Another orbit? Why, you don’t look a rotation older than 4.56 billion years!

Start the new year with SCIENCE!  I'm going to take a quick break from posting QualiaSoup videos because this is also really cool.  Dr. Plait (The Bad Astronomer) has a great explanation on orbital phenomenon.  Generally people don't care about things like that, except when an arbitrary demarcation point has been made (such as a calendar).  So last night a lot of folks stayed up late, drank champagne, and kissed their loved ones.  But what does it actually mean?  Not a whole heck of a lot really, but astronomically speaking, there's a few opportunities to discuss how our planet behaves, and the effects it causes here.  And I really think that's cool!

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?

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