The moon is hungry my friends and will happily devour any star or planet in its path. Of course, it doesn’t really eat stars, but might as well from appearances. When on celestial body covers another — when the two appear along the exact same line of sight — it’s called an occultation. The moon has the largest apparent size of any night sky object, so it does most of the occulting.
Occultations occur in a blink of an eye because stars are so far away that they’re essentially points of light. Coupled with the fact that the moon has next to no atmosphere, a star remains bright right up to its edge, and then blinks out of view in an instant. The best thing I can compare the star’s sudden disappearance to is a frog snatching prey with its long, sticky tongue. Yes, it’s that fast. Here’s a video.
When the moon sucks up a bright star like Aldebaran in Taurus or Antares in Scorpius, you can watch it disappear with your naked eye or a pair of binoculars. Fainter stars require binoculars or a small telescope. Telescopes and spotting scopes are best because they sit still, making it easier and more comfortable to catch the action. Point your instrument at the moon 10 minutes before the occultation time. Then wait and watch. Gradually the moon will approach the star until it hovers a final moment at the edge before it’s snuffed from view.
Sometimes the bright edge — called the lunar limb — is visible during an occultation, so you can watch the moon march right up to the star. Other times the part of the moon approaching the star is still in darkness, so the star disappears in “empty space” near the bright edge. After its disappearance behind the approaching moon, the star will pop out the opposite side minutes to an hour or so later depending on your latitude.
That brings us to the two occultations that are happening Wednesday night, Feb. 5. The waxing gibbous moon will first occult Eta Geminorum, a moderately bright, magnitude 3.3 star located in the toes of Castor in the Gemini Twins. That happens between around 6-7 p.m. Central Standard Time (CST) depending on your location.
For Duluth, Minn., where I live, the moon will cover the star at 6:19 p.m. From Chicago it happens at 6:21 p.m. and at 6:56 p.m. EST for Bostonians. The occultation will be visible across the eastern half of the U.S. from the southern states (except Florida and the southern areas of its neighboring states) north through Canada. Observers in western Europe and western Africa will also see it, but around 12:30 a.m. Feb. 6 London time. You can look up the specific time the star disappears for your city here. Times shown are Universal Time (UT). To convert UT to your time zone, subtract 5 hours for EST; 6 for CST; 7 for MST and 8 for PST.
Take a break after the Eta Gem event and then return around 11 p.m. CST (midnight EST, 10 p.m. MDT and 9 p.m. PDT) when the moon occults the star Mu Geminorum (magnitude 2.9). Observers across the northern third of the U.S., Alaska and Canada will witness this event. It’s unusual for the moon to occult two fairly bright stars in a night — I hope you’ll get to see at least one. Again, look up the specific time of disappearance and reappearance of the star for your city here or just click on the map.
Both occultations may be visible in binoculars, but a small telescope or spotting scope will guarantee that you’ll see them. In both, the star will disappear behind the dark, unilluminated part of the moon ahead of the bright limb. Good luck and enjoy seeing how fast the moon really moves!