Abstract: Parallax is explained astronomically, and its significance is discussed. Parallax correction for the Moon's position in horoscopes is recommended since astrology begins with accurate astronomical observation.

If you hold up a finger at arm's length, look at it with your left eye only, then both eyes, and then with just your right eye, it will appear to move to the left with respect to a faraway background. This apparent change in position is known as "parallax." It is one of the methods used to measure the distance of stars.

We are all on the surface of the Earth-not at its center, fortunately. When we observe the sky, we take measurements in angles. Stars and planets are so far away that only sophisticated equipment can measure a difference in angle from one side of the Earth to the other. The Earth's maximum diameter is 12,756 km, which is very tiny compared to the distance to the Sun (149,597,870,960 km), or even the minimum distance to our closest planet Venus (38,050,900 km). The difference between looking at a planet from one side of Earth or the other is only a second or two of arc, which is usually astrologically negligible. The Moon, however, is so close to the Earth that its zodiacal position depends slightly where we are on the Earth's surface.

For example, an astronomer in Singapore (point A) might measure the Moon's position to be 11° Aries. At the exact same moment, an observer in Edounga, Africa (point B), would measure the Moon to be at 12 Aries°. Simultaneously, an observer in La Tagua, Columbia (point C) would measure the Moon to be at 13° Aries. The Moon is in exactly the same physical position, but three observers measure it at slightly different positions because they are on the surface of the Earth, not at its center. (Fig. 1)

The Moon's position as calculated from an ephemeris would be 12° Aries, because this is the direction from the center of the Earth to the center of the Moon. However, if a star or planet is at 11° Aries, it will be exactly in the same direction as the Moon only if we are in Singapore, not if we are in Edounga or La Tagua.

If the Moon is at the Midheaven (as from point B) or Nadir (point D), no parallax correction applies. If the Moon is 90° away from the Midheaven, when it is rising or setting, the parallax correction can be as much as 1° 0' 37. The maximum correction in the Moon's position due to parallax occurs when the Moon is rising or setting, when it is closest to the Earth at its perigree, when we are on the Equator, and when the Moon's celestial latitude is greatest-in other words, when the Moon is square its Nodes.* The Moon's parallax correction is often quite close to 1°. This can make a difference in its astrological subdivision, its degree, its Nakshatra, or more rarely, its sign or rashi.

Parallax cannot be calculated from readily available tables. When our astronomical data was less precise and computers were yet to be invented, such a small discrepancy in the position of the Moon was ignored. However, we now observe the heavens with greater accuracy and precision than every before. Now we have clocks that automatically update to the correct time-to the second-via satellite, so the times of events are recorded more accurately than in previous centuries. Now we can utilize more accurate longitudes and latitudes than ever before. Now we also have astrological theories that utilize very small arcs of the zodiac, some of them less than one degree. These astrological tools are very exciting.

The Moon is certainly of great astrological importance. It is so close to us that it has already been found to affect us physically, not only psychologically or vibrationally. A billion people follow a lunar calendar, meditations for world peace are held on Full Moon days, and devout Muslims listen anxiously to find out whether to break their month-long holy fast according to whether the crescent Moon is visibly seen by the regional mulla. Newspapers in Calcutta and Bombay report eclipse times and other lunar events according to the actual times they will be seen locally, not when they would be seen from the Earth's center. The Moon's position at birth corresponds with many personality traits and is critical in astrological predictive techniques. The position of the Moon always was, and still is, its actual observed position, not a theoretical position as if we were at the center of the Earth.

It is not possible for parallax correction to be tabulated in an ephemeris because it depends on the exact time of day, our latitude, the Moon's celestial latitude, and the Moon's position in its orbit. However, now that computers can calculate it, we are lax if we don't use parallax correction!

With the help of AstroDataBank software, here are three examples in which the parallax correction to the Moon's position results in the Moon appearing in the 1st house rather than the 12th. Bernadette Peters was the youngest actress to be elected to the Theater Hall of Fame and has had a lengthy and successful career. Without parallax correction, her Moon position is 9° 17' 20 Scorpio in the 12th house. With parallax correction, it is 10° 10' 17 in her first house. Actress and model Farah Fawcett's Moon position without parallax correction is 4° 37' 56 Cancer, in her 12th house; with parallax correction it is 5° 31' 19 in her first house. Astrologer Joy Hunimer, the first president of Kepler College, has her Moon in 1° 29' 5 Sagittarius without parallax correction, which falls in her 12th house. With parallax correction, her Moon is in 2° 10' 57 in her first house. These cases all have accurate birth times (10:45pm AA rating, 3:10pm AA rating, and 6:22pm A rating, respectively). Their biographies are more consonant with traditional interpretations of a 1st house Moon than 12th. Joy's astrological intuition is adequately accounted for by Mercury closely conjunct Ketu and Saturn in the 8th house, and Moon trine to Pluto in the 8th house. Further research regarding small arcs such as parallax correction requires birth data that is extremely accurate, which is now being accumulated thanks to modern technology.

If your astrology software calculates a parallax Moon correction, make sure to verify its accuracy. One professional astrology program had an error in the parallax Moon correction that occasionally caused the Moon's position to jump 180 across the wheel from one second to the next, and then back again later, which obviously did not happen to the real Moon. There was no way to know that the reported Moon's position was severely inaccurate except by checking an ephemeris. It took the company four years to fix this calculation error. Hundreds of people may have this inaccurate calculation in their software without knowing it. (I now use Kepler 7, and have not discovered an inaccuracy in its parallax Moon correction.) Many astrology software programs are now available, some of them free; always verify the accuracy with a trustworthy source.

The formula for the maximum Moon parallax correction is:

max = arctan [Rmax / (dmin @ cos Bmax)] = 1.010382° = 1 0' 37

where: Rmax = maximum radius of Earth (at the Equator, including elevation) = 6,378.135 km

dmin = minimum distance of Moon to Earth (at perigree) = 363,104 km

Bmax = maximum celestial latitude of Moon = 5.1333

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