Astronomical Unit (AU)

I have to say, I don’t think I’ve heard the term “Astronomical Unit” before. I do use the word “astronomical” when referring to big things, so I thought I’d do a little research. I anticipated there would be some correlation between the two.

Astronomical — of or relating to astronomy, extremely large

An Astronomical Unit is approximately the mean distance between the Earth and the Sun. It is used to indicate distances within our solar system.

The recent (suspected) comet that hit Jupiter is thought to be from a part of the solar system known as the Oort Cloud that sits well beyond Pluto and the Kuiper Belt. This region of space spans a zone between 1,000 and 20,000 AU away from the Sun.

Here is the formal definition that makes my brain hurt. The radius of an unperturbed circular orbit a massless body would revolve about the sun in 2*(pi)/k days (i.e. 365.2568983... days), where k is defined as the Gaussian constant exactly equal to 0.01720209895. Since an AU is based on radius of a circular orbit, one AU is actually slightly less than the average distance between the Earth and Sun. (I’m guessing this is because the Earth does not orbit in a perfect circle, rather an oval where the Sun is not exactly in the middle. See my blog on Perihelion, Aphelion and Precession.)

History of Earth/Sun Distance — In the late 1500’s Tycho Brahe estimated the distance at 5 million miles. The early 1600’s Johannes Kepler estimated 15 million miles. In the late 1600’s Giovanni Cassini estimated 87 million miles by observing Mars and estimating our distance from that planet and then was able to determine the Earth to Sun distance. Pretty close for so long ago. Funny that I cannot find who was able to determine the current distance–will keep looking.

So what is Earth’s distance from the Sun? 93 million miles, or 1 Astronomical Unit. Wow. What an astronomical distance!

Now you’re a little smarter, Girlfriend — And so am I.