Can an Astronomical New Moon Be Seen by "Earthshine?"

Some calendar systems use an astronomical new moon or conjunction to begin each month. This event occurs when the moon is closest to the straight line between centers of the Earth and Sun. Today, this event is possible to calculate mathematically with a high degree of precision. Historical evidence indicates that this calculation could not be performed until about 330 BC.

"The months beginning with the conjunction will be called exact lunar months or conjunction months. These months are a theoretical construction; they could not be used in practice in classical times, because before Kallippos (330 B.C.) astronomers were not able to predict the true conjunction." (B.L. van der Waerden, The Journal of Hellenic Studies, vol 80, 1960, p. 169, "Greek Astronmical Calendars and Their Relation to the Athenian Civil Calendar").

There can be up to three successive nights between the last old crescent and the first new crescent (see reference, above). Without the ability to calculate the true conjunction, there is no way a person could know which day is the astronomical new moon. How then, can anyone justify using a calculated calendar system today that we know could not have been used by the righteous men of the Bible? The answer that has sometimes been put forth is "earthshine." Is it possible to actually observe the astronomical new moon by earthshine?

Because earthshine is not light from the sun to the moon to the observer on earth, but instead is light from the sun to the earth to the moon to the observer on earth (a double reflection from the sun rather than a single reflection from the sun), it is observed as a faint dark blue-gray color rather than white. Earthshine does not have the shape of a crescent, but instead fills out the remaining part of a circle beyond the crescent. Earthshine together with the crescent make a full circle. As the crescent is being observed and twilight grows darker, it becomes easier to see earthshine, but sometimes earthshine will not be seen at all when the crescent is seen, especially when the moon sets during the early part of twilight. This is my personal observation, yet pictures and brief discussions in astronomy books corroborate my sightings.

Claims have been made that ancient observers could see earthshine during the astronomical new moon (conjunction) due to the lack of air pollution and the lack of lights from cities. (My comment for now is that if a person travels far from city lights, light effect is canceled, so the only issue of difference between today and anciently is air pollution.)

In order to evaluate claims about observing earthshine during the conjunction, one must first note that when the time of conjunction arrives, the time in Jerusalem may be night, daytime, or twilight. When it is night in Jerusalem during conjunction, the moon is between (although not exactly "between" except during an eclipse) the earth and sun, and Jerusalem is hidden in darkness on the side of the earth not facing the sun and moon, so it is certainly not even imaginable that someone in Jerusalem could see earthshine. When it is daytime in Jerusalem during conjunction (assuming it is not a total solar eclipse, which is very rare), the angle from the sun to Jerusalem to the moon is less than 6 degrees so the brightness of the visible sun, being so close to the moon, will surely block out the faint earthshine. The cases of conjunction during night and daytime from Jerusalem constitute the vast majority of times of conjunction.

The last choice to consider is twilight from Jerusalem during conjunction. Here the circle of the sun is always slightly below the horizon and only some of the time the moon is slightly above the horizon, yet the angular separation between sun and moon is less than 6 degrees. The moon's crescent is too small to see at this time because the world's record for a documented shortest time for seeing a new crescent after conjunction with the unaided human eye is 15.53 hours (Sky and Telescope, vol. 92, December 1996, page 104; since the observation was made near the Mauna Kea Observatory in Hawaii, it was at a very high altitude and hence not likely to have been seen near sea level). The difficulty in seeing the new crescent at conjunction has nothing to do with air pollution, but instead the difficulty involves too thin a bent hair of reflected light to be detectable by the human eye with the twilight glare from the sun still significant. It is this same twilight glare that should hinder seeing the full circle of faint earthshine. While lack of air pollution helps one to see earthshine, this same lack of air pollution also increases twilight glare which hinders seeing earthshine. These two effects of lack of pollution are likely to cancel each other, unless we are near a major center of air pollution. Thus I am not convinced that ancient observations of earthshine would have been easier when compared to places of today that are far from city lights and major centers of pollution. It should be emphasized that this discussion is only relevant to the time of conjunction in Jerusalem when it is early twilight; at other times of conjunction in Jerusalem it is not imaginable to consider seeing earthshine (except during a total solar eclipse).

I have examined about two dozen astronomy books concerning earthshine, and they all take it for granted that the observer is looking for earthshine at a time when the crescent is already visible. In Sky and Telescope, volume 42, August 1971, page 78, Joseph Ashbrook wrote the following below a photograph of a crescent moon showing earthshine, "A 29-hour-old crescent moon is setting behind trees a mile away in this photograph taken on January 17, 1961, at Mount Pinos, California, by Alan McClure of Los Angeles. When the moon is much nearer new [i.e, the conjunction], naked-eye viewers never see the earthshine because the twilight glow is too bright." Joseph Ashbrook authored the regular column in Sky and Telescope titled "Astronomical Scrapbook" until his death in 1980. From his statement it is clear that he did not believe that earthshine was visible at or very near conjunction.

During a total solar eclipse, which is only observable from a small portion of the earth's surface, it is possible to see a conjunction by earthshine (article "Eclipse Earthshine" by Bradley E. Schaefer, Publications of the Astronomical Society of the Pacific, vol. 103, pages 315-316, March 1991).

In summary, there are no documented cases of anyone seeing a conjunction by earthshine without an eclipse. Even if atmospheric conditions were at one time better, Earthshine would not have been a reliable way to determine the beginning of months. —Herb Solinsky


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