00:01:54 state transitions. And they do it over
00:01:54 --> 00:01:56 weeks and months,
00:01:56 --> 00:01:58 >> which means we've watched the whole
00:01:58 --> 00:02:02 process start to finish many times over.
00:02:02 --> 00:02:05 many many times. And what we've learned
00:02:05 --> 00:02:07 is that the jets switch on and off at
00:02:07 --> 00:02:09 particular points in that cycle. And
00:02:09 --> 00:02:11 those points are tied to how fast the
00:02:11 --> 00:02:14 black hole is feeding, measured against
00:02:14 --> 00:02:16 something called the Edington limit.
00:02:16 --> 00:02:19 >> And this is worth spelling out because
00:02:19 --> 00:02:21 everything else today rests on it.
00:02:21 --> 00:02:24 >> It really does. So material falling
00:02:24 --> 00:02:27 toward a black hole gets extremely hot
00:02:27 --> 00:02:29 and hot material radiates. That
00:02:29 --> 00:02:32 radiation pushes outward. gravity pulls
00:02:32 --> 00:02:34 inward. The Edington limit is the point
00:02:34 --> 00:02:36 where those two balance, where the
00:02:36 --> 00:02:38 radiation coming off the infalling
00:02:38 --> 00:02:41 material is pushing outward just as hard
00:02:41 --> 00:02:43 as the black hole's gravity is pulling
00:02:43 --> 00:02:44 in.
00:02:44 --> 00:02:46 >> So if you try to feed a black hole
00:02:46 --> 00:02:49 faster than that, the radiation starts
00:02:49 --> 00:02:50 blowing the meal away.
00:02:50 --> 00:02:54 >> Anna, roughly, yes. Though nature has
00:02:54 --> 00:02:56 ways around it, and we'll come back to
00:02:56 --> 00:02:57 that. The useful thing about the
00:02:58 --> 00:02:59 Edington limit is that it scales with
00:02:59 --> 00:03:02 mass. A black hole a million times
00:03:02 --> 00:03:04 heavier has a limit a million times
00:03:04 --> 00:03:06 higher. So instead of talking about
00:03:06 --> 00:03:08 absolute brightness, astronomers talk in
00:03:08 --> 00:03:12 fractions of Edington, 10% of Edington,
00:03:12 --> 00:03:15 1% of Edington. And that gives you a way
00:03:15 --> 00:03:17 to compare black holes of wildly
00:03:17 --> 00:03:19 different sizes on the same scale.
00:03:19 --> 00:03:21 >> Which brings us to the super massive
00:03:21 --> 00:03:24 black holes. And the reason this has
00:03:24 --> 00:03:25 been so stubborn,
00:03:25 --> 00:03:28 >> the reason is simply time. Everything
00:03:28 --> 00:03:30 about an accretion flow scales with the
00:03:30 --> 00:03:32 mass of the black hole and that includes
00:03:32 --> 00:03:35 how fast it evolves. A stellar mass
00:03:35 --> 00:03:38 system cycles in months. A super massive
00:03:38 --> 00:03:41 black hole in an ordinary active galaxy
00:03:41 --> 00:03:42 takes something on the order of
00:03:42 --> 00:03:45 thousands of years to do the equivalent.
00:03:45 --> 00:03:48 >> So you can't watch one. You get a single
00:03:48 --> 00:03:51 frame of a film that runs for millennia.
00:03:51 --> 00:03:53 >> Exactly that. You can look at thousands
00:03:53 --> 00:03:56 of active galaxies and take a census.
00:03:56 --> 00:03:57 Here's one with a jet. Here's one
00:03:58 --> 00:04:00 without. But you can never watch a
00:04:00 --> 00:04:02 single object cross a threshold and see
00:04:02 --> 00:04:04 what happens. Which means the critical
00:04:04 --> 00:04:06 accretion rates that we know so
00:04:06 --> 00:04:08 precisely for stellar mass black holes
00:04:08 --> 00:04:10 have simply been unmeasurable for the
00:04:10 --> 00:04:12 big ones.
00:04:12 --> 00:04:14 >> Unless something gives a super massive
00:04:14 --> 00:04:17 black hole a sudden well-defined meal.
00:04:17 --> 00:04:20 >> And that's the move. Tidal disruption
00:04:20 --> 00:04:23 events. A star wanders too close to the
00:04:23 --> 00:04:24 super massive black hole at the center
00:04:24 --> 00:04:27 of its galaxy. The difference in
00:04:27 --> 00:04:28 gravitational pull across the star
00:04:28 --> 00:04:31 exceeds what's holding the star together
00:04:31 --> 00:04:33 and it comes apart. Roughly half the
00:04:33 --> 00:04:35 debris gets flung away and the other
00:04:35 --> 00:04:37 half falls back and builds a brand new
00:04:37 --> 00:04:40 accretion disc from nothing.
00:04:40 --> 00:04:43 >> And that one plays out fast,
00:04:43 --> 00:04:46 >> years, not millennia. You get to watch
00:04:46 --> 00:04:48 an accretion flow around a super massive
00:04:48 --> 00:04:52 black hole be born, rise, peak, and
00:04:52 --> 00:04:54 decline. And you can do it inside a
00:04:54 --> 00:04:56 single research career. That is the
00:04:56 --> 00:04:59 entire reason this result was possible.
00:04:59 --> 00:05:02 >> So Dr. Adele Goodwin at Curtain
00:05:02 --> 00:05:04 University's International Center for
00:05:04 --> 00:05:08 Radioastronomy Research in Perth, who's
00:05:08 --> 00:05:10 also a forest research foundation
00:05:10 --> 00:05:13 fellow, working with Dr. Andrew Mumry at
00:05:13 --> 00:05:15 the Institute for Advanced Study in
00:05:15 --> 00:05:18 Princeton, published in Nature Astronomy
00:05:18 --> 00:05:20 yesterday, Thursday the 17th of
00:05:20 --> 00:05:21 September.
00:05:22 --> 00:05:23 >> And what they show is that title
00:05:23 --> 00:05:25 disruption events actually launch
00:05:25 --> 00:05:28 outflows twice in two physically
00:05:28 --> 00:05:31 distinct episodes. The first comes early
00:05:31 --> 00:05:32 while the black hole is feeding above
00:05:32 --> 00:05:35 its Edington limit. That brief ferocious
00:05:35 --> 00:05:37 phase right after the star comes apart
00:05:37 --> 00:05:39 when there's far more material trying to
00:05:39 --> 00:05:41 get in than the black hole can
00:05:41 --> 00:05:43 comfortably swallow.
