Astro Bob: The full-flower moon meets Zubenelgenubi

I spotted my first wildflowers over the weekend — hepaticas, bloodroot, marsh marigolds, anemones and yellow bellflowers. It always feels so good to see these friends again each spring. In consort with the blooming Earth the moon will bloom full on Thursday, May 7. Its traditional name of Full Flower Moon is well chosen. Because the moment of greatest fullness occurs at 5:45 a.m. on the 7th, the moon will appear nearly full for two nights in a row.

On Wednesday the 6th it will be 99.8 percent full; on the 7th, 99.2 percent. Close enough that I doubt we’ll see the difference except in a binoculars or a telescope. Find a place with a nice view to the east-southeast and you’ll see the moon rise about 35-45 minutes before sunset on Wednesday. A true full moon rises very close to sunset but on that day we’ll be seeing the moon nearly 10 hours before full phase, so it’s technically a gibbous moon, not quite full. Because it rises in a pre-sunset sky it will appear very pale until it climbs a few degrees above the horizon.

On Thursday night — roughly 12 hours past full phase — it will rise a half-hour after sunset and be very easy to see at the moment of rising because the sky will be much darker. On both nights it will shine from Libra the scales . Libra is shaped like a diamond and represents the pans of a balance used for weighing. Although the constellation has no especially bright stars it does have three of the coolest-ever named stars ever — Zubenelgenubi (zoo-BEN-el-je-NEW-bee), Zubeneschmali (zoo-BEN-ess-sha-MAH-lee) and Zubenelhakrabi (zoo-BEN-el-hak-RAH-bee) otherwise known as Alpha, Beta and Gamma Librae, respectively.

The three “Zubens” were once part of the neighboring constellation Scorpius; their names are Arabic for the “northern claw” (Z-genubi), “southern claw” (Z-schmali) and simply “claws” (Z-hakrabi) of the scorpion. On Wednesday the 6th if you point binoculars at the moon you’ll see Zubenelgenubi in the same field of view to the south. Take a closer look and you’ll discover that it’s a double star. This is no happenstance alignment. The two suns are physically connected in space and revolve about their common center of gravity.

The companion is named Alpha-1 and shines just to the upper right (northwest) of the brighter star, Alpha-2, also called the primary in double star parlance. Any pair of binoculars will cleave the pair.

Both are 77 light years away and moving together through space separated by 420 billion miles, equal to about 140 times Pluto’s distance from the sun. Alpha-2 keeps a little secret – it’s double again! But the two suns are so close together they appear as one even to an observer orbiting a hypothetical planet around Alpha-1.

Notice, too, how incredibly brilliant the moon appears in your binoculars. It shines by the light it reflects from the sun. Although the moon reflects just 13.6 percent of that light — equal to a moderately-weathered asphalt parking lot — it’s more than enough to find your way at night. The same light drowns out the fainter stars giving the sky a gray, anemic appearance.

If the moon were close by it would only light up a part of the sky the same way a distant shopping mall only lights up the sky in that direction. Imagine the sight. You’d see fewer stars near the moon but the sky would still be dark and starry in the opposite direction.

Because the moon is so far away its light scatters off the entire, moon-facing atmosphere of the Earth, spreading its light across the sky and dimming stars no matter which direction you look.

I hope you’ll find time to catch a moonrise this week. Click here to find the time when the Full Flower Moon rises for you.

*** SPECIAL EVENT: Stuck at home because of COVID-19? You can still enjoy the sky! Join me tomorrow (May 5) from 4-4:30 p.m. Central Time for the second live episode of Night Sky Explorer — Launch into Skywatching on Zoom and on Facebook Live brought to you by Voyageurs National Park. We’ll be talking about how you can see a bright comet later this month and other interesting facts about the full moon. See you then!