Author's Note: This article first appeared in the Special Moon Edition of Welcome to Planet Earth Magazine (Libra/Scorpio, 1996). It provides background information on occultations of the moon. A companion article on occultations title "Hot Fun in the Summertime" also appears on this site.
The Astronomy of Occultations
A Primer for Astrologers
by Gloria Lalumia
Transits, eclipses, and occultations all involve a body which moves between two others, and, as a result, obscures some or all of the light of one of the bodies as seen from the other. A transit, in purely astronomical terms, occurs when, as observed from Earth, a relatively small body crosses in front of a larger body. This movement usually involves the Sun or a planet. For example, Mercury and Venus transit the Sun, tracing a very small area of shadow across the Sun's face. Similarly, a satellite may transit its planet.

In the broadest sense, the term occultation refers to the complete or partial obscuring of an astronomical object by another. A solar eclipse, therefore, is an example of an occultation, and an occultation may be considered to be an eclipse event. The difference in terminology reflects the difference in the apparent sizes of the bodies involved. Eclipses occur with bodies which appear to be similar in size; solar and lunar eclipses, eclipses of satellites of a planet as the satellites move into the planet's shadow, and the periodic eclipsing of binary stars by one another as observed from Earth are examples of this configuration.

An occultation, in more specific astronomical terms, occurs when there is complete or partial obscuration of a body by another of larger apparent diameter. Examples include the disappearance of a planetary satellite or space probe behind a body in the solar system, but the term is most often used by astrononers to describe the disappearance of a star, nebula, or planet behind the Moon. A lunar occultation may be considered a small scale eclipse event, not as spectacular as a solar or lunar eclipse, but an event which occurs much more frequently.

RESEARCH USING ECLIPSES AND TRANSITS

The observance of all three types of alignments has led to major astronomical discoveries. For example, transits of Mercury and Venus across the Sun's face have been used to accurately determine the diameter of each planet by measuring the amount of sunlight that each blocks. The rare Venus transits have been useful in mapping distances in the solar system.

In 1679 Edmund Halley hypothesized that using the parallax--the apparent difference in direction of an object when observed in different positions--astronomers could find the distance of Venus from the Sun by using relatively simple calculation and observations of the times of contacts between Venus and the Sun from locations widely spaced across the Earth. Many of the observations during the 1761 transit were not successful, but in 1769 observations were made in such far-flung locations as the South Pacific, which had not been attempted before and where there was a good chance of clear skies. Captain James Cook, the famous navigator and explorer, who had also observed solar eclipses, used his experience with parallax observation gained from years of marine surveying to complete the vital Pacific observations for the Royal Society.

The study of solar eclipses has contributed a great deal of knowledge about the chronosphere and corona, the thin external layers of the Sun that are usually lost in the glare of the Sun's shining surface. Until the recent invention of the coronagraph, which allows studies of these layers without waiting for an eclipse to occur, the Moon served to screen out the direct rays from the Sun's surface, thereby allowing observations. This type of study resulted in the discovery of an unknown gas in 1868, which wasn't identified on Earth until 1895 as helium.

The exact timing of the contacts between Moon and Sun is also important for measuring the Earth, because these moments depend on where the observations take place, and therefore, on the shape of the Earth. Solar eclipse studies have also been used to verify Einstein's prediction that starlight is deflected by a gravitational field such as the Sun's, thus making a star appear slightly further from the Sun than it actually is.

Lunar eclipses have enabled astronomers to gain more knowledge of the Moon's surface. As the Moon is hidden in the Earth's shadow, the lunar surface cools. Studies of this cooling have resulted in a better understanding of how the surface conducts heat and how the lunar soil is constructed.

DISCOVERIES MADE BY OBSERVING OCCULTATIONS

Observations of occultations by planets have lead to some amazing recent discoveries. In 1977, while astronomers were observing an occultation of a star by Uranus, they noticed its brightness dropped in distinct pulses. Voyager 2 confirmed in 1986 that these pulses were caused by a ring system around Uranus. Then, in 1984, while Neptune was occulting a star, scientists were able to observe what appeared to be an incomplete ring system, which was again confirmed by Voyager 2.