00:05:43 --> 00:05:45 >> The prompt outflow,
00:05:45 --> 00:05:47 >> the prompt one. And then later,
00:05:47 --> 00:05:49 sometimes months or years later, there's
00:05:50 --> 00:05:53 a second separate outflow. And that one
00:05:53 --> 00:05:54 arrives as the accretion rate falls
00:05:54 --> 00:05:57 through a specific value about 2% of the
00:05:57 --> 00:05:58 Edington limit.
00:05:58 --> 00:06:02 >> 2%. And that number is familiar. That
00:06:02 --> 00:06:05 number is the same critical accretion
00:06:05 --> 00:06:07 rate at which stellar mass black holes
00:06:07 --> 00:06:09 in binaries make their state
00:06:09 --> 00:06:11 transitions. The same value we've
00:06:11 --> 00:06:13 measured over and over in objects
00:06:13 --> 00:06:16 millions of times smaller. Goodwin and
00:06:16 --> 00:06:17 Mumy are arguing that the coupling
00:06:18 --> 00:06:20 between how a black hole feeds and how
00:06:20 --> 00:06:23 it throws material back out is scale
00:06:23 --> 00:06:25 invariant. That it doesn't care about
00:06:25 --> 00:06:26 mass at all
00:06:26 --> 00:06:29 >> across roughly seven orders of
00:06:29 --> 00:06:30 magnitude.
00:06:30 --> 00:06:32 >> Which, if it holds, is a genuinely
00:06:32 --> 00:06:34 unifying statement about a class of
00:06:34 --> 00:06:37 object we usually treat as two separate
00:06:37 --> 00:06:38 populations.
00:06:38 --> 00:06:40 >> And it also clears up a mess, doesn't
00:06:40 --> 00:06:42 it? Because radio astronomers have had
00:06:42 --> 00:06:45 an awkward problem with these events for
00:06:45 --> 00:06:45 a while.
00:06:46 --> 00:06:48 >> A very awkward one. Some title
00:06:48 --> 00:06:50 disruption events produce a radio flare
00:06:50 --> 00:06:53 almost immediately. Some produce one
00:06:53 --> 00:06:55 much later out of nowhere. Sometimes
00:06:56 --> 00:06:57 years after everyone has stopped paying
00:06:57 --> 00:06:59 attention. Some appear to produce
00:06:59 --> 00:07:02 nothing at all. That looked like three
00:07:02 --> 00:07:05 different phenomena or worse like noise.
00:07:05 --> 00:07:07 >> And the two outflow picture makes it one
00:07:07 --> 00:07:10 phenomenon. one phenomenon with two
00:07:10 --> 00:07:12 stages where whether you see either of
00:07:12 --> 00:07:14 them depends on when you happen to be
00:07:14 --> 00:07:17 looking and how the accretion rate was
00:07:17 --> 00:07:20 falling. The prompt and delayed outflows
00:07:20 --> 00:07:22 stop being a puzzle and start being a
00:07:22 --> 00:07:23 prediction.
00:07:23 --> 00:07:25 >> And Goodwin makes a very practical point
00:07:25 --> 00:07:26 about that.
00:07:26 --> 00:07:28 >> She does and it's my favorite part of
00:07:28 --> 00:07:31 the release. Her line is quote, "Radio
00:07:31 --> 00:07:34 telescopes are incredibly powerful, but
00:07:34 --> 00:07:35 knowing when to look is just as
00:07:35 --> 00:07:38 important as knowing where to look." And
00:07:38 --> 00:07:40 she goes on, "If we can anticipate when
00:07:40 --> 00:07:42 a black hole is more likely to launch a
00:07:42 --> 00:07:44 jet, we can run better targeted
00:07:44 --> 00:07:47 campaigns, waste fewer observations, and
00:07:47 --> 00:07:49 improve our chances of catching these
00:07:49 --> 00:07:51 rare events at the moment they matter
00:07:51 --> 00:07:52 most,
00:07:52 --> 00:07:54 >> which is not a small thing when you're
00:07:54 --> 00:07:56 competing for time on the world's big
00:07:56 --> 00:07:57 dishes.
00:07:57 --> 00:07:59 >> It's the difference between a survey and
00:07:59 --> 00:08:01 a stakeout. And it matters enormously
00:08:01 --> 00:08:04 for what's coming because the Vera Rubin
00:08:04 --> 00:08:06 Observatory is about to start finding
00:08:06 --> 00:08:09 tidal disruption events in numbers we've
00:08:09 --> 00:08:11 never had before. If you can look at the
00:08:11 --> 00:08:13 optical light curve and predict roughly
00:08:13 --> 00:08:16 when the radio outflow should arrive,
00:08:16 --> 00:08:18 you can have the radio telescopes ready.
00:08:18 --> 00:08:20 Without that, you're guessing.
00:08:20 --> 00:08:23 >> And this one is properly ours.
00:08:23 --> 00:08:26 >> It is led from Curtain University in
00:08:26 --> 00:08:29 Perth through. And the underlying data
00:08:29 --> 00:08:31 set is the product of years of
00:08:31 --> 00:08:33 multi-wavelength campaigns drawing on
00:08:33 --> 00:08:35 telescopes in Australia, the United
00:08:35 --> 00:08:39 States, India, South Africa, and in
00:08:39 --> 00:08:41 space. This is not a single instrument
00:08:41 --> 00:08:44 result. It's a decade of patient radio
00:08:44 --> 00:08:47 follow-up on rare events assembled into
00:08:47 --> 00:08:49 one picture. And it lands in the same
00:08:49 --> 00:08:51 week that we're talking about the square
00:08:52 --> 00:08:54 kilometer arrays southern half taking
00:08:54 --> 00:08:57 shape out in Western Australia,
00:08:57 --> 00:08:58 >> which is precisely the instrument you'd
00:08:58 --> 00:09:00 want pointed at the next one.
00:09:00 --> 00:09:02 >> Two things to keep straight, though.
00:09:02 --> 00:09:05 >> Yes. The first is that this is a
00:09:05 --> 00:09:08 threshold, not a switch. saying black
00:09:08 --> 00:09:11 holes launch outflows at around 2% of
00:09:11 --> 00:09:14 Eddington is a statement about where the
00:09:14 --> 00:09:16 transition sits, not a promise that
00:09:16 --> 00:09:19 every black hole crossing that line will
00:09:19 --> 00:09:21 produce a jet you can detect. Whether
00:09:21 --> 00:09:23 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
00:09:28 --> 00:09:30 pointed in the right direction.
