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Lunar Parallax



When you refer to the Moon's declination north or south of the Earth's equator (or the celestial equator, which an extension of the Earth's equator into outer space), you pretty much have to talk about geocentric declination, unless you're talking about a specific place on the Earth's surface. Geocentric declination is the angular distance of the Moon from the equator as seen from the center of the Earth. But as viewed from the Earth's surface (topocentric declination), the Moon's position relative to the background stars differs from place to place. Topocentric declination varies because of something called lunar parallax.

Looking at the illustration, the Moon is at a geocentric declination of 29 and 3/4 degrees north of the equator. (I had meant to write 28 and 3/4 degrees, but no matter. Many thousand years ago, when the inclination of the Earth's axis was greater, the Moon might haved crawled this far north.) The three people in the drawing are residing upon same meridian of longitude, with the Moon and star at zenith for the person at 29 and 3/4 degrees north latitude.

The star, like the Moon, is at a geocentric declination of 29 and 3/4 degrees north. But since the star is millions (if not billions) of times farther away than the Moon, the star -- unlike the Moon -- displays no discernible parallax. As seen from 60 degrees north latitude, the moon appears south of this star -- or less than 29 and 3/4 degrees north of the celestial equator. As seen from 0 degrees latitude (the Earth's equator), the Moon appears to be north of this star -- or more than 29 and 3/4 degrees north of the celestial equator.

Though the Sun and planets also exhibit parallax, their parallaxes are so small that they are extremely hard to detect and measure. Since the Moon is relatively close to Earth, its parallax readily betrays itself. That is why astronomers in centuries B.C. were able to determine the distance and the size of the Moon with reasonably good precision.

Keep in mind, if the illustration were drawn to scale, the Moon would be about 60 times the distance of the Earth's radius away from us. As seen at 60 degrees north latitude, the center of the Moon would be about 1/2 degree south of the star, whereas from the equator (0 degrees latitude), the center of the Moon would be about 1/2 degree north of the star. At 29 and 3/4 degrees north latitude, the star wouldn't be visible, because the Moon would be occulting (or covering over) the star.

The Moon's angular diameter is equal to about 1/2 degree. The Moon's geocentric and topocentric positions for your area are available at the U.S. Naval Observatory web site.

copyright 2006 by Bruce McClure

More Parallax Articles:

Cassini and Richer team up to find Mars' parallax in 1672
Wilhelm Struve measures the parallax of the star Vega in 1836

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