JWST M51 image credit: ESA/Webb, NASA & CSA, A. Adamo (Stockholm University) and the FEAST JWST team

This is issue 724 of the BAN, and this is a cool number because it’s the sum of four consecutive prime numbers (173 + 179 + 181 + 191) as well as the sum of six consecutive prime numbers (107 + 109 + 113 + 127 + 131 + 137). I don’t know why I love stuff like this, but sometimes it’s enough to just love it anyway.

Another weirdness: Imagine you have ten queens on a 10 x10 chess board. How many ways can you arrange the queens so that none threatens another? Yup: 724.

Astronomers are faced with a very weird problem: 95% of the universe is made of something we don’t understand. I’m talking dark energy and dark matter, two (probably) unrelated phenomena that do have one thing in common: they’re dark, which means we can’t see them directly. We can only see their effects on objects we can observe.

What are they? In this week’s episode of Big Picture Science, the SETI Institute’s radio show/podcast, hosts Molly Bentley and Seth Shostak talk to a pair of scientists investigating this problem. Even better (for some definition of the word “better”) they invited me to be on the show as color commentary, adding my own take on these issues after I heard what the scientists had to say. It’s a fun episode, asking more questions than it answers, but hey, that’s science. If we didn’t have more things to wonder about after we solve a problem, then it wouldn’t be as much fun!

Global warming is changing the rotation of the Earth. And that may affect computers and the economy around the world.

That’s a weird thing to say, but it’s true. And it all ties into leap seconds.

These are different than leap years, when we add a day to the calendar every four years due to a mismatch between Earth’s spin and the time it takes to orbit the Sun.

Earth’s rotation rate changes all the time for lots of reasons (though these changes are very small). The main one is tidal friction due to the Moon which slows Earth’s rotation very gradually, about 2 milliseconds every 100 years (though this is far from constant; there are many sources that affect Earth’s spin). We say that there are 86,400 seconds in one day, but that’s not really true. It was back in 1820 or so, but the planet has slowed since then, and now takes about 86,400.002 seconds.

That means, very roughly, every year there’s an extra second in the year. Again, this changes all the time, so it’s not always one second, and not every year. There’s a group, the International Earth Rotation and Reference Systems Service, that looks at Earth’s rotation and, if it starts to lag by more than 0.9 seconds in a given year, decides to add an extra second to the calendar. At midnight December 21 of that year, the official clocks read 23:59:60, instead of ticking over to 00:00:00. That keeps our calendars aligned with, say atomic clocks, which measure time much more accurately than Earth’s spin. Since the year 1972, 27 leap seconds have been added to keep the clocks in sync. They’ve all been positive; that is, we’ve always added a second to the clocks.

As weird as that is, of course it gets more complicated. Other things affect Earth’s spin too. They all boil down to angular momentum, a property of objects that are either spinning (rotating) or revolving around another object.

A basic property of physics is that in a closed system (like a planet spinning in space) angular momentum is conserved: It cannot be changed. Simply put, the rotation rate of an object and the way its mass is distributed are related. Change the shape of the object and the rotation rate will change to keep angular momentum the same.

A canonical example of this is an ice skater who has their arms out wide and goes into a spin. At first they spin slowly, but as they draw their arms in their rotation rate speeds up.

Same with Earth. If the material making up our planet shifts in some way, it can change the planet’s spin. So, for example, a landslide on a mountain brings material closer to the ground, so Earth’s rotation speeds up a little. We see this effect in weird ways; for example damming up rivers redistributes water, which is heavy, and that can affect Earth’s spin too. Not much, but it’s there, and can change the way we measure time.

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