# Astronomical Unit (AU)

1. The astronomical unit (AU) is the mean Earth-Sun distance. It is a mean in the sense of being the average of closest and farthest distances from the Sun (i.e., perihelion and aphelion ). This mean is also the semi-major axis of the elliptical orbit of Earth. The AU in centimeters is given by
```    1 AU=1.49597870*1013cm=1.496*1013cm=1.5*1013cm.
```
NOT a memorable quantity---though it's better than * using miles. 1 is much easier to remember. >

2. Why use AUs? Well up until about the 1660s solar system distances other than the distance to the Moon (which is about 60 Earth radii) could not be related accurately to terrestrial distances, but they could be known in terms of AUs (No-351). It was one of the great triumphs of Copernicus that he was able to determine the RELATIVE DISTANCES of the solar system and thus determine its structure.

3. Nowadays the AU scale is just convenient for thinking about the solar system. The solar system isn't huge or minute in AUs. And also, although our position in the solar system is not special in physics, it's special to us and perhaps to LIFE AS WE KNOW IT. So it is convenient to distances in terms of where we are.

Table of Planet Distances and Eccentricities
```
Planet           Mean Distance (AU)        Eccentricity

Mercury            0.38710                   0.2056
Venus              0.72333                   0.0068
Earth              1.00000                   0.0167
Mars               1.52369                   0.0933
Jupiter            5.20283                   0.048
Saturn             9.53876                   0.056
Uranus            19.19139                   0.046
Neptune           30.06107                   0.010
Pluto (ex-planet) 39.52940                   0.248

```
NOTE: Eccentricity is the RELATIVE AMOUNT that the planet-Sun distance varies from the mean distance. For example, Mercury goes 20.56 % nearer and farther from the Sun than the mean distance. Pluto and Mercury have the most eccentric orbits. See
ellipse for more on eccentricity.