This Saturday evening, March 4, 2017, the First Quarter Moon passes in front of a bright red-orange star by the name of Aldebaran. Visible from across most of the United States, the sight of a bright star disappearing behind the Moon adds to the fascination of star watchers.
The Moon will be situated to the upper right of the famed constellation Orion, which has its own bright, fiery red-orange star, Betelgeuse. Aldebaran is in the neighboring constellation Taurus the Bull.
One can say that the star is undergoing a “total stellar eclipse.” Actually, astronomers use another term, “occultation” when it involves night sky stars.
In its orbit around the Earth, the Moon “occults” (let’s say “cover ups”) hundreds of stars you can watch with a small telescope. Truthfully, the Moon covers up not only hundreds, and not even merely thousands, millions or billions, but an untold number of stars as far as the distant reaches of the Universe.
The Moon’s glare, however, keeps us from seeing only the brighter stars, ones that are easily seen in a small telescope. On a rare occasion, the Moon will pass over a bright star, one you can witness with unaided eyes.
The Moon’s path around the sky closely follows the “ecliptic,” the imaginary line the Sun appears to follow as seen from Earth. Similarly, all of the main planets from Mercury to Neptune follow close to this line. On occasion, the Moon will go in front of other bright stars of at least first magnitude, including Castor, Pollux, Regulus, Spica, and Antares. The bright planets also get their turn of being hidden by the Moon passing in front.
The Moon will take about 20 minutes to an hour to pass in front of Aldebaran, depending on where you are. Some sample locations include New York City where Aldebaran disappears at 11:10 p.m. EST and reappears at 11:31 p.m.; from St. Louis, Missouri, Aldebaran disappear at 9:52 p.m. CST and reappears at 10:42 p.m.
While you won’t need a telescope, binoculars will greatly enhance your view. From the Eastern Time Zone, start looking between 10:30 and 11 p.m. You will see the bright red star to the left of the darkened portion of the Moon. As you keep looking, the Moon will creep up and the star will suddenly vanish behind the dark lunar limb. It’s a little harder to see it reappear on the bright side because of the glare but soon the star will be nicely placed just to the right.
Many amateur astronomers follow occultations and make accurate timings. They then report what they see to the International Occultation Timing Association (IOTA). Occultations are predicted far in advance. With a telescope you can appreciate how airless the Moon really is, since the star does not dim as it is covered up.
Grazes are very interesting, where the Moon’s limb skims a star; the star can be seen blinking off behind mountains and craters on the Moon.
Careful telescopic observation of stars as they are occulted by asteroids yield important information about the latter’s size and shape.
Keep looking up.
— Peter Becker is Managing Editor at The News Eagle in Hawley, Pa. Notes are welcome at email@example.com. Please mention in what newspaper or website you read this column.