The Moon Occults a Star

You've probably seen illustrations of a crescent moon with a star within its pointed cusps, such as in the children's book P.J. Funnybunny Camps Out. What's wrong with that picture?

Well, consider that the moon is a solid object that is only partially illuminated most times. Complete the circle of the moon, and it becomes more obvious that stars within the moon are not possible.

Shortly after midnight on the opening hour of September 5, you can watch a star pop out from behind the dark part of the moon. As Friday night segues into Saturday morning, the moon occults the star Aldebaran in the constellation of Taurus the Bull. From the perspective of South Bend, IN, the star reappears around 12:38 a.m. EDT, but you'll need a low eastern horizon to see it.

As the moon orbits earth, the moon appears to move from right to left against the background stars for observers in the United States. Every now and then it's going to move in front of a prominent star. Unfortunately, the last quarter moon is still below the eastern horizon late Friday night in South Bend when 1st-magnitude Aldebaran disappears behind the bright leading edge. The moon rises at 12:20 a.m. in South Bend, so it's only two degrees (about four moon diameters) above the eastern horizon when Aldebaran reappears around 12:38 a.m.. When the star pops back into view from behind the dark side, which may be faintly visible from earthshine, it's a fun and obvious sight.

The reddish star Aldebaran is the bull's eye of Taurus. It also is the end of a V of stars that make up the Hyades open star cluster. Prominent above Taurus at this hour is the Pleiades, a tight group of stars known as the Seven Sisters. In Japan the star cluster is known as Subaru (look at the logo of the car company of the same name).

Amateur astronomers, led by the International Occultation Timing Association (IOTA), traditionally timed the duration of occultations to reconstruct the moon's profile. In the past decade modern spacecraft have refined our knowledge of the moon's size and shape. Times for the disappearance and reappearance of Aldebaran for other cities are listed on the IOTA webpage Occultation of ZC692 on 05 Sep 15.

Many children's books have shortcomings in their illustrations of the moons. For some people, finding fault is nit-picking; for others, it's an observation of common errors. I've listed other books with faux phases at Bad Moons Rising.

Image : P.J. Funnybunny Camps Out (Author: Marilyn Sadler Illustrator: Roger Bollen Random House, Inc.)

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