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.

  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