Happy equinox! Half-moon dabbles in the occult tonight

Yellow sugar maple leaves contrast with the bark of a paper birch yesterday. Photo: Bob King

Happy first day of fall! The sun crossed the celestial equator on its way south at 9:49 a.m. (CDT), sending a chill up the spines of those who love summer. Many of us look forward to cooler days and nights and that first frost. Here in Duluth, Minn. leaves began changing as early as the end of August. We’re now well into the color season with every hue from lemon to scarlet to brown falling to the ground.

Tonight all those colorful trees will cast shadows under a first quarter moon. First quarter refers to the moon having completed a quarter of its orbit around the Earth. This takes about 7 days. In another 7 days the half moon will fill out into a full moon. We get a special edition of the full moon this month – the Harvest Moon. I’ll have more on exactly what makes it special early next week.

Don’t pass up the chance to look at the moon through a small telescope over the next few nights.  The number of craters visible is incredible.

Illustration of the moon tonight (September 22) showing the star Mu Sagittarii hovering above the earthlit half of the moon minutes before occultation as viewed from Duluth, Minn. North is up. Created with Christ Marriott’s SkyMap software

Let’s return to tonight. The moon will occult the star Mu Sagittarii (Sa-ji-TARE-ee-eye) this evening around 8:20 p.m. (CDT). An occultation is the astronomical term to describe one celestial object covering up another.

Mu is a moderately bright star – magnitude 3.8 – and should be visible next to the moon this evening in binoculars. A telescope will show it plainly. If your sky’s free of haze you may even be able to make out the dim, earthlit half of the moon. Try putting the bright half out of the field of view to see it best.

The fun begins when the moon draws very close to the star. Minutes before the cover-up, you can see the moon move in real time as it moves in for the kill. With just seconds remaining,  Mu may seem to hover forever at the precipice, and then – PFFFT! – it’s gone. Whether you’re looking through telescope or binoculars, the star will blink out with surprising suddenness because the moon lacks an atmosphere.  If there was air up there, Mu would gradually dim and disappear. Even without special instruments, early astronomers could be certain there was little air on the moon by observing occultations.

An 8th magnitude star appears to sit on the moon’s edge moments before it’s occulted on April 21, 2007. Credit: Herbert Raab

Times for the occultation vary depending on where you live. In Duluth, Minn. it happens very close to 8:20 p.m., in Chicago 8:21 p.m. Central time, Atlanta at 9:21 p.m. Eastern time, Denver at 6:53 p.m. Mountain time and LA at 5:15 p.m. Pacific time. Depending on your latitude, Mu will disappear at a slightly different spot along the moon’s left or eastern edge.

In the northwestern U.S., the star will just miss the moon, appearing to graze its northern limb. If you’re keen to observe the event, write to me in the Comments section with the name of the nearest medium-sized or large city to your home, and I’ll send you back a time.

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About astrobob

My name is Bob King and I work at the Duluth News Tribune in Duluth, Minn. as a photographer and photo editor. I'm also an amateur astronomer and have been keen on the sky since age 11. My modest credentials include membership in the American Association of Variable Star Observers (AAVSO) where I'm a regular contributor, International Meteorite Collectors Assn. and Arrowhead Astronomical Society. I also teach community education astronomy classes at our local planetarium.

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