Time Roundup

A couple of cool science/technology news items just dropped:

• There’s going to be a new time zone for the moon (and non-Earth places, generally)
• The Earth was going to need an unprecedented negative leap second, but we’re not (yet)

Coordinated Lunar Time (LTC)

Remember that list of Falsehoods Programmers Believe About Time? You know, that migraine-inducing laundry list of gotchas for anyone who needs to deal with something as exotic as time in an app? Well, it’s going to get a whole lot worse. Be sure to tack on something like “a second is not always the same length of time.” Any sentence beginning with “Due to general and special relativity…” ought to make your blood run cold as a programmer.

But, and this is the nifty part, it’s cool to think about as a concept. If you were standing on the moon, in your moonboots, and looking at those sloppy Earthlings, you’d notice that each day they lose about 58.7 microseconds. It would seem that their clocks just run more slowly. Clearly not a huge difference, but if you’re a sciency type that can really amount to some mustard on your shirt–scientifically speaking.

Navigation is something that’s notoriously sensitive to pesky old Einstein. In fact, if the ol ‘Stein weren’t accounted for in the GPS system circling Earth, the cumulative error would amount to around 10km per day. Part of this effort then is to come up with a system that’s going to be robust for use outside of the range of terrestrial GPS.

There’s some other fun stuff in this report. It mentions one of my favorite time standards International Atomic Time (or TAI). This is the standard that behaves much more like the naieve “Falsehoods Programmers Believe…” list seeks to demolish. It just lets you have your delicious falsehoods. TAI works by averaging hundreds of atomic clocks. It differs from UTC by some whole number of leap seconds, because it doesn’t have them (UTC does). Basically, Earth is a great place to live but not great at being a clock.

Negative Leap Second

Everyday experience with things that spin should give you a intuitive understanding things slow down. A top will start to wobble and then eventually tip over. The Earth isn’t different. It’s just really big and it’s where all my stuff is stored. People also happen to have some misguided sentimental attachment to the sun being out around noon. These two facts:

• people wanting certain times to correspond to stuff like the sun being roughly overhead
• the Earth slowly losing that property

…means that we have to play some funny games with how our clocks work. In the past it meant slowly adding something called a leap second. Like a leap day, but much shorter. And they’ve always been added; there’s occasionally some minute at some time (often when we least expect it) that has 61 seconds rather than 60. Which is what you expect if you want a steadily marching metronome to keep up with a gradually slowing Earth.

But! For various theorized reasons Earth has actually been spinning more quickly of late. And in the reverse of the above reasoning, that would mean that eventually we’d need to remove a second from the clock to keep pace with a faster Earth. Nobody’s quite sure what the full implications of this would be.

And this is where software comes into the picture. Generally we’d assume code like this:

t1 = now();
// do some stuff
t2 = now();
diff = t2 - t1;

would always yield a positive, or depending on the timer resolution, zero value. We might be surprised if it would be negative. And when software developers are surprised, we don’t get angry, we just make bugs. Sorry, but I’m not sure what this means for the upcoming surprise party that you were thinking of throwing for your software developer friend.

And in the final twist here, there’s some extra slowing that we can attribute to climate change. And that extra slowing is counteracting the somewhat-mysterious acceleration, and thus staving off the need for a negative leap second. Which would be unpopular.

So… for various competing reasons it looks like we’re safe from the hypothetical scourge of the negative leap second for at least a little while longer. But, you know, watch this space.

Sources

• White House Cislunar Technology Strategy Memo (Office of Science and Technology)
• A global timekeeping problem postponed by global warming (Nature)
• Leap Second (Wikipedia)
• Real-World Relativity: The GPS Navigation System

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