Observer's Notebook for May 1999

A Stellar Disappearing Act

by John Rummel

Most casual watchers of the sky know that the moon moves each night relative to the background stars. In fact, each night, the moon moves a bit to the left, or east, as it orbits around the earth. This movement is also responsible the monthly changes of the moon's phase. Nothing illustrates this gradual drift better than when the moon eclipses or "occults" another object.

The best example of this phenomenon is a solar eclipse, when the moon passes directly between the Earth and the Sun. Though the Sun is about 400 times larger than the moon, it is also about 400 times more distant, making the two disks appear to be almost the same size. Thus, the moon is able to just cover the disk of the sun, making a total solar eclipse one of the rarest and most sought after of all astronomical phenomenon, and consequently, one that most people have never seen (the last solar eclipse to occur within reasonable distance of Madison was an annular eclipse on May 10, 1994; the next will be total, on August 21, 2017).

During its travel across the sky, the moon eclipses other objects too. When any heavenly body eclipses another, astronomers call it an occultation. The easiest kind of occultations to see are those of the moon eclipsing planets or bright stars. Since the moon follows the sun's path (the ecliptic) pretty closely, there are only four 1st magnitude stars that can be occulted by the moon; Aldebaran in Taurus, Spica in Virgo, Antares in Scorpius, and this month, Regulus in Leo the lion.

Since the moon has no atmosphere, occultations of stars are dramatic events to view - the light from the star may seem to flatten momentarily, then winks off in an instant, there one moment, gone the next. It is especially rewarding to view such an occultation when the moon's darkened edge occults the star, as is the case with Regulus this month. Robert Burnham, in his classic Celestial Handbook, notes that "the disappearance of the star on such occasions is startlingly abrupt, one of the most nearly instantaneous phenomena which the eye can observe."

Observer's Challenge for May, 1999

On Friday evening, May 21, as the sun sets, the first quarter moon (almost a perfect "half" moon) will be well above the western horizon. The occultation will actually begin at about 11 pm CDT, so go out to take your first look at about 10 pm. By this time, Regulus will be very close to the moon's left edge, the shadowed portion. Because this region is very dark, it is difficult to tell exactly when the star will pass behind the moon, so as 11:00 approaches, your best bet is to glue your eyes to the eyepieces and try to avoid blinking!

Look closely at Regulus through a small telescope and you'll notice that it is a binary - it has a small companion star nearby. The companion will disappear 5-7 minutes before Regulus. If you have access to binoculars on a tripod, or a small telescope, catching the companion's disappearance will be a cinch.

Regulus is probably about 5 times the diameter of our own sun, but its great distance reduces it (as with all stars) to a mere point of light. Though our moon is much, much smaller, it can easily cover Regulus because it is so close by comparison.

To appreciate the contrast in distances, consider the following: light travels at about 186,200 miles every second. The light leaving our sun takes about 8 minutes and 20 seconds to reach the moon, and then after bouncing off the moon, takes 1.28 seconds to reach your eyes. By way of contrast, the light reaching your eyes from Regulus began its journey in 1914 when World War I was just getting started in Europe. Your grandparents probably weren't even born yet.

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