00:09:30 --> 00:09:31 >> And the second,
00:09:31 --> 00:09:33 >> the second is about timing. And we want
00:09:34 --> 00:09:35 to be straight with you. This was
00:09:35 --> 00:09:37 published in Nature Astronomy yesterday.
00:09:38 --> 00:09:40 That's real and the peer review is done.
00:09:40 --> 00:09:42 But the preprint went up on the archive
00:09:42 --> 00:09:45 back in February. So while this is newly
00:09:45 --> 00:09:47 published, it is not newly written. And
00:09:47 --> 00:09:49 if it feels vaguely familiar to anyone
00:09:49 --> 00:09:51 who follows the preprint servers
00:09:51 --> 00:09:53 closely, that's why.
00:09:53 --> 00:09:55 >> Which is a pattern we've flagged before
00:09:55 --> 00:09:56 on this show.
00:09:56 --> 00:09:59 >> It is, and we'll keep flagging it. The
00:09:59 --> 00:10:00 science is no less good for having
00:10:00 --> 00:10:03 waited seven months in review, but
00:10:03 --> 00:10:05 published this week and discovered this
00:10:06 --> 00:10:08 week are different claims, and we try
00:10:08 --> 00:10:10 very hard not to blur them.
00:10:10 --> 00:10:11 >> Longtime listeners will hear some
00:10:11 --> 00:10:13 threads here, too. We covered a
00:10:13 --> 00:10:15 wandering black hole revealed by a tidal
00:10:15 --> 00:10:19 disruption event back in episode 153 and
00:10:19 --> 00:10:21 a quazar driving turbulence across
00:10:21 --> 00:10:25 300 lightyear in 154. and the
00:10:25 --> 00:10:27 partial disruptions that fade and come
00:10:27 --> 00:10:31 back in episode 185. This result is the
00:10:31 --> 00:10:33 framework those individual objects have
00:10:33 --> 00:10:34 been waiting for.
00:10:34 --> 00:10:37 >> Now on to story two. That phrase super
00:10:37 --> 00:10:40 Edington came up in the lead and it's
00:10:40 --> 00:10:42 about to come up again because the
00:10:42 --> 00:10:44 second story this week is also about a
00:10:44 --> 00:10:46 black hole eating faster than it should
00:10:46 --> 00:10:49 be able to just 11 billion years
00:10:49 --> 00:10:50 earlier.
00:10:50 --> 00:10:52 >> The little red dots.
00:10:52 --> 00:10:54 >> The little red dots. And if you've been
00:10:54 --> 00:10:56 with us a while, you'll know these have
00:10:56 --> 00:10:58 been one of the most irritating things
00:10:58 --> 00:11:01 the James Web Space Telescope has found.
00:11:02 --> 00:11:05 Extremely compact, extremely red,
00:11:05 --> 00:11:07 extremely numerous in the early
00:11:07 --> 00:11:09 universe. They show the broad hydrogen
00:11:09 --> 00:11:12 emission lines you'd expect from gas
00:11:12 --> 00:11:14 whipping around a black hole, but there
00:11:14 --> 00:11:16 are far too many of them. And the black
00:11:16 --> 00:11:19 holes implied are far too heavy for the
00:11:19 --> 00:11:21 galaxies they sit in,
00:11:21 --> 00:11:23 >> which has produced a small industry of
00:11:23 --> 00:11:25 explanations. Some of them fairly
00:11:25 --> 00:11:26 exotic,
00:11:26 --> 00:11:30 >> some very exotic. So, here's what's new.
00:11:30 --> 00:11:33 A team led by Sunyan Chon with Shingo
00:11:33 --> 00:11:36 Hiano, Tommoaki Ishyama, Sukjun Chang,
00:11:36 --> 00:11:39 and Vulkar Springle published in Nature
00:11:39 --> 00:11:42 on the 16th of September ran fully
00:11:42 --> 00:11:44 cosmological radiation hydrodnamic
00:11:44 --> 00:11:47 simulations of a dense protocluster
00:11:47 --> 00:11:50 region in the early universe using
00:11:50 --> 00:11:53 Japan's Aeroi3 supercomput.
00:11:53 --> 00:11:55 >> And crucially, without putting the
00:11:55 --> 00:11:56 answer in by hand,
00:11:56 --> 00:11:58 >> that's the point. They didn't seed the
00:11:58 --> 00:12:00 simulation with the black holes they
00:12:00 --> 00:12:03 wanted. They let the physics run. And
00:12:03 --> 00:12:06 what happens is this. In that intensely
00:12:06 --> 00:12:08 overdense environment, the giant gas
00:12:08 --> 00:12:11 clouds are bathed in so much ultraviolet
00:12:11 --> 00:12:14 radiation that they can't fragment into
00:12:14 --> 00:12:17 ordinary stars. Normally, a big cloud
00:12:17 --> 00:12:19 breaks up into lots of little stellar
00:12:19 --> 00:12:22 nurseries. Here it can't. So instead,
00:12:22 --> 00:12:25 the whole thing collapses as one object,
00:12:25 --> 00:12:28 a primordial super massive star, which
00:12:28 --> 00:12:30 then promptly collapses into a black
00:12:30 --> 00:12:32 hole.
00:12:32 --> 00:12:33 >> A heavy seed,
00:12:33 --> 00:12:36 >> a heavy seed of around a million solar
00:12:36 --> 00:12:38 masses, which is roughly 10 times
00:12:38 --> 00:12:41 heavier than theory typically expects.
00:12:41 --> 00:12:43 And then those seeds develop dense,
00:12:43 --> 00:12:46 optically thick discs, so thick that
00:12:46 --> 00:12:48 light bounces around inside them,
00:12:48 --> 00:12:50 scattering off free electrons on the way
00:12:50 --> 00:12:53 out. And that electron scattering smears
00:12:53 --> 00:12:56 the hydrogen emission into exactly the
00:12:56 --> 00:12:59 broad lines we see in little red dots.
00:12:59 --> 00:13:01 >> So the broad lines aren't necessarily
00:13:01 --> 00:13:03 telling you the gas is moving as fast as
00:13:03 --> 00:13:04 you'd assumed.
00:13:04 --> 00:13:07 >> That's the elegant bit. Part of the
00:13:07 --> 00:13:10 width is scattering, not velocity. and
00:13:10 --> 00:13:12 then sustained super Edington accretion
00:13:12 --> 00:13:15 drives the thing up to around 30 million
00:13:15 --> 00:13:18 solar masses by a red shift of eight
00:13:18 --> 00:13:20 which puts it right in the population of
00:13:20 --> 00:13:23 over massive quazars web keeps finding.
