Time’s Tipping Point: The End of Leap Seconds and a Lunar Clock Revolution

[00:00:00] [SPEAKER_00] Hi everyone, it's Saturday the 17th of October and this is the Weekend Wrap from Astronomy Daily. I'm Anna.

[00:00:07] [SPEAKER_01] And I'm Avery. On Thursday, in a palace just outside Paris, the world decided that its clocks will stop waiting for the Earth.

[00:00:15] [SPEAKER_00] No more leap seconds from May next year. And in the same session, the first formal step towards an agreed time for the Moon.

[00:00:23] [SPEAKER_01] Then we look back over the week. Weather on a world without a sun, Hera breaking hard for an asteroid, five flashes on the far side of the Moon, a cloud on Mars that our models said couldn't form, and the first green star.

[00:00:38] [SPEAKER_00] And in the Skywatch, Mercury's last good week for the South, Venus about to vanish, and why the Orionids are a set an alarm shower this year. Let's get into it. To understand Thursday's decision, you need to know that the world has been running two clocks side by side for more than 50 years.

[00:01:00] [SPEAKER_00] The first is atomic time. Hundreds of atomic clocks in laboratories around the world are averaged together into a timescale that ticks with extraordinary steadiness. The civil version of that, the one your phone ultimately follows, is coordinated universal time, UTC. The second clock is the Earth itself. Astronomers call it UT1. It's simply the angle the planet has turned through, measured against the distant sky.

[00:01:27] [SPEAKER_00] And the Earth is a terrible clock. Its spin wobbles with tides, with the weather, with ocean currents, with the slow sloshing of the liquid core. Over the long run, tidal friction from the Moon has been slowing it down.

[00:01:42] [SPEAKER_01] So the two clocks drift apart. Since 1972, the rule has been that UTC is never allowed to get more than nine-tenths of a second away from the Earth's rotation. When the gap gets close to that, the International Earth Rotation and Reference System Service announces a leap second. And every clock on the planet that follows UTC has to count a 61st second in one particular minute.

[00:02:06] [SPEAKER_00] That has happened 27 times. The last one was at the end of 2016. In July, the IERS confirmed there won't be one at the end of this year either. So we'll reach a full decade without one.

[00:02:18] [SPEAKER_01] It's worth remembering that, for most of history, these were the same clock. Until 1956, the second was defined as a fraction of the average solar day. Then, because the day wasn't steady enough, it was redefined as a fraction of the year. Still astronomy. Only in 1967 did the second leave the sky entirely and move into the cesium atom. Leap seconds were the compromise that kept the two loosely tied together. So, why now?

[00:02:47] [SPEAKER_01] That decade without one is actually the reason for Thursday's vote. For most of the leap second era, the Earth was running slow, so the correction was always the same. Add a second. But in recent years, the planet's rotation has been running slightly fast.

[00:03:02] [SPEAKER_00] Which raises a possibility nobody has ever had to deal with. A negative leap second. A minute with only 59 seconds in it. The resolution itself puts a number on it. A workshop held in March last year by the Consultative Committee for Time and Frequency and the IERS, including experts on Earth's rotation, estimated that the probability of a negative leap second rises rapidly in the near future, reaching about 30% by 2035.

[00:03:31] [SPEAKER_00] And the delegates were blunt about the risk. A negative leap second has never been applied. The resolution describes it as a high risk to critical infrastructure that is largely unprepared. And says the cost of getting ready would be similar to what industry spent preparing for the millennium bug.

[00:03:49] [SPEAKER_01] There's a quieter problem, too. Positive leap seconds already cause trouble. So big service providers have each invented their own workarounds, smearing the extra second across hours of the day in different ways. The resolution notes that this leaves different systems disagreeing with each other about what time it is. And that some users were starting to abandon UTC altogether for satellite navigation time.

[00:04:11] [SPEAKER_00] So, on Thursday, the 15th of October, at the 28th General Conference on Weights and Measures in Versailles, member states adopted what's called Resolution 5. It was carried by a majority vote. Two decisions.