But occultations by the Moon are by far the most familiar to skywatchers. Although the Moon only occasionally occults a planet, it frequently occults a fixed star. Because the Moon moves quickly most stars disappear almost instantaneously (which indicates the lack of atmosphere on the Moon), while the disappearance (or reappearance) of the giant planet Jupiter takes more than one minute. A minor planet takes under one second to disappear, while even a giant star like Aldebaran or Antares disappears at a rate ten times faster than that of a minor planet!

Observing the Moon's occultations of stars has also resulted in the discovery of binary star systems (double stars) which previously had been missed by telescopes. In these observations, stars appear to fade gradually, or actually in two stages, rather than instantly, thus indicating that there are actually two stars, which may not be clearly evident by telescopic viewing.

Observing the Moon's occultations of stars yields basic data on the disturbances of the Moon's orbit and on the irregularities in the Earth's rotation. During this century the realization that the Earth's rate of rotation is not constant has led to the concept of ephemeris time, a method of time determination which is based on the orbital motion of the Sun, Moon, and planets (and given in ephemeris tables) as opposed to the unsatisfactory earlier method of time measurement using the Earth's axial rotation.

Occasionally the Moon will just "skim" a star. This event is called a "grazing occultation." During these occurrences the star appears to move across the limb (edge) of the Moon, disappearing behind lunar mountains and then reappearing through the lunar valleys. Because of the irregularity of the lunar surface, the star may disappear and reappear several times. Where an observer is positioned is critical because small differences in location (even a few yards) may mean the difference between seeing the occultation or missing it completely. Because it is hard to make an exact prediction, grazes are usually observed by a team of observers who space themselves 100 yards apart along a line, ideally at right angles to the predicted graze path. Observers can obtain a series of timings as the occultation occurs, thus allowing a detailed profile of the Moon's edge and providing a highly accurate knowledge of the Moon's position at that moment in time. Before the start of manned lunar exploration, this was the only way to determine the topography along the Moon's visible edge.

THE ASTRONOMY OF THE MOON'S OCCULTATIONS OF FIXED STARS

The Moon appears to travel more slowly across the sky than the stars, appearing to drift eastward in relation to the stars, though its general direction is actually westward. Like the Sun and stars, the Moon rises east and sets west, as a result of the Earth's rotation from west to east. But the Moon also revolves around the Earth from west to east, which visually reduces the effect of the Earth's own rotation on the Moon. The result is that the Moon appears to wander across the sky more slowly than the Sun and stars, which makes its schedule erratic. (The daily retardation of moonrise and moonset by approximately 50 minutes each day brings the Moon out and in step with the Sun over a period of a month, or more exactly, 29 1/2 days.)

In the process of the Moon's apparent eastward movement against the sky, the stars catch up with the Moon (or the Moon falls behind the stars) and are occulted (hidden) by the Moon. An occulted star always disappears at the Moon's eastern limb and reappears at the western. This process is often visible with the naked eye if one watches when a star is near and to the west of the Moon and the Moon is not full or close to fullness.

The Moon moves quickly, about its own diameter in 1 hour. Occultation is best observed at the dark limb of the Moon. If the Moon and star are at the proper positions, the star seems to creep near the Moon, and when it reaches the Moon's unlit edge (in the case of a waxing Moon, the unlit edge is the first to cover the star) the star seems to disappear as though a switch were turned off. After an hour at most, it pops up on the opposite side of the Moon just as suddenly as it disappeared. With a waning Moon, the lighted half is the first to cover the star, while a full Moon is too bright to create the "switched off" effect.

The inclination of the Moon's orbit is a little more than 5 degrees off the plane of the ecliptic (the apparent path of the Sun). Due to the gravitational attraction of the Sun, the Moon's orbit moves backwards along the ecliptic. This movement is traced by the Nodes, the point where the Moon's orbit intersects the ecliptic, which take 18.6 years to complete one cycle around the ecliptic. As a result, all the stars in a belt of 10 degrees around the ecliptic are occulted at some time during a period of about 19 years. Among these are the brightest stars in the sky including the Pleiades star cluster, the Crab Nebula, and the four "Royal Stars" -- Aldebaran, Regulus, Spica, Antares.