00:13:23 --> 00:13:27 So it's one continuous story. Seed dot
00:13:27 --> 00:13:30 quazar and eventually the super massive
00:13:30 --> 00:13:32 black holes sitting in galaxies today.
00:13:32 --> 00:13:35 >> One story with the little red dot phase
00:13:35 --> 00:13:38 being short and dust shrouded, a stage
00:13:38 --> 00:13:41 rather than a species. And the timing is
00:13:41 --> 00:13:43 rather lovely because Nature Astronomy
00:13:43 --> 00:13:45 published a whole focus issue on little
00:13:45 --> 00:13:48 red dots the same week, including a
00:13:48 --> 00:13:51 review by Hannah Ubler on massive black
00:13:51 --> 00:13:54 holes in the first billion years. The
00:13:54 --> 00:13:56 field is consolidating and this happens
00:13:56 --> 00:13:58 to be the week it did it.
00:13:58 --> 00:14:00 >> One caution though, and it's the same
00:14:00 --> 00:14:02 one we gave for the black hole feedback
00:14:02 --> 00:14:03 work last month.
00:14:03 --> 00:14:04 >> Go on.
00:14:04 --> 00:14:06 >> This is a simulation that reproduces the
00:14:06 --> 00:14:09 observations. That's a demonstration
00:14:09 --> 00:14:11 that the physics is sufficient. You
00:14:11 --> 00:14:13 don't need anything exotic to get little
00:14:13 --> 00:14:16 red dots. It is not on its own proof
00:14:16 --> 00:14:18 that this is what actually happened.
00:14:18 --> 00:14:21 Other roots may also produce them. What
00:14:21 --> 00:14:23 would settle it is a direct measurement
00:14:23 --> 00:14:25 of one of these objects that
00:14:25 --> 00:14:27 distinguishes scattering broadened lines
00:14:27 --> 00:14:29 from genuinely fast gas,
00:14:29 --> 00:14:31 >> which is exactly the kind of thing Web
00:14:31 --> 00:14:32 could do.
00:14:32 --> 00:14:35 >> Right. Next up, let's come all the way
00:14:35 --> 00:14:38 back. about 6 kilometers in fact to
00:14:38 --> 00:14:39 the middle of the Earth,
00:14:39 --> 00:14:41 >> which is not where you think it is.
00:14:41 --> 00:14:43 >> It really isn't. And this is one of
00:14:43 --> 00:14:45 those facts that sounds like a trick
00:14:45 --> 00:14:47 until you sit with it. There are two
00:14:47 --> 00:14:49 different centers of the Earth. There's
00:14:49 --> 00:14:51 the center of figure, the geometric
00:14:51 --> 00:14:53 middle of the solid planet surface,
00:14:53 --> 00:14:56 which is essentially fixed. And there's
00:14:56 --> 00:14:58 the center of mass, the balance point of
00:14:58 --> 00:15:00 everything the planet is made of.
00:15:00 --> 00:15:02 >> And the planet is partly made of water
00:15:02 --> 00:15:04 and air, which move
00:15:04 --> 00:15:07 >> constantly. Snow falls across Siberia
00:15:07 --> 00:15:10 and Canada. The Amazon basin floods. The
00:15:10 --> 00:15:13 monsoon arrives over Southeast Asia.
00:15:13 --> 00:15:15 Every one of those shifts an enormous
00:15:15 --> 00:15:17 amount of mass from one part of the
00:15:17 --> 00:15:19 planet to another. And the balance point
00:15:19 --> 00:15:22 moves to follow it. So the center of
00:15:22 --> 00:15:24 mass and the center of figure drift
00:15:24 --> 00:15:25 apart and back together over the course
00:15:26 --> 00:15:26 of a year.
00:15:26 --> 00:15:28 >> And we care because
00:15:28 --> 00:15:30 >> because the center of mass is what
00:15:30 --> 00:15:32 satellites orbit. It's the origin of the
00:15:32 --> 00:15:35 reference frame that underpins satellite
00:15:35 --> 00:15:37 navigation and every precise elevation
00:15:38 --> 00:15:40 measurement on the planet. If you're
00:15:40 --> 00:15:41 wrong about where it is, you're wrong
00:15:41 --> 00:15:44 about where everything else is.
00:15:44 --> 00:15:46 >> So, how big is the wobble?
00:15:46 --> 00:15:48 >> That's the news. The traditional figure
00:15:48 --> 00:15:52 has been around 11 mm. A team led by
00:15:52 --> 00:15:54 Donald Argus at NASA's Jet Propulsion
00:15:54 --> 00:15:57 Laboratory now puts it at roughly 4 to
00:15:57 --> 00:16:00 5, about half what we thought. published
00:16:00 --> 00:16:03 in Geoysical Journal International with
00:16:03 --> 00:16:04 the JPL release on Tuesday.
00:16:04 --> 00:16:07 >> Half is a big correction for something
00:16:07 --> 00:16:09 people have been measuring for decades.
00:16:09 --> 00:16:11 >> It is. And here's how uncertain it's
00:16:11 --> 00:16:14 been. The last two international
00:16:14 --> 00:16:16 estimates, one from 2017 and one from
00:16:16 --> 00:16:20 2023, disagree with each other by about
00:16:20 --> 00:16:23 7 mm, which is almost as large as the
00:16:23 --> 00:16:24 entire motion they're trying to measure.
00:16:24 --> 00:16:27 >> So, the error bar was the same size as
00:16:27 --> 00:16:29 the signal. Essentially,
00:16:29 --> 00:16:31 Argus' own line is that the movement now
00:16:32 --> 00:16:34 looks like about half of what we
00:16:34 --> 00:16:37 believed 8 years ago, and that the mass
00:16:37 --> 00:16:39 of water and air sloshing between the
00:16:39 --> 00:16:42 hemispheres is smaller than we thought.
00:16:42 --> 00:16:44 >> And where does the movement actually
00:16:44 --> 00:16:45 come from?
00:16:45 --> 00:16:47 >> Three main contributors, ocean,
00:16:47 --> 00:16:51 atmosphere, and land water. In March,
00:16:51 --> 00:16:53 snow sitting across Eurasia and North
00:16:53 --> 00:16:56 America pulls the center of mass about 3
00:16:56 --> 00:16:59 millimeters toward the northern pole.
00:16:59 --> 00:17:01 Then in April, the Amazon hits its
00:17:02 --> 00:17:05 annual water maximum, around 2
00:17:05 --> 00:17:08 gatons, and tugs at roughly 2.2 mm
00:17:08 --> 00:17:10 towards South America. Later in the
00:17:10 --> 00:17:12 year, monsoon water across Southeast
00:17:12 --> 00:17:15 Asia peaks at around 600 gatons and
00:17:15 --> 00:17:16 pulls in its own direction.