[00:04:27] [SPEAKER_01] First, UTC becomes a continuous timescale on the 20th of May, 2027. No more leap seconds after that. In either direction.

[00:04:36] [SPEAKER_00] And second, the allowed gap between UTC and the Earth's rotation grows more than a year. It goes from under a second to one hour. 3,600 seconds. The resolution says that ensures UTC's long-term continuity for several centuries.

[00:04:52] [SPEAKER_01] Back in 2022, the same conference had agreed to do this by 2035. What changed is that the negative leap seconds stopped looking theoretical. So they brought it forward by eight years.

[00:05:03] [SPEAKER_00] The president of the Time Committee, Noelle DeMarc, said it came after 30 years of debate. Now, the honest caveats. Because the headlines this week have been a bit dramatic. First, this is a decision about the clock, not about the Earth. The planet will keep turning exactly as it was going to.

[00:05:22] [SPEAKER_01] Second, the drift is slow. We're talking seconds per decade, not minutes. Nobody will notice the sun rising at the wrong time in their lifetime. The resolution itself makes the point that most of us already live an hour or more away from local solar time because of time zones and daylight saving.

[00:05:40] [SPEAKER_00] And third, what happens if the gap ever approaches that one hour limit is left to people several centuries from now? The resolution sets the limit. It doesn't spell out the fix.

[00:05:53] [SPEAKER_01] Here's the bit I find most interesting for this show. Astronomers still need UT-1. If you want to point a telescope at a star or work out where a satellite is over the ground, you need to know exactly how far the Earth has actually turned. That number keeps being published by the IERS and broadcasts in satellite navigation signals. It just stops being fed back into your clock every few years.

[00:06:18] [SPEAKER_00] And the way the Earth's rotation is measured is pure astronomy. The technique that's unique in its ability to pin down UT-1 is very long baseline interferometry. Radio dishes on different continents all observe the same distant quasars, billions of light years away, and time the arrival of the signal at each dish.

[00:06:38] [SPEAKER_01] Because those quasars are so far away, they're effectively fixed. They make the most stable reference frame we have. The tiny differences in arrival time tell you how the dishes, and the whole Earth under them, have rotated.

[00:06:52] [SPEAKER_00] And Australia is part of that network. Oscope's 12-meter dishes at Hobart in Tasmania, Catherine in the Northern Territory, and Yerriga Dee in Western Australia observe in the International Geodetic VLBI sessions. So the length of your day is quite literally partly measured from Australia against quasars.

[00:07:11] [SPEAKER_01] Which is a nice callback to episode 197, when we covered the Earth's center of mass wobbling with the seasons. Same idea. The ground we stand on is not a fixed platform, and it takes the whole sky to measure it.

[00:07:26] [SPEAKER_00] Resolution 5 got the headlines, but the same session adopted Resolution 6. And for space exploration, it may matter more. It's the first international step towards a lunar reference timescale. The moon's equivalent of UTC.

[00:07:41] [SPEAKER_01] The problem is relativity. A clock on the moon sits in weaker gravity and moves differently from one on Earth, so it runs fast. A widely cited estimate puts that at roughly 56 millionths of a second a day.

[00:07:56] [SPEAKER_00] That sounds tiny, but light travels about 17 kilometers in 56 microseconds. If you're building navigation on the moon, landers and rovers and astronauts working near each other, an error that grows by kilometers a day is not acceptable.

[00:08:12] [SPEAKER_01] And several agencies are building lunar navigation systems at the same time. The Resolution's stated aim is to avoid a proliferation of competing lunar timescales, and to keep whatever is chosen traceable back to UTC. For now, it asks space agencies to share how they handle the relativistic corrections. It doesn't define the timescale yet. That work is still to come.

[00:08:36] [SPEAKER_00] One more. Resolution 4 starts the groundwork for redefining the second itself. Since 1967, the second has been defined by cesium atoms. The best optical clocks now beat the best cesium fountain clocks by around a factor of 100.

