This should be fun to watch. On Monday night, Sept. 21, the waxing crescent moon will briefly cover up the bright star Acrab in the head of Scorpius during evening twilight.
Acrab, also known as Beta Scorpii, will disappear along the "dark" edge of the moon — the part lit by sunlight reflected off the Earth — and reappear on the opposite or bright side. Astronomers call these stellar peek-a-boo events occultations. What makes this one extra special is that Acrab is a double star with a 5th magnitude companion. An absolutely gorgeous sight in a small telescope I consider it one of the best binaries in the summer sky.
That means we'll see two occultations one right after the other. Very cool! Scorpius is low in the southwestern sky this time of year, so be sure to find a place where you can get a clear look at the moon without interference from buildings, trees or mountains.
The event will be visible during evening twilight for much of the U.S., Canada, Mexico and Central America. Unfortunately, the moon sets along the East Coast before the occultation begins. Similarly, observers along the West Coast from about San Francisco north will miss the show because it wraps up before sunset. But if you're in the east-central to west-central U.S. and Canada you'll see the star's dramatic disappearance along the upper left side of the moon.
Folks living in the western third of the U.S. and Canada will miss the disappearance but will see the star return to view along the moon's bright edge during evening twilight. South of Central America the moon narrowly misses the star — skywatchers will see a pretty conjunction instead.
To find out the times of disappearance and / or reappearance for your location consult this list of hundreds of cities prepared by IOTA, the International Occultation Timing Association. The times shown are Universal Time or UT. To convert to Eastern, subtract 4 hours; 5 hours for Central; 6 hours for Mountain and 7 hours for Pacific. For example, disappearance for Chicago occurs at 1:44:57 (rounded to 1:45) on Sept. 22. Back up 5 hours and that becomes 8:45 p.m. on Sept. 21.
You should have no problem seeing Beta's disappearance with binoculars, but a telescope will split the star in two for an extra dimension of enjoyment. Be sure you're out at least 10 minutes before Acrab's farewell and return so you don't miss a thing. Minutes before it disappears, the brighter star will hover seemingly forever at the moon's edge ... and then vanish in a breathtaking instant followed by its companion.
Stars disappear and reappear along the lunar edge with such suddenness for two reasons — they're so far away that they're essentiallypoints of light, and the moon lacks a substantial atmosphere. Without air to filter the star's light it remains bright right up to the edge and then blinks off.
I hope you get to see this!