Apply Parallax Correction to Moon

Traditionally, the planet’s positions are calculated as if they were observed from the center of earth (hence “geocentric”). However, as the moon is much closer to the earth than the other planets, certain geometrical effects come into play that are negligible for the other planets. In particular, due to the moon’s proximity to the earth, it appears to be in a slightly different zodiacal position depending on where on earth it is observed from. This effect is known as lunar parallax (or also as a “topocentric” coordinate system).

Typically, this can make a difference to the moon’s longitude of the order of 1 degree. Thus, if the moon is near the end of a sign, applying this correction can actually put the moon into the next sign in same cases.

In the "Preferences" dialog, "Calculations" tab you can specify whether to apply parallax correction or not.

Any charts opened or calculated, or dynamic reports generated are calculated according to the current setting of this option. When this option is switched on for a particular chart, the chart details text has an additional line indicating that the parallax correction has been made.

Note that, although the altitude of the observer also makes a small difference to the parallax calculation, it is negligible in comparison with the observer’s location, so is ignored by Solar Fire. For example, an altitude of 3000 meters makes a maximum difference of only 2 to 3 seconds of arc in the moon’s position, and it is usually less than this.