Our Tiny Moon Hides A Supergiant Star Tonight

The half moon will cover the star Rho Leonis this evening for much of the U.S., Canada and Mexico. In this simulation for Kansas City, Missouri, Rho Leonis hovers at the dark limb at 11:36 p.m. local time, one minute before occultation. Created with Stellarium

How does the moon do it? Only 2,160 miles across, tonight it will cover a supergiant star 75 times the diameter of the sun,  23 times more massive and 165,000 times more luminous. Amazing. The moon does its trick because it’s so close to the Earth, the same way you can raise your thumb and hide the moon.

Skywatchers will be treated to a fine occultation of Rho (ρ) Leonis Wednesday night, May 31st across the U.S., Canada and Mexico. Cyan = occultation at moonrise/moonset; red dotted = daytime occultation; blue = twilight occultation; and white = nighttime occultation. Credit: Occult 4.2

The star getting blanked from view is Rho Leonis, an unassuming magnitude +3.8 star in Leo the Lion. This should be a fun event to watch in a small telescope. The star will disappear — occulted in astrospeak — at the moon’s dark limb. The dark limb is the left or east side of the moon, the part that’s still in shadow.

The moon travels east as it revolves around the Earth, so the dark edge will slowly approach the bright star. In the moments before the edge covers the star, Rho will seem to hover right at the limb for what feels like eternity. Then all at once – pffft – the star disappears!

Here are a couple additional views from Duluth, Minn. and San Francisco. The times shown are about 1 minute before the moon covers up the star. Created with Stellarium

The suddenness and brevity of its disappearance really makes an occultation a brief but dramatic event worth watching. You not only experience how fast the moon moves — 2,280 mph (3,683 kph) for an average — but how truly tiny stars are due to their immense distances from Earth. Rho Leonis lies about 3,650 light years away. Were it as close as the bright summer star, Vega (25 light years), it would far outshine Venus! Everyone would know Rho.

Since the moon’s position in the sky varies a little depending on your location, time for viewing the occultation vary, too. Click here for a list of over 700 cities with times of the disappearance of the star and its reappearance at the moon’s bright limb about an hour later. Note that the times will be in UT or Universal Time. To convert to Easter Daylight, subtract 4 hours; Central Daylight, 5 hours; Mountain, 6 hours and Pacific, 7 hours. The first three numbers under the U.T. heading give the disappearance time in hours, minutes and seconds. To give an example: the star disappears from Chicago at 4:31:25 U.T. Subtract 5 hours to get 11:31:25 CDT. You can ignore all those other columns of numbers if you like.

Occultation of Lambda Geminorum on Feb. 5, 2015

To view it, you’ll need a small telescope, nothing fancy. It may possibly be visible in binoculