Pre-Halloween treat – moon makes two stars disappear tonight, Saturday

Small telescope view of the moon from Minnesota at 7:15 p.m. tonight Oct. 11 just minutes before it covers up Rho Sagittarii.  Stellarium

Like a disappearing act, the half moon hides and then reveals two bright stars in as many days as it travels east through the constellations Sagittarius and Capricornus.¬†Tonight around 7:15 p.m. CDT the moon will slink up to 4th magnitude Rho Sagittarii (Roe Sa-ji-TARE-ee-eye) and blank it from view around 7:15 p.m. The event, called an occultation, will be visible from the U.S., Canada and Mexico. In Minneapolis, the star will blink out at 7:17 p.m. and in Chicago at 7:25 p.m. Since the moon moves its own diameter in about an hour, the star will reappear along the moon’s bright edge approximately an hour later. Disappearances are much easier to see since the star stands in stark contrast to the dark edge of the moon.

View of Beta Capricorni minutes before it’s covered up Saturday night around 8 p.m. Stellarium

To find out what time the occultation happens where you live, click HERE. Look down the list and find your city or the one nearest to it. Times shown are UT or Universal Time. Subtract 4 hours for Eastern Daylight Time, 5 for Central, 6 for Mountain and 7 for Pacific. For example, if the time reads 0 hours 20 minutes UT (12:20 a.m. UT) Oct. 12 and you live in the Eastern zone, YOUR viewing time is 8:20 p.m. Oct. 11.

Saturday night the moon occults a brighter star, 3rd magnitude Beta Capricorni, around 8 p.m. CDT. Again, the U.S., Canada and Mexico are favored. Click HERE for times.

The thick lunar crescent occulted members of the Pleiades or Seven Sisters Clusters in 1990. Credit: Bob King

Both events should be visible in binoculars, but a small telescope is ideal, because you’ll be able to watch the star creep up to the moon’s edge and better anticipate the moment of its disappearance. Because the moon has next to no atmosphere, each star will vanish with a sudden snap – no gradual fading. Air absorbs light, thicker air more so, causing a star approaching the edge of a planet like Mars or Jupiter to gradually dim.

Occultations are much fun to watch. You get to see the moon move in real time, something not easy to appreciate in casual viewing. As Rho and Beta vanish behind its dark edge, remember you’ll watching a body speed along its orbit at some 2,280 mph (3,660 km/hr).

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