00:17:16 --> 00:17:18 >> None of which wins.
00:17:18 --> 00:17:21 >> None of which wins. They combine into an
00:17:21 --> 00:17:23 annual oscillation that never settles.
00:17:23 --> 00:17:25 And my favorite complication in the
00:17:25 --> 00:17:27 whole study is this. When you pile
00:17:27 --> 00:17:29 trillions of tons of water onto a
00:17:29 --> 00:17:32 continent, the crust flexes under the
00:17:32 --> 00:17:34 load, which means the ground stations
00:17:34 --> 00:17:36 you're using to measure the movement are
00:17:36 --> 00:17:37 themselves moving.
00:17:37 --> 00:17:39 >> You have to subtract the observatory
00:17:39 --> 00:17:41 from the observation.
00:17:41 --> 00:17:43 >> You do. They handled it by combining
00:17:43 --> 00:17:45 laser ranging to satellites. And that's
00:17:45 --> 00:17:48 the Legios technique which Australia
00:17:48 --> 00:17:49 contributes to from the Mount Stromlo
00:17:50 --> 00:17:53 station outside Canberra with GPS and
00:17:53 --> 00:17:55 with the Grace follow-on gravity mission
00:17:55 --> 00:17:58 then modeling the custal deformation out
00:17:58 --> 00:17:59 >> and the practical end of it.
00:17:59 --> 00:18:02 >> Felix Lander, one of the co-authors put
00:18:02 --> 00:18:03 it well. He said that while these
00:18:04 --> 00:18:06 movements might appear tiny, our modern
00:18:06 --> 00:18:08 world relies on extremely accurate
00:18:08 --> 00:18:10 positioning and that by understanding
00:18:10 --> 00:18:13 what changes the reference system, we
00:18:13 --> 00:18:15 can build better reference systems. He
00:18:15 --> 00:18:16 lists the beneficiaries as everything
00:18:16 --> 00:18:18 from global shipping logistics to
00:18:18 --> 00:18:20 precision agriculture,
00:18:20 --> 00:18:22 >> which is a long way from black holes.
00:18:22 --> 00:18:25 And I rather like that about this job.
00:18:25 --> 00:18:26 >> Me, too.
00:18:26 --> 00:18:29 >> Last story. And it's a small, clever
00:18:29 --> 00:18:32 one. 20 light years away, there's an
00:18:32 --> 00:18:36 object called SIMP0136.
00:18:36 --> 00:18:39 It's a brown dwarf, too heavy to be a
00:18:39 --> 00:18:41 planet in the ordinary sense, too light
00:18:41 --> 00:18:44 to have ever ignited hydrogen fusion and
00:18:44 --> 00:18:47 become a star. It sits right on the
00:18:47 --> 00:18:51 boundary and it has no host star at all.
00:18:51 --> 00:18:52 It's just drifting,
00:18:52 --> 00:18:56 >> which makes it unusually easy to study,
00:18:56 --> 00:18:59 >> enormously easier. Normally, if you want
00:18:59 --> 00:19:01 to look at a giant planet's atmosphere,
00:19:02 --> 00:19:03 you're fighting the glare of the star
00:19:03 --> 00:19:07 next to it. Here there's no star. You
00:19:07 --> 00:19:09 just point and look. Which is why
00:19:09 --> 00:19:12 SIMP0136
00:19:12 --> 00:19:14 has become the reference object for what
00:19:14 --> 00:19:16 directly imaged giant planets are
00:19:16 --> 00:19:18 probably like.
00:19:18 --> 00:19:19 >> And it varies.
00:19:19 --> 00:19:22 >> It spins once every 2 hours and 25
00:19:22 --> 00:19:25 minutes. And as it spins, its brightness
00:19:25 --> 00:19:28 changes by a few%. Different amounts at
00:19:28 --> 00:19:30 different wavelengths, which tells you
00:19:30 --> 00:19:33 there's weather, clouds, hotspots,
00:19:33 --> 00:19:35 chemistry, all of it changing as
00:19:35 --> 00:19:38 different faces rotate into view. The
00:19:38 --> 00:19:39 trouble has been that it's looked
00:19:40 --> 00:19:42 fantastically complicated. Multiple
00:19:42 --> 00:19:45 mechanisms, multiple layers, all tangled
00:19:45 --> 00:19:46 together.
00:19:46 --> 00:19:47 >> So, what did they do differently?
00:19:48 --> 00:19:51 >> They stopped assuming. Merl Schrader, a
00:19:51 --> 00:19:54 PhD candidate at Trinity College Dublin
00:19:54 --> 00:19:56 with colleagues there and elsewhere,
00:19:56 --> 00:19:58 took one full rotation of web
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,
00:20:06 --> 00:20:08 how many independent things are actually
00:20:08 --> 00:20:11 changing 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 >> And the answer was
00:20:17 --> 00:20:19 >> two. Two components are enough to push
00:20:19 --> 00:20:22 what's left over down to the noise floor
00:20:22 --> 00:20:24 of the instrument. Which means that
00:20:24 --> 00:20:26 within what web can detect, there is
00:20:26 --> 00:20:28 nothing else going on.
00:20:28 --> 00:20:29 >> And what are the two?
00:20:29 --> 00:20:32 >> The first is broadband. It moves the
00:20:32 --> 00:20:34 whole spectrum together and that's
00:20:34 --> 00:20:37 temperature. The second is chromatic
00:20:37 --> 00:20:39 wavelength dependent and that traces the
00:20:39 --> 00:20:42 vertical structure of the clouds, how
00:20:42 --> 00:20:45 high and how thick they are. And between
00:20:45 --> 00:20:47 them, those two resolve into three
00:20:47 --> 00:20:49 recurring conditions. Patches that are
00:20:50 --> 00:20:52 hotter with thinner cloud, patches that
00:20:52 --> 00:20:54 are cooler with thick vertically
00:20:54 --> 00:20:56 extended cloud, and transitional regions
00:20:56 --> 00:20:58 between the two.
00:20:58 --> 00:21:00 >> So it looks chaotic and it's actually
00:21:00 --> 00:21:01 organized.
00:21:01 --> 00:21:04 >> Lowdimensional is the term. A famously
00:21:04 --> 00:21:06 messy atmosphere turns out to be running
00:21:06 --> 00:21:09 on two dials. And the team then went
00:21:09 --> 00:21:11 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
00:21:19 --> 00:21:21 models are capturing the right physics
00:21:21 --> 00:21:24 rather than accidentally agreeing.
