Earlier this week, skywatchers in the southern U.S. got to see the crescent moon cover or “occult” the star Zeta Tauri. Friday night (May 10) the moon will be at it again but this time will pass in front of the Beehive, a bright star cluster in Cancer. Good news. Occultations will be visible over a much wider area — across the eastern two-thirds of the U.S. and Canada, Central America and eastern South America.
On a dark night, the Beehive looks like a fuzzy spot with the naked eye and a beautiful splash of stars in a pair of binoculars. Friday evening, the 6-day-old moon will slowly glide over the cluster from west to east, covering and uncovering stars as it goes. About a dozen brighter stars — between magnitude 6 and 7.5 — lie in its path. All of these should be easy to watch disappear through a small telescope.
The entire show lasts between 2 and 3 hours. East Coast viewers will see it happen in a dark sky starting around 9:30 p.m. local time and ending around midnight. If you live in the Midwest you can start watching as soon as mid-twilight or about 9 p.m. By the time darkness falls in the mountain states, the moon will have already made it halfway through the cluster. Observers there should begin their watch around 8:30 p.m. to make the best of the remaining occultations. Only those living in the Pacific time zone won’t see the event — by the time it’s dark enough, the moon will have departed the Beehive.
During an occultation, the dark edge of the moon (called the “limb”) slowly approaches a star. Before it’s occulted, the star seems to hover over the limb for an eternity, and then just like that, it’s gone in a split second. The slow approach vs. the sudden disappearance makes for exciting viewing. Although the moon’s phase is approaching half, there will still be enough earthshine to make the dark limb of the moon stand out against the sky, the better to anticipate when a star will be covered. One tip. When observing occultations, place the bright part of the moon out of the field of view to better see the earth-lit limb without the glare.
Depending on your location the moon will take a slightly different path across the Beehive. From the northern U.S., it passes a little south of the cluster’s core; from the Caribbean, it passes directly over the center, and from the northern half of South America, north of center. Each of us has a unique perspective! That’s because the moon is close enough to Earth that its position shifts relative to the background stars when seen from different places across the planet.
A star disappears so suddenly when it’s occulted because the moon has very little atmosphere. If an astronaut on the moon were watching Earth occult a star, it would slowly fade before it disappeared, filtered by the air, water and dust that comprise our planet’s atmosphere.
Watching occultations also allows us to see the moon move in real time. It orbits the Earth with an average speed of 2,288 mph (3,680 kph), but we’re normally not aware of how fast that is because the moon is so far away. We only detect its slow eastward creep over the hours. But when the moon covers a star you see that rapid motion in real time, with the star covered in an instant! That said, remember it will take time for the moon to crawl toward each star, so the whole show will last a long time.
Cross your fingers for clear skies Friday, May 10. If you don’t have a scope you can still see a bunch of stars near the moon, but if you do have a telescope, take some time and see if you can watch a stellar bee or two disappear.