These first magnitude (brightest) stars are on or near the ecliptic. Of the four Royal Stars, Spica is the furthest off the ecliptic while Regulus lies practically on it. Aldebaran and Antares (halfway around the sky from Aldebaran) are both south of the ecliptic, with Antares being slightly further from the ecliptic than Aldebaran, which is 6 degrees off. Consequently, because of variations in the Moon's movement and the backward movement of the Nodes around the ecliptic, the 5 degree tilt of the Moon's orbit causes the Moon to occult Aldebaran every 18 to 19 years, staying in the right geometry for such occultations to occur over the span of about four years. (Antares also is occulted on a regular 18-19 year cycle.) The last series of Aldebaran occultations took place from 1978 to 1981; a new series will begin in August of this year. A series of occultations of Antares which began in 1986 ended in 1991; the next series will start in 2005. Because Spica and Regulus, like the other stars within the 10 degree belt around the ecliptic, are not as close to the Moon's orbital path as Aldebaran and Antares are, they are not occulted with the same regularity or in series that last as long as those that occur with Aldebaran or Antares.

FINDING OUT MORE ABOUT LUNAR OCCULTATIONS

The most comprehensive source of information on occultations by the moon to planets and stars is Astronomical Tables by Jean Meeus published by Willman-Bell. It is available at major university libraries, although many do not have the latest edition which lists the dates of all occultations from 1980 through 2020. The tables also include the locations where each occultation will be visible and the nearest hour of ephemeris time when the least angular distance between the center of the Moon and the body occurs.

Two of the more accessible general reference sources of basic information on occultations are almanacs. The World Almanac and Book of Facts list of "Planetary Configurations" highlights some of the occultations which occur during the year, as does the Old Farmers Almanac. Neither details the areas of visibility of the events as the Meeus book does, although the Farmers Almanac does note if an occultation is not visible in the U.S. The World Almanac information is unique, however, in that it notes the apparent location of the Moon in relation to the body it is occulting, i.e. using a range of 1 degree, it specifies, for example, that the Moon will "pass 0.9 degrees North" as it occults a particular body on a given date.

A general search of the Web using the key words "eclipses and occultations" will send you to sites with many potentially useful links. Sky Online's "What's Up in the Sky" website (from Sky Publishing, the publishers of Sky and Telescope) provides a list of celestial events, although it is not as complete as the almanac list (http://www.skypub.com/whatsup/hmtl#). The International Occultation Timing Association Home Page Master Index offers several articles which provide basic information on occultations (http://www.sky.net/~robinson/iotandx.htm). Finally, the Smithsonian Astrophysical Laboratory's Telescope Date Center links to "Stellar Occultations," which focuses on occultation of stars by planets (http://tdc-www.harvard.edu/TDC.html).

Clearly, more research needs to be done by astrologers to fully understand the implications of occultations by the moon, particularly with regard to the fixed stars and planets. Perhaps, like solar and lunar eclipses and transits, the moon's occultations will enhance our ability to understand the events occurring in the world and in our lives. As astrologers, we may find that these common celestial events may offer new approaches to interpretation if we take the time for closer examination. Now may be the time...

BIBILIOGRAPHY
Alter, Dinsmore et al. Pictorial Astronomy. Thomas Y. Crowell Company, New York, 1963.
The Audubon Society Field Guide to the Night Sky. Alfred A. Knopf, New York, 1991.
Bondi, Victor, ed. American Decades 1970-1979; 1980-1989. Gale Research, Detroit, MI, 1995.
Hough, Richard. Captain James Cook, A Biography. W.W. Norton & Company, New York, 1994.
Illingworth, Valerie, ed. Facts on File Dictionary of Astronomy. 3rd edition, Facts on File, Inc., New York, 1994.
Meeus, Jean. Astronomical Table of the Sun, Moon & Planets. Willmann- Bell, Inc., Richmond, VA, 1983; 2nd edition, 1995.
Ronan, Colin, ed. The Golden Book of Astronomy. Golden Press, New York, 1984.
Roth, Gunter-Dietmar, ed. Compendium of Practical Astronomy. Vol. 2., Springer-Verlag, Berlin, 1994. Chapter 17, W.D. Heinz, "Occultations of Stars by the Moon".
Watters, Thomas R. Planets: A Smithsonian Guide. Macmillan-USA, New York, 1995.
Information Please Almanac
World Almanac Book of Facts
Old Farmer's Almanac.

Gloria Lalumia lectures and writes on astrological topics and currently serves on the Board of Directors of the Astrological Society of Princeton, N.J. grlnj@aosi.com

Copyright 1996 by Gloria R. Lalumia. All Rights Reserved. Republished here by permission of the author.

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