00:21:24 --> 00:21:26 >> Two notes of care on this one,
00:21:26 --> 00:21:26 >> please.
00:21:26 --> 00:21:28 >> The first is that some of the coverage
00:21:28 --> 00:21:30 has described these patterns as
00:21:30 --> 00:21:32 persisting over more than a dozen
00:21:32 --> 00:21:35 rotations. The paper's core analysis is
00:21:35 --> 00:21:37 one rotation. That's a real and
00:21:37 --> 00:21:39 interesting result, but a second
00:21:39 --> 00:21:41 highquality rotation is precisely the
00:21:42 --> 00:21:43 test that would confirm the patterns
00:21:43 --> 00:21:46 hold. So, we'll describe it as the test
00:21:46 --> 00:21:47 rather than the finding.
00:21:47 --> 00:21:50 >> And the second is our usual one.
00:21:50 --> 00:21:52 >> The preprint went up in late July, so
00:21:52 --> 00:21:54 there's about a 7-week gap before the
00:21:54 --> 00:21:56 journal version and the Trinity release
00:21:56 --> 00:21:58 this week. Shorter than yesterday's, but
00:21:58 --> 00:22:00 worth saying.
00:22:00 --> 00:22:03 >> And one lovely human detail to finish.
00:22:03 --> 00:22:06 SIMP0136
00:22:06 --> 00:22:09 is 20 light years away. The web data
00:22:09 --> 00:22:12 Shraider analyzed was gathered in 2023.
00:22:12 --> 00:22:14 So the light she was working with left
00:22:14 --> 00:22:17 that object in the year she was born.
00:22:17 --> 00:22:19 >> That's a very good reason to go into
00:22:19 --> 00:22:20 astronomy.
00:22:20 --> 00:22:22 >> Now we have one more quick one before
00:22:22 --> 00:22:24 Skywatch.
00:22:24 --> 00:22:26 >> We do indeed. The sun has gone very
00:22:26 --> 00:22:28 quiet indeed.
00:22:28 --> 00:22:29 >> How quiet?
00:22:29 --> 00:22:32 >> Very nearly blank. As of yesterday,
00:22:32 --> 00:22:34 there was exactly one numbered active
00:22:34 --> 00:22:36 region left on the Earth-facing side,
00:22:36 --> 00:22:38 AR4528,
00:22:38 --> 00:22:40 and it's rotating out of view as we
00:22:40 --> 00:22:43 speak. If nothing new emerges behind it,
00:22:43 --> 00:22:45 we're about to get the sun's first
00:22:45 --> 00:22:47 spotless day since the 24th of February
00:22:47 --> 00:22:49 this year.
00:22:49 --> 00:22:51 >> And February was itself notable.
00:22:51 --> 00:22:55 February ended a streak of 1
00:22:55 --> 00:22:57 consecutive days with at least one
00:22:57 --> 00:23:00 sunspot going all the way back to June
00:23:00 --> 00:23:01 2022.
00:23:02 --> 00:23:05 Solar cycle 25 peaked in October 2024.
00:23:05 --> 00:23:07 And this is what the downhill side looks
00:23:07 --> 00:23:11 like. For scale, the last solar minimum
00:23:11 --> 00:23:14 between 2018 and 2020 delivered
00:23:14 --> 00:23:17 something like 700 spotless days.
00:23:17 --> 00:23:19 >> And minimum itself is still some way
00:23:19 --> 00:23:23 off. not expected before about 2030. But
00:23:23 --> 00:23:25 here's the part worth holding on to, and
00:23:25 --> 00:23:27 it connects two stories we've run
00:23:27 --> 00:23:29 recently. A quiet sun is not a harmless
00:23:30 --> 00:23:32 sun. It's a differently hazardous one.
00:23:32 --> 00:23:35 We talked in episode 192 about the
00:23:35 --> 00:23:37 energy a big active region can store for
00:23:37 --> 00:23:40 a super flare. And in 193 about cosmic
00:23:40 --> 00:23:43 radiation at aviation altitudes. And
00:23:43 --> 00:23:45 that second one runs the opposite way.
00:23:45 --> 00:23:48 When the sun is quiet, its magnetic
00:23:48 --> 00:23:50 field does less to shield the inner
00:23:50 --> 00:23:52 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%
00:23:58 --> 00:24:01 higher at solar minimum than at maximum.
00:24:01 --> 00:24:03 >> Same dial, opposite end.
00:24:03 --> 00:24:06 >> Fewer auroras, more cosmic rays.
00:24:06 --> 00:24:08 >> And that's a very good excuse to talk
00:24:08 --> 00:24:11 about the sky because tonight there is
00:24:11 --> 00:24:13 something genuinely worth walking
00:24:13 --> 00:24:15 outside for. and it works from
00:24:15 --> 00:24:17 everywhere.
00:24:17 --> 00:24:18 >> Venus.
00:24:18 --> 00:24:21 >> Venus at greatest brilliancancy.
00:24:21 --> 00:24:24 Tonight, the 18th of September, Venus
00:24:24 --> 00:24:25 reaches its peak brightness for this
00:24:26 --> 00:24:28 entire evening apparition, magnitude
00:24:28 --> 00:24:31 minus4.8.
00:24:31 --> 00:24:32 There is nothing else in the night sky
00:24:32 --> 00:24:35 that comes close except the moon.
00:24:35 --> 00:24:37 >> And the reason it peaks tonight rather
00:24:37 --> 00:24:39 than when Venus is full is genuinely
00:24:39 --> 00:24:41 counterintuitive.
00:24:41 --> 00:24:43 It's the best bit of physics in the
00:24:43 --> 00:24:45 whole segment. Venus is not full
00:24:45 --> 00:24:49 tonight. 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 --> 00:24:58 nearly 40 arcsec across. Brightness is
00:24:58 --> 00:25:00 lit fraction multiplied by apparent
00:25:00 --> 00:25:03 size. And right now the disc is growing
00:25:03 --> 00:25:05 faster than the illuminated fraction is
00:25:05 --> 00:25:06 shrinking.
00:25:06 --> 00:25:09 >> A big thin crescent beats a small full
00:25:09 --> 00:25:12 disc. every time. And if you have
00:25:12 --> 00:25:15 binoculars, hold them steady and you'll
00:25:15 --> 00:25:18 actually see the crescent shape. At 40
00:25:18 --> 00:25:21 arcsec, it's within reach, which
00:25:21 --> 00:25:22 surprises people because we don't think
00:25:22 --> 00:25:25 of Venus as something you can resolve.
