Late tonight and early tomorrow morning, the waning gibbous moon will occult Aldebaran, the bloodshot eye of Taurus the Bull. This wonderfully orange-red giant star is the brightest in the constellation and 14th brightest in the night sky. An occultation occurs when the moon, moving eastward in its orbit around the Earth, slides over and covers a bright star in its path.
Because the moon is only ½° across, depending where you live, it may glide north or south of Aldebaran, just missing the star. This shift in the position of object depending on your perspective is called parallax. The easiest way to see parallax in action is to hold your fist at arm’s length and stick up an index finger. Now close one eye and note the position of your finger against the more distant backdrop. Then close that eye and open the other. You’ll see your finger jump from side to side. Earth’s a big place and the moon is close — just like your finger — so it appears in a slightly different position against the starry backdrop depending where on the planet you view it from.
Across the northwestern section of the U.S., the moon will just miss covering the star. Instead, those folks will see a very close conjunction of the two celestial bodies with Aldebaran just missing the northern edge of the moon. But across a broad swath of the East and South and into Central America, Aldebaran will be completely hidden for a time, up to about an hour, before reappearing on the opposite (western) side of the moon.
The most interesting place to watch the event is along the so-called graze line, a narrow path several hundred yards wide. Here, the rugged, mountainous edge of the moon doesn’t quite cover the star but “grazes” it instead. Observers there can watch Aldebaran flash in and out of view in binoculars and small telescopes as it’s alternatively hidden and exposed to view by a succession of peaks and valleys. By good fortune, the graze line passes directly through or very close to four large cities: Minneapolis, Denver, Las Vegas and Los Angeles. For details of the exact location of the graze line, click on the International Occultation Timing Association (IOTA) Aldebaran graze path page and scroll to near the bottom of the page for links to maps.
The moon will be in waning gibbous phase with its bright, sunlight edge moving east (left) in the sky, so Aldebaran will disappear at the bright edge, be covered for a time and then reappear at the dark, shaded western limb of the moon. This is a wonderful and easy-to-view event in binoculars or any telescope. Because of the brightness of the moon, it will be difficult to see with the naked eye. The only downside is the time. For East Coast skywatchers, the graze/occultation/conjunction happens around 2 a.m. Wednesday (Oct. 19). That’s 1 a.m. Central time, midnight Mountain and 11 p.m. Pacific.
To find exactly when Aldebaran will disappear and reappear for your location, head over to IOTA’s Aldebaran occultation page where you’ll find times listed for over 1,000 cities. The times are given in UT or Universal Time. Be sure to subtract 4 hours for Eastern, 5 hours for Central, 6 hours for Mountain and 7 hour for Pacific times.
Before Aldebaran is occulted, observers in the eastern half of the U.S.can watch the 4th magnitude double star Theta-1, Theta-2 Tauri reappear post-occultation along the moon’s west-facing limb around 10 p.m. local time. Watch for both stars in the wide pair to suddenly pop into view one after the other.
The moon and Aldebaran are currently in a 5-year series of occultations; this is one of the better ones. I hope you’ll get to see it!