Thin Crescent Moons

by John P. Pratt
©1998 by John P. Pratt. All rights Reserved.

Astronomy Home Page

It can be a really fun and almost magical experience to see a extremely thin crescent moon. We have all seen God's Fingernail, as my mother used to call it: the thin crescent moon seen in the west shortly after sunset, a day or so after the new moon. For morning people, a thin crescent can also be seen in the east just before sunrise, a day or two before the new moon. It can be a beautiful sight, especially when the dark part of the moon is illuminated by "earthshine," which is light reflected off the earth that in turn illuminates the dark part of the moon.

So what is "almost magical" about a really thin crescent moon? It is that it can almost mysteriously disappear very shortly after it is seen. Poof, and it's gone, as if a magician made it disappear! In the evening, this happens because the sun must set far enough for the sky to become just dark enough for the very thin crescent to appear. Then, sometimes only minutes later, the moon sets enough to disappear into the hazier lower sky near the horizon. Only once in my life have I had this experience where it could only be seen for a matter of minutes, but it is a striking phenomenon. Sometimes this occurs on the very day of the new moon, and sometimes the day after, depending on the exact conditions. Then, only one day after that, the usual crescent is obvious for everyone to see in a clear sky, and it stays up for about an hour. Remember that the moon rises and sets nearly an hour (50 minutes on the average) every night.

Similar magic happens in the morning. Once I was driving to work and I chanced to see an extremely thin crescent in the eastern morning sky just before sunrise. I stopped the car to get a better look, and when I hunted for it, it had disappeared in the light of the rising sun. One might think he had imagined the sighting if unfamiliar with the moon's motion in the sky.

Although trying to see the thin crescent is fun to do, it was much more serious for several ancient peoples because their calendar depended on such observations. An observational lunisolar calendar was used by most ancient people before the solar calendar became popular. "Lunisolar" means that the month was a lunar month, which began at the new moon; the "solar" part meant that the year was based on the length of the sun's cycle of four seasons.

For example, at the time of Christ, the first day of a month began after the sunset on which the very thin crescent of the new moon could first be observed. If two reliable witnesses saw the thin crescent, and their testimony passed the scrutiny of the Sanhedrin, who knew where and when and how it should appear, then the new month was declared. In order to give people an incentive to look, a dinner was sometimes held for those who would at least try to see it. That might not be a bad idea to do today, at the beginning of a star party.