00:25:25 --> 00:25:27 >> And if you see a different date quoted
00:25:27 --> 00:25:30 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, the
00:25:36 --> 00:25:40 lit fraction drops from about 26% to 23,
00:25:40 --> 00:25:42 while the disc grows from roughly 40
00:25:42 --> 00:25:46 arcsec to 42. And those two changes very
00:25:46 --> 00:25:48 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 >> Now, how well you do tonight depends
00:25:53 --> 00:25:55 enormously on where you're standing, and
00:25:55 --> 00:25:58 the gap is dramatic.
00:25:58 --> 00:26:00 >> From Sydney, this is a spectacle. Sunset
00:26:00 --> 00:26:03 is at4 to 6 and at that moment Venus is
00:26:03 --> 00:26:06 39° above the western horizon. That's
00:26:06 --> 00:26:09 more than a third of the way up the sky.
00:26:09 --> 00:26:11 It doesn't set until just before 9:00,
00:26:11 --> 00:26:14 which gives you 3 hours and 13 minutes
00:26:14 --> 00:26:17 of Venus after sunset. You do not need a
00:26:17 --> 00:26:20 clear horizon. You do not need to hurry.
00:26:20 --> 00:26:22 >> And from the northern hemisphere, it's a
00:26:22 --> 00:26:25 different evening entirely. From Los
00:26:25 --> 00:26:28 Angeles, sunset is at 10 to 7 and Venus
00:26:28 --> 00:26:32 is 14° up. It sets an hour and 19
00:26:32 --> 00:26:35 minutes after the sun. From New York,
00:26:35 --> 00:26:39 10° up, 1 hour and 3 minutes. From
00:26:39 --> 00:26:43 London, 3 1/2° at sunset and gone 28
00:26:43 --> 00:26:44 minutes later,
00:26:44 --> 00:26:47 >> which is not Venus being fainter. It's
00:26:47 --> 00:26:49 exactly as bright everywhere. It's the
00:26:49 --> 00:26:50 geometry.
00:26:50 --> 00:26:53 >> It's the tilt of the ecliptic. At this
00:26:53 --> 00:26:55 time of year, the plane of the solar
00:26:55 --> 00:26:57 system stands almost vertically out of
00:26:57 --> 00:26:59 the western horizon at dusk from the
00:26:59 --> 00:27:02 southern hemisphere. So anything on that
00:27:02 --> 00:27:04 plane climbs steeply and takes a long
00:27:04 --> 00:27:06 time to set. From the northern
00:27:06 --> 00:27:09 hemisphere in September, that same plane
00:27:09 --> 00:27:11 lies down almost flat against the
00:27:11 --> 00:27:14 horizon and everything on it skims
00:27:14 --> 00:27:16 sideways and sets quickly.
00:27:16 --> 00:27:18 >> So northern listeners, here's the
00:27:18 --> 00:27:20 practical version. Find a spot with a
00:27:20 --> 00:27:24 genuinely clear, flat western horizon.
00:27:24 --> 00:27:27 No trees, no buildings. Start looking 20
00:27:27 --> 00:27:29 minutes after sunset and don't leave it
00:27:29 --> 00:27:31 much past 45. It'll be the brightest
00:27:31 --> 00:27:33 thing in that part of the sky by an
00:27:33 --> 00:27:35 enormous margin, so you won't be in any
00:27:35 --> 00:27:36 doubt once you've got it.
00:27:36 --> 00:27:39 >> And a bonus for the south, Mercury is up
00:27:39 --> 00:27:43 there, too. From Sydney, Mercury is 16
00:27:43 --> 00:27:46 degrees above the horizon at sunset and
00:27:46 --> 00:27:48 doesn't set for an hour and 20 minutes,
00:27:48 --> 00:27:50 which for Mercury is a comfortable
00:27:50 --> 00:27:53 viewing window. From Los Angeles, it's
00:27:53 --> 00:27:58 9° and 43 minutes. From London, 4° and
00:27:58 --> 00:28:01 26 minutes. That one really is a
00:28:01 --> 00:28:03 southern target this week.
00:28:03 --> 00:28:05 >> And then there's the moon, which is
00:28:05 --> 00:28:07 doing something rather precise tonight.
00:28:07 --> 00:28:10 >> First quarter, and it's exact. The moon
00:28:10 --> 00:28:13 reaches first quarter at 20:43 universal
00:28:13 --> 00:28:16 time tonight. That's quarter to 5 on
00:28:16 --> 00:28:19 Friday afternoon in New York,4 to 2 in
00:28:19 --> 00:28:22 Los Angeles to 10 on Friday evening in
00:28:22 --> 00:28:26 London. And for Australia, 20 to 7 on
00:28:26 --> 00:28:27 Saturday morning.
00:28:27 --> 00:28:29 >> And from Sydney tonight, the moon is
00:28:29 --> 00:28:32 very nearly overhead. 82 degrees up at
00:28:32 --> 00:28:34 sunset, which is close enough to
00:28:34 --> 00:28:35 straight up that you'll find yourself
00:28:35 --> 00:28:37 leaning back to look at it. which
00:28:37 --> 00:28:40 matters because tomorrow night, Saturday
00:28:40 --> 00:28:42 the 19th, is international observe the
00:28:42 --> 00:28:44 moon night.
00:28:44 --> 00:28:45 >> And the timing of that is not an
00:28:45 --> 00:28:47 accident. It's deliberately scheduled
00:28:47 --> 00:28:50 near first quarter because first quarter
00:28:50 --> 00:28:51 is when the moon is at its most
00:28:52 --> 00:28:54 interesting through any optical aid.
00:28:54 --> 00:28:57 Along the terminator, the line dividing
00:28:57 --> 00:28:59 day from night, the sun is striking the
00:28:59 --> 00:29:01 surface at a grazing angle. So every
00:29:01 --> 00:29:03 crater rim and mountain throws a long
00:29:03 --> 00:29:06 shadow across the ground behind it. A
00:29:06 --> 00:29:09 full moon looks flat. A half moon looks
00:29:09 --> 00:29:10 three-dimensional.
00:29:10 --> 00:29:12 >> Completely three-dimensional. Run the
00:29:12 --> 00:29:14 terminator with even a small pair of
00:29:14 --> 00:29:16 binoculars and the whole landscape
00:29:16 --> 00:29:19 stands up out of the surface. If you've
00:29:19 --> 00:29:20 only ever looked at a full moon and
00:29:20 --> 00:29:22 found it disappointing, this is the
00:29:22 --> 00:29:25 night to try again. NASA's own event is
00:29:25 --> 00:29:27 at the US Space and Rocket Center in
00:29:27 --> 00:29:29 Huntsville, Alabama. But the whole point
00:29:29 --> 00:29:32 is that it's global. You just need to go
00:29:32 --> 00:29:33 outside.
