Parallax arises from the different points from which which two observers view an object. A simple example of parallax in our everyday lives provides an easy way to understand what parallax is.
An example of parallax is this: Two people are standing 30 feet apart and are looking at the same object 50 feet away. When each of the two people points to the object, an imaginary line drawn from person A to the object and an imaginary line drawn from person B to the object are not parallel. The object is in a different place relative to the background.
Measuring parallax: The closer the object is relative to the distance between the people, the greater is the angle of the two yellow lines in the diagram where they meet at the object being viewed. This angle is a measure of parallax. As the object is moved farther away, parallax decreases and at an infinite distance becomes zero. Note, however, that size of parallax depends on two things: the distance by which the two observers are separated and the distance of the object being viewed. Note that given the distance between two people and the angle of parallax, the distance of the object can be determiend so parallax can be used to determine the distance of an object. To do this, we need two points of viewing the object and a background to reference where the object is from these two points of view.
There are two basic kinds of parallax in astronomy that are relevant to astrology, and I describe them below:
- Stellar Parallax: Astrologers often tend to think of fixed stars as having a given position in a particular year. In the tropical zodiac the only motion of the fixed stars that we usually consider in astrology is the precession of the fixed star forward in the tropical zodiac by an amount of 1 degree in approximately 72 years. However, stars also have a motion of their own, known as their proper motion, but the proper motion of a star is very slow. Proper motion is not completely negligible, however. The star with the fastest proper motion is Barnard's star and it moves more than 1 minute of arc every 6 years. This 1 minute of arc, however, is its direct motion and measured along any given celestial plane is likely to be less than this. However, our topic in this article is parallax and not proper motion. As the Earth revolves around the Sun, the fixed stars are being viewed from the Earth at a different point in space. However, the fixed stars are so far away, that this parallax results in a difference of less than 1 second of arc in the position of the fixed star! This very small amount can be disregarded for astrological purposes, so stellar parallax is not of great importance to astrologers.
- Lunar Parallax: If I look at the Moon right now where I am in Florida and someone half way around the world looks at the Moon (assuming the Moon is above the horizon so you physically see it but even if not physically observable because below the horizon the same principle applies), the Moon is seen as being about 2 degrees different in its zodiac position by the two of us. Even though the circumference of the Earth may not seem great on a celestial scale, it is significant in relationship to viewing the Moon because the Moon is relatively very close to Earth as compared to other celestial objects. The Moon position normally used in astrology is based on where the Moon is from the point of view of the center of the Earth, and is therefore a "compromise" position so that the maximum difference of the parallax-corrected Moon position from the usual position that astrologers use is about one degree. You can approximate lunar parallax by measuring the angle of the Moon from the MC-IC axis. When the Moon is conjunct the MC or IC parallax is near zero. When the Moon is square the MC and the Moon is on the east side, i.e. near the Ascendant, then the parallax-corrected Moon approaches about one degree ahead of the usual Moon position. When the Moon is square the MC and the Moon is on the west side, i.e. near the Descendant, then the parallax-corrected Moon approaches about one degree behind the usual Moon position. The parallax-corrected Moon is therefore, you might say, "lowered" in the chart wheel. You can also calculate parallax-corrected positions of the Sun and the planets, but the difference between the parallax-corrected positions and the usual positions is very small (less than one minute of arc).
Parallax is a big issue. It is possible that astrologers are using an incorrect position of the Moon by not using the parallax-corrected Moon position. In research that I have done in the past amazingly the parallax-corrected Moon seemed to not work as well as the plain old Moon and this surprised me. I suspected that the parallax-corrected Moon would be more accurate because it is the position of the Moon from the person's point of view. In a birth chart it is where the Moon is if one looks up at the night sky. The research that I have done or that anyone else has done up to this point in time is not definitive. There are no research studies that clearly and unambiguously support astrological theories, so we are still at a stage of building models, gathering anecdotal evidence, and doing exploratory research.
The parallax-corrected Moon affects everything that an astrologer may interpret: the birth chart, forecasts, AstroMaps, etc. The parallax-corrected Moon is simply at a different position than the usual Moon, and, as noted above, is up to about one degree different in its placement.
The elevation of a place from sea level also has an effect on parallax. At the top of a mountain parallax is greater than at sea level. The difference in elevation can change the parallax-corrected position of the Moon by up to a few minutes of arc.
Note that there are other important issues that are also regarded as unimportant by astrologers, such as refraction of light and the speed of light, for example. These topics can be addressed in a separate article in the future.