Optional Correction Factors

The two following correction factors may be applied when chart are calculated.

•Apply Geocentric Correction to Latitude - It is possible to calculate charts based either on geographic (also known as geodetic) latitude or geocentric latitude. Atlases such as the ACS Atlas and Solar Writer’s own place databases, contain geographic latitudes. Geographic latitudes are based on the measurement of the angle of the local horizon (also known as the geodetic horizon) onto the celestial sphere. It is assumed that all latitudes entered into Solar Writer are geographic latitudes, and all displayed latitudes, such as in chart details text, are geographic latitudes. However, because the earth is not perfectly spherical it is also possible to define a latitude based on where the horizon would be if the earth was a perfect sphere, which also corresponds to the angle of a line from that location to the centre of the earth. This is known as a geocentric latitude. The geocentric latitude is never more than 12 minutes of a degree different from the geographic latitude. There are arguments for and against applying the geocentric correction to latitude, and astrologers are divided about which is the most appropriate to use. When this option is on, any new chart calculated or opened will have a correction applied to its latitude to convert it from geographic to geocentric. In this case the chart’s latitude will de displayed with a ‘(G)’ following it. If the ‘(G)’ is absent, then the chart has been calculated without applying this correction.

•Apply Parallax Correction to Moon - Traditionally, the planet’s positions are calculated as if they were observed from the centre of earth. 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. 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. (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 Writer. For example, an altitude of 3000 metres makes a maximum difference of only 2 to 3 seconds of arc in the moon’s position, and it is usually less than this.) The chart display indicates when this correction has been applied by inserting the text “Parallax Moon” at the end of the chart details.