Moon Occults The Beehive Star Cluster Friday Night

The Beehive cluster is a fine sight in binoculars. Located about 577 light years from Earth it contains at least 1,000 members. Many will be occulted by the moon Friday night. Bob King

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.

From Duluth, Minn. the moon is already inside the cluster at 9:15 p.m. CDT about 45 minutes after local sunset. Stars are occulted at the earth-lit edge of the moon. They’ll reappear later at the bright limb later but will be lost in the glare. Stellarium

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.

The Moon enters the Beehive in a dark sky for Eastern observers and in twilight for those in the Midwest. Stellarium

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.

Views of the Moon’s progress through the Beehive as seen from Denver and Portland. Times are approximately 30 minutes after sunset for both locations. Notice that the moon has moved beyond the cluster for the West Coast by the time the sky gets dark enough for a look. Stellarium

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.

This time-lapse animation depicts the Moon moving across the Beehive between 9:15 p.m. and 12:15 a.m. May 10–11 from Dayton, Ohio.

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.



2 Responses

  1. kevan hubbard

    I loved your animation of the Moon occulting m44 probably all I’ll get to see of it been a diabolical May here with temperatures half the seasonal average and loads of cloud too although I did see a nice sundog a few days ago.even the swallows are late coming up from Africa only seen about 10 this year.

    1. astrobob

      Sorry about your weather. May’s been crazy around here — we just got 7 inches of fresh snow overnight. Tomorrow it’ll be 60°!

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