[00:08:53] [SPEAKER_01] Which means the definition has become the limitation, not the technology. Two options are on the table. Pick one optical transition or average several. The aim is a vote at the next conference in 2030.

[00:09:05] [SPEAKER_00] So, in a single afternoon in Versailles, the world agreed to stop chasing the Earth's spin with its clocks, started building a clock for the moon, and began retiring cesium. Not bad for a meeting most people have never heard of.

[00:09:19] [SPEAKER_01] And next week, we'll be watching a spacecraft head for Mars' moons. More on that in the outro.

[00:09:25] [SPEAKER_00] Next, let's move on to our wrap of the space news from the past week.

[00:09:29] [SPEAKER_01] Monday's lead was Wyze 0855, the coldest brown dwarf we know, about seven and a half light years away. With no star to warm it. A team led from the University of Arizona used JWST's NEAR-Spec to take a spectrum every 15 minutes for 11 hours.

[00:09:47] [SPEAKER_00] And the atmosphere changed while they watched. Carbon monoxide and phosphine rose and fell together, which points to convection dredging deep gas upward. Higher up, patchy water clouds thickened and thinned.

[00:09:59] [SPEAKER_01] The careful bit. Some headlines said these were the first water clouds seen outside the solar system. They're not. Water clouds were claimed for this object in 2014 and 2016. What's new is seeing them change. The paper is a preprint, posted about three weeks before we covered it.

[00:10:17] [SPEAKER_00] Tuesday, we led on ESA's HERA, closing on the Didymos binary asteroid. The system NASA's DART mission struck in 2022.

[00:10:25] [SPEAKER_01] On the 7th of October, its second launch anniversary, HERA caught its target for the first time. Didymos and its moonlit dimorphos together as a single pixel from about 236,000 kilometers away.

[00:10:39] [SPEAKER_00] Then early on Thursday, the 15th, the big one, three thrusters burning continuously for 93 minutes to shed a few hundred meters per second. With ESA's new Norsha antenna in Western Australia lined up to pick up the signal afterwards.

[00:10:53] [SPEAKER_01] As we record, ESA hasn't yet published the result, which is normal while the flight dynamics team checks the new trajectory. We'll bring it to you when they do.

[00:11:02] [SPEAKER_00] Next up is a second shorter burn of about 31 minutes on Thursday, the 22nd. Then three small maneuvers to bring HERA almost to a stop beside the asteroids ahead of close-up observations from 25 to 30 kilometers away in November and December.

[00:11:17] [SPEAKER_01] Wednesday, NASA's preliminary lunar science report from Artemis 2. During a 54-minute stretch when the sun was hidden behind the moon, the crew watched a dark lunar surface and reported five faint flashes of light.

[00:11:30] [SPEAKER_00] Rocks, each only a few centimeters across, hitting the moon. One of those flashes was marked independently by all three crew members who logged them, which is about as good as eyewitness evidence gets. They may be the faintest lunar impact flashes ever seen by human eyes.

[00:11:45] [SPEAKER_01] Also on Wednesday, Anmul Agarwal and Ralph Schoenrich at University College London, in two papers in the monthly notices of the Royal Astronomical Society, went after one of the lines of evidence for hypernovae, chemical fingerprints in very old stars.

[00:12:00] [SPEAKER_00] They found that ordinary supernovae, enriching the gas unevenly, explain three of the four stars thought to carry a hypernova signature. The fourth, a giant rich in silver and uranium, fits a neutron star merger plus one ordinary supernova.

[00:12:19] [SPEAKER_01] And the honest framing? That doesn't mean hypernovae don't exist. Very energetic explosions linked to gamma ray bursts have been observed directly. What's in question is whether these old stars require them.

[00:12:31] [SPEAKER_00] Thursday took us to Mars. Every morning, in the southern spring and summer, a thin cloud up to 1,800 kilometers long, streams from the 20-kilometer volcano Archea Mons, then vanishes by afternoon.

