Tag: anomalistic year

Happy New Year! Again!

By Phil Plait | January 1, 2011 12:02 am

I don’t usually repost blog entries, because it’s lazy. But it’s 2.5 hours before New Years as I sit here, and you know what? Tonight I’m lazy (though not so lazy to make a few edits to bring the post up to date). Plus, this post was last seen three years ago, on December 31, 2007, and I have a lot of new readers since then so it’s new to them. Also? This post is one my favorites I’ve ever written. So enjoy it, but fair warning: if you’re hungover I can almost guarantee this’ll make it worse.


Yay! It’s a new year!

But what does that mean, exactly?

The year, of course, is the time it takes for the Earth to orbit the Sun, right? Well, not exactly. It depends on what you mean by "year", and how you measure it. This takes a wee bit of explaining, so while the antacid is dissolving in your stomach to remedy last night’s excesses, sit back and let me tell you the tale of the year.

First, I will ignore a few things. For example, time zones. These were invented by a sadistic watchmaker, who only wanted to keep people in thrall of his devious plans. So for now, let’s just ignore them, and assume that for these purposes you spend a whole year (whatever length of time that turns out to be) planted in one spot.

However, I will not ignore the rotation of the Earth. That turns (haha) out to be important.

Let’s take a look at the Earth from a distance. From our imaginary point in space, we look down and see the Earth and the Sun. The Earth is moving, orbiting the Sun. Of course it is, you think to yourself. But how do you measure that? For something to be moving, it has to be moving relative to something else. What can we use as a yardstick against which to measure the Earth’s motion?

Well, we might notice as we float in space that we are surrounded by zillions of pretty stars. We can use them! So we mark the position of the Earth and Sun using the stars as benchmarks, and then watch and wait. Some time later, the Earth has moved in a big circle and is back to where it started in reference to those stars. That’s called a "sidereal year" (sidus is the Latin word for star). How long did that take?

Let’s say we used a stopwatch to measure the elapsed time. We’ll see that it took the Earth 31,558,149 seconds (some people like to approximate that as pi x 10 million (31,415,926) seconds, which is an easy way to be pretty close). But how many days is that?
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CATEGORIZED UNDER: Astronomy, Cool stuff, Geekery, Humor
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