Here’s something very fun and enjoyable to see with a small telescope or even a pair of 50mm binoculars. This Thursday night across North America the crescent moon’s dark, earth-lit edge will cover up to three stars in the familiar V-shaped Hyades star cluster.
Never heard of the Hyades? Its next door neighbor is the Pleiades cluster, the one shaped like a little dipper and better known as the Seven Sisters. At just 153 light years away, the Hyades is the closest star cluster to Earth, one of the reasons it covers a nice-sized chunk of sky and is plainly visible to the naked eye. The bright orange giant Aldebaran helps to complete the cluster’s nifty V-shape, but isn’t a true member; the star simply happens to lie along the same line of sight.
The moon passes near the Hyades every month but only passes through the cluster for a six-year period every 18.6 years, the time it takes the moon’s orbit to precess or cycle once around the ecliptic. This last happened from 1995 to 2001. We begin a new cycle this year.
The sun’s gravitational pull on the moon forces its orbit to slowly rotate westward. Combined with the 5-degree tilt of the lunar orbit, the moon’s track across the zodiac constellations varies continuously to the attentive observer over an 18.6 year cycle. During part of that cycle, it crosses the Hyades; during another part it swings north and misses them.
Thursday night the dark edge of the moon will cover one, two or even three bright Hyades depending where you live. Eastern and central U.S. and Canadian observers will see the moon blot out Delta 1 followed by Delta 2 for observers in the northern U.S. and Canada. The final bright star, Delta 3, slides behind the moon for much of the central and western U.S. and Canada.
Because the moon is close to the Earth compared to the planets and stars, observers in different locations see it against a slightly different background of stars. Travel north in North America, the moon slides south. Travel south and the moon’s path shifts north.
To find out which stars and when they’ll be covered for your city or region, click the links for each below:
When you visit these sites, select the disappearance times of the star. Note that the times are given in Greenwich or Universal Time for April 4. Subtract 4 hours for Eastern, 5 for Central, 6 for Mountain and 7 for Pacific. For example, 2 hours UT April 4 = 9 p.m. CDT April 3.
The fun in watching occultations is to see how suddenly the star disappears when it touches the edge of the moon. Were there a substantial lunar atmosphere, it would gradually fade away instead. It’s also just plain cool to see the moon move in real time as it approaches and then blinks out the star.
Some of you will be able to see one or more of the Deltas graze the edge of the moon, popping in an out of view as they’re hidden by crater walls and mountains along the lunar profile.