[00:12:46] [SPEAKER_01] For years, the models couldn't make it. Jorge Hernandez Bernal and colleagues at LMD in Paris, writing in Nature Geoscience, only reproduced it by letting ice form directly from vapor with no dust to seed it. The air over the volcano cools by around 30 degrees in 10 minutes, and the humidity soars to more than 100,000 times everyday Earth levels before it freezes.

[00:13:11] [SPEAKER_00] It's a modeling result. Sufficient, not yet observed directly.

[00:13:15] [SPEAKER_01] And Friday? Color. Mark Hollins and colleagues at the University of Warwick took white dwarf spectra astronomers already had and ran them through a model of human color vision.

[00:13:25] [SPEAKER_00] WDJ 1330 plus 6435 comes out green, chartreuse, because sodium from the rocky remains of its own planetary system absorbs the orange light. WDJ 1011 plus 2845, 48 light years away, may be the reddest star ever recorded. A deep salmon pink.

[00:13:49] [SPEAKER_01] Two caveats. These colors are computed, not seen. No white dwarf is close enough to see its color by eye. And the typical white dwarf turns out to be white. Of the 151 within about 65 light years, only around 1 in 30 has an unusual color.

[00:14:06] [SPEAKER_00] A few more from the week, quickly. Monday. VTF J0440 plus 2325. A red dwarf pulling gas off a brown dwarf of about 25 Jupiter masses on an 87-minute orbit. The MIT-led team in Nature Astronomy calls it the first confirmed case of stable mass transfer from a substellar object onto a star.

[00:14:31] [SPEAKER_01] Also Monday, the dead Soviet weather satellite Meteor-27, launched in 1981, broke apart on the 5th of October, about 850 kilometers up. And the breakup was picked up by LEO lab's radar in Western Australia. The cause is still unknown.

[00:14:46] [SPEAKER_00] Wednesday, Juno found Jupiter's vast magnetic disk is crumpled, rippling as it rotates, crossing the spacecraft four times in 10 minutes in one pass. And simulations from the Indian Institute of Science suggested strong magnetic fields could let a white dwarf grow to around 2.4 solar masses, well past the usual limit near 1.4.

[00:15:12] [SPEAKER_01] Thursday. JWST mapped the edge of the hot Jupiter WASP-17b, about 860 Kelvin at the morning edge and 1520 at the evening edge, the largest contrast yet measured. And an INF-led team found that nearly 300 well-measured multi-planet systems fall into two distinct families.

[00:15:34] [SPEAKER_00] And Friday, a southern one, reprocessing archived Mariah data from CSIRO's Parks Telescope turned up six pulsars, five of them in the Large Magellanic Cloud, plus the slowest radio pulsar yet found in the Small Magellanic Cloud. Both are preprints, and both are reminders that old data still has surprises in it.

[00:15:55] [SPEAKER_01] That's the week that was.

[00:15:56] [SPEAKER_00] Moving on to Skywatch, starting with the Moon. First quarter is at 1612 Universal Time on Sunday. That's 312 on Monday morning in Sydney, 912 Sunday morning in Los Angeles, 1212 in New York, and 512 Sunday afternoon in London. So all week, the Moon is in the evening sky and growing.

[00:16:18] [SPEAKER_01] Southern Hemisphere, this is Mercury's last strong week. Tonight from Sydney, it's 24 degrees up in the west at sunset and doesn't set until two hours after the sun. Even by Wednesday, it's still 22 degrees up.

[00:16:32] [SPEAKER_00] From Los Angeles, it's under 10 degrees at sunset and sets within the hour. New York, 7 degrees. London, under 3. Northern listeners, you'd need a flat western horizon and binoculars. And please wait until the sun is fully down.

[00:16:49] [SPEAKER_01] Venus is the bigger story. From the northern hemisphere, it's already gone, below the horizon by sunset. From Sydney tonight, it's still 11 degrees up at sunset and sets an hour later. But by Wednesday, it's down to 5 degrees in half an hour.

[00:17:03] [SPEAKER_00] Venus reaches inferior conjunction, passing between us and the sun, around 3 Universal Time next Saturday, the 24th. That's early Saturday afternoon in Australia and Friday evening in the Americas. It slips about 6 and a half degrees from the sun, then reappears in the morning sky.

