The geocentric view of a planet is its location as seen from an observer in the center of the Earth. The parallax view is the location as seen from an observer on the surface of the Earth. For most planets, these two locations are virtually the same. The location of the Moon can vary by as much as 1 degree between these two views.
The algorithms used in the following calculations were obtained from the Lunar Parallax Calculator written by Brian Conrad in 1999. His webpage is related to Indian Astrology (sidereal astrology), and uses the Lahiri Ayanamsha.
I modified the routines so that the results are given in terms of Western Astrolgy (zero Aries occurring on the Vernal Equinox). This script will read the date, time, and timezone from your computer. The default location is New York City.
You may need to modify the input parameters before calculating the Lunar parallax. The program will accept any year between 1800 and 2100.
I will compare the answers generated by Astrolog32 1.10 and the Lunar Parallax Calculator. For our test, we will use April 30, 2005 at 10:00 am EST in New York City (74:00:00 W, 40:43:00 N). We will calculate the longitude of the Moon.
|Type of Calculation||Astrolog||Lunar Par Calc|
|Tropical, Non-parallax:||1 Aqu 50 45||1 Aqu 50 12|
|Tropical, Parallax:||N/A||1 Aqu 00 28|
|Sidereal, Non-parallax:||7 Cap 54 51||7 Cap 54 25|
|Sidereal, Parallax:||N/A||7 Cap 04 41|
You will notice that Astrolog32 1.10 and the Lunar Parallax Calculator agree to within one minute of longitude. At this date and time, an observer in NYC would see the Moon at a slightly smaller longitude than that reported by Astrolog32 1.10. This would mean that the transiting Moon would reach the NYC Midheaven a little later than reported by Astrolog32 1.10.