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In yesterday’s BAN, I wrote this:

“…all days are the same length — 24 hours, depending on what kind of day you mean, and surprise, there are a lot.”

But I didn’t really follow up on that, though I promised I would today. So here we are!

You might think the meaning of the word “day” is pretty obvious — though in yesterday’s post I already showed it can be a bit confusing, since it means the time the Sun is up in the sky, as well as the time it takes Earth to rotate once. 

But even that latter bit isn’t really specific enough. That’s because there are different ways to measure that rotation. For example, you could lie down on the surface of our planet at night and note when a specific star passes directly overhead (or, say, passes the vertical line connecting due south to due north, what astronomers call the meridian — this is actually how we do measure star positions, in fact). Then you count seconds until the next night when the star is in the same position in the sky. How long did that take?

You’ll find you get to 86,164 of them. If you convert to more convenient units, that’s 23 hours 56 minutes and 4 seconds.

But wait. That’s not right, is it? Shouldn’t that be 86,400 seconds, which is exactly 24 hours?

Nope. We measured the length of time it takes for that star to go around Earth once, or, more properly, how long it takes Earth to spin once relative to that (or any) star. This is called a sidereal day, from the Latin word “sidus” for star. But while astronomers are happy with such a measurement, normal people tend to use the Sun as a benchmark, since that rules our daily lives. If we measure how long it takes the Sun to complete one circuit around the sky, we get the familiar 24 hours.

Why the difference? Because over the course of one sidereal day, Earth moves in its orbit around the Sun by about 1 degree. That means it has to spin a little more, by 1°, to move the Sun back to its position in the sky where it was yesterday. That difference is the 3 minutes and 56 seconds to go from a sidereal to a solar day.

Either one of these units is perfectly legit. It just depends on what you’re wanting to measure. If you like to have lunch at noon every day, go for the solar day. If you’re an astronomer who wants to observe the same celestial object every night and want to know where it is, you use a sidereal clock. In some ways it’s like the difference between mile and kilometer; they both measure length, they’re just slightly different units.

Incidentally, I lied a second ago: I said a solar day is 24 hours, but that’s not really true either. Earth’s orbit is elliptical, and when it’s closer to the Sun (like it is now around the end of the year) it moves faster in its orbit, so it takes a little bit longer for the rotation to catch up, making a solar day a little longer in turn. On the other hand Earth’s tilt also affects this, since the height of the Sun above the horizon also plays a role here. It’s a little confusing, though Wikipedia covers it well enough. But this changes the length of a solar day by several minutes over the course of a year. It’s a mess, but that’s nature!

By the way, you could make length-of-day measurements like this using any number of different celestial clocks, in fact. You could measure the time it takes Jupiter to go around the sky once, which is much closer to a sidereal day but not exactly equal to it because Jupiter is orbiting the Sun as well, so again we have to spend an extra (very short) amount of time rotating a wee bit more to catch up to it in the sky. This kind of thing has a name too: it’s called a synodic period, the general term for the time it takes two objects moving relative to each other to line up.

I wrote about this in some detail back on The Old Blog™ at SYFY. I suppose I could’ve waited until January 1 to post this, since that article is about the weird and confusing different kinds of years there are, but a promise is a promise, so here you go. Feel free to read that whole thing on Thursday, if you like.

Go ahead. Make some time.

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