[00:17:23] [SPEAKER_01] It's the same geometry we keep coming back to. In southern spring, the evening ecliptic stands up steeply, so the south keeps Venus and Mercury longer.

[00:17:31] [SPEAKER_00] And after dark, everyone gets sadder. It's only a couple of weeks past opposition, rising in the east around sunset and well-placed by mid-evening from all four cities. Its rings are tilted only about 7 degrees towards us at the moment. So in a small telescope, they look like a thin, bright line across the planet. They were exactly edge on in March last year and are slowly opening again, with the south face of the rings towards us. So each year, they'll look a little wider.

[00:18:01] [SPEAKER_01] Now, what about the Orionids?

[00:18:03] [SPEAKER_00] The Orionid meteor shower peaks on the night of the 21st into the 22nd. These are dust grains from Halley's Comet, the same comet that gives us the Aida Aquarids in May.

[00:18:14] [SPEAKER_01] The catch this year is a bright, roughly 80% moon, but it sets in the small hours. About 3.40 a.m. in Sydney, 3.20 in Los Angeles, 3.10 in New York and 2.50 in London. That leaves a dark window before dawn.

[00:18:30] [SPEAKER_00] And at 4.30 a.m., the radiant in Orion is around 70 degrees up from Los Angeles, 63 from New York, 53 from London and 39 from Sydney. So the north is favored, but the south is perfectly respectable. Typical rates under dark skies are around 20 an hour. Go after moonset, look away from the radiant and give your eyes 20 minutes.

[00:18:57] [SPEAKER_01] While you're up, Mars and Jupiter. This morning, they're 11.5 degrees apart, closing to under 9 by next weekend and to just over 1 degree on the 16th of November.

[00:19:08] [SPEAKER_00] At nautical dawn, this is a northern showpiece. Mars around 60 degrees up and Jupiter around 50 from Los Angeles and New York, 50 and 42 from London. From Sydney, they're lower, 23 and 19 degrees, but easy. Jupiter is the bright one. Mars is the orange one above it.

[00:19:29] [SPEAKER_01] Finally, the sun has woken up. A large active region, AR-4551, has developed the most complex magnetic classification there is, beta, gamma, delta, which gives it the potential for strong flares. If it launches an Earth-directed eruption, aurora watchers in Tasmania and southern Victoria and across Canada, the northern U.S. and Scotland will want to check the Bureau of Meteorology's Space Weather Services and NOAA's Space Weather Prediction Center. It's a possibility, not a forecast.

[00:19:58] [SPEAKER_00] And if all that sunspot talk tempts you to look, never look at the sun directly or through binoculars, a telescope or a camera without a certified ISO 12312-2 solar filter covering the front of the instrument. Ordinary sunglasses are not safe. Projection or a properly filtered telescope is the way to see sunspots.

[00:20:18] [SPEAKER_01] On Monday, we'll be on launch eve for JAXA's Martian moons exploration mission, MMX, which is aiming to bring back the first sample from Phobos with the capsule due to land in South Australia in 2031. It's a fitting follow on to today's lead. MMX will spend years working at Mars, kept on time by atomic clocks on Earth and tracked by the deep space stations that serve every mission out there.

[00:20:42] [SPEAKER_00] Launch is set for 4.41 Tuesday morning Japan time. That's 6.41 Tuesday morning in Sydney, 12.41 Monday lunchtime in Los Angeles, 3.41 Monday afternoon in New York, and 8.41 Monday evening in London.

[00:20:58] [SPEAKER_01] If you have a question you'd like us to chase, the contact form is at astronomydaily.io, and you'll find us at astrodailypod on social media. Astronomy Daily is part of the Bytes.com podcast network.

[00:21:11] [SPEAKER_00] Thanks for spending part of your weekend with us. I'm Anna.

[00:21:14] [SPEAKER_01] And I'm Avery. Clear skies, and we'll see you Monday.