How accurate are planetary positions?
The horizon line does not include the effects of astronomical refraction, which raises stars along the horizon by about one half of a degree, and should therefore depress the horizon line by that amount. (It does however include the much smaller effects of terrestrial refraction on the altitude of the terrain that forms the horizon, as described in the technical FAQ.) We currently store position data for the Sun, Moon and planets only for 2007 through 2010. If you specify a time earlier or later than that, you won’t see any solar system objects. We store those positions — right ascensions and declinations computed by NASA JPL’s HORIZONS System — at one minute intervals for the Moon and ten minute intervals for all other objects and interpolate linearly within those intervals. We compute the topocentric position for the Moon and Earth shadow, but use geocentric positions for all the other objects (i.e. we place the Moon in its proper place in the sky based on the observer’s location o