00:29:33 --> 00:29:35 >> And to balance the ledger because the
00:29:35 --> 00:29:37 south has had the better of the evening.
00:29:37 --> 00:29:39 The morning sky belongs decisively to
00:29:40 --> 00:29:40 the north.
00:29:40 --> 00:29:43 >> It does. Mars and Jupiter are both
00:29:43 --> 00:29:45 climbing in the pre-dawn east. And the
00:29:45 --> 00:29:47 same ecliptic geometry that flattened
00:29:47 --> 00:29:50 Venus for northern viewers works the
00:29:50 --> 00:29:52 other way around before sunrise. At
00:29:52 --> 00:29:54 nautical dawn tomorrow morning, Mars is
00:29:54 --> 00:29:58 48° up from Los Angeles and 46 from New
00:29:58 --> 00:30:01 York and 41 from London. Avery and from
00:30:01 --> 00:30:02 Sydney,
00:30:02 --> 00:30:05 >> 20°. So that's very much your sky, not
00:30:05 --> 00:30:09 ours. Jupiter is 26° up from Los Angeles
00:30:09 --> 00:30:10 and 9 from Sydney.
00:30:10 --> 00:30:13 >> And those two are closing on each other.
00:30:13 --> 00:30:16 They're about 23° apart this morning. By
00:30:16 --> 00:30:20 midocctober, that's down to 12. And by
00:30:20 --> 00:30:22 the middle of November, they'll be a
00:30:22 --> 00:30:24 little over a degree apart. Close enough
00:30:24 --> 00:30:26 to cover with a fingertip at arms
00:30:26 --> 00:30:31 length. That's the one to diarize.
00:30:31 --> 00:30:33 Saturn, meanwhile, is up essentially all
00:30:33 --> 00:30:35 night from everywhere. It doesn't set
00:30:35 --> 00:30:37 until nearly 7 in the morning from
00:30:37 --> 00:30:39 Sydney and just before 8 from Los
00:30:39 --> 00:30:41 Angeles. and it's building toward
00:30:41 --> 00:30:44 opposition on the 4th of October when
00:30:44 --> 00:30:46 the disc will be about 19 and a half
00:30:46 --> 00:30:49 arcsec across with the rings roughly 7°
00:30:49 --> 00:30:51 open
00:30:51 --> 00:30:53 >> and the equinox next week
00:30:54 --> 00:30:55 >> which we'll flag carefully because we
00:30:55 --> 00:30:57 got this slightly wrong in an earlier
00:30:57 --> 00:31:00 episode and corrected it. The September
00:31:00 --> 00:31:03 equinox is a single instant, not a day.
00:31:03 --> 00:31:05 5 minutes past midnight universal time
00:31:05 --> 00:31:08 on the 23rd. That's the evening of the
00:31:08 --> 00:31:10 22nd across the Americas and midm
00:31:10 --> 00:31:13 morning on the 23rd in Australia. So,
00:31:13 --> 00:31:15 the date depends entirely on where
00:31:15 --> 00:31:16 you're standing.
00:31:16 --> 00:31:18 >> And one last thing which follows
00:31:18 --> 00:31:20 directly from that quick hit about the
00:31:20 --> 00:31:23 blank sun because every time we mention
00:31:23 --> 00:31:25 sunspots, people quite reasonably want
00:31:25 --> 00:31:27 to go and look.
00:31:27 --> 00:31:30 >> And you can, but never ever with
00:31:30 --> 00:31:32 unprotected eyes and never through
00:31:32 --> 00:31:34 binoculars or a telescope that isn't
00:31:34 --> 00:31:37 purpose-built for it. The only safe way
00:31:37 --> 00:31:39 to look directly at the sun is through
00:31:39 --> 00:31:41 filters certified to the international
00:31:41 --> 00:31:45 standard ISO12312-2.
00:31:45 --> 00:31:48 That's eclipse glasses or a proper solar
00:31:48 --> 00:31:49 filter fitted over the front of the
00:31:49 --> 00:31:53 instrument. Never on the eyepiece end.
00:31:53 --> 00:31:55 >> And ordinary sunglasses are not solar
00:31:55 --> 00:31:58 filters. Neither is smoked glass,
00:31:58 --> 00:32:01 exposed film, a CD, or stacking several
00:32:01 --> 00:32:04 pairs of sunglasses together.
00:32:04 --> 00:32:06 >> None of those are safe. Check your
00:32:06 --> 00:32:09 eclipse glasses for the ISO12312-2
00:32:09 --> 00:32:11 marking. And if they're scratched,
00:32:11 --> 00:32:13 punctured, or you can't find the
00:32:13 --> 00:32:15 marking, don't use them. The safest
00:32:15 --> 00:32:18 option of all, and honestly the best one
00:32:18 --> 00:32:21 for a group, is projection. Put the
00:32:21 --> 00:32:23 sun's image onto a white card and
00:32:23 --> 00:32:24 everybody can look at once without
00:32:24 --> 00:32:26 anyone looking up.
00:32:26 --> 00:32:28 >> Although this week there may be nothing
00:32:28 --> 00:32:31 to see, which is rather the point.
00:32:31 --> 00:32:33 >> That's Astronomy Daily for Friday the
00:32:33 --> 00:32:35 18th of September. Everything we've
00:32:35 --> 00:32:37 talked about today, the papers, the
00:32:37 --> 00:32:39 DOIs, the full skywatch figures for all
00:32:39 --> 00:32:42 four cities is in the show notes and on
00:32:42 --> 00:32:45 the website at astronomyaily.io
00:32:46 --> 00:32:47 >> where you'll also find the full back
00:32:48 --> 00:32:50 catalog, the daily newsletter, and the
00:32:50 --> 00:32:53 contact form. And do use that contact
00:32:53 --> 00:32:55 form. Some of our favorite segments this
00:32:55 --> 00:32:58 year have come from listener questions,
00:32:58 --> 00:33:00 and we read every one of them.
00:33:00 --> 00:33:03 >> Go out tonight and look west. Venus will
00:33:03 --> 00:33:05 not be this bright again this year.
00:33:05 --> 00:33:08 >> And tomorrow night, look at the moon.
00:33:08 --> 00:33:09 >> We'll be back on the weekend with this
00:33:09 --> 00:33:12 week's weekend rap. Until then, clear
00:33:12 --> 00:33:24 skies.
00:33:24 --> 00:33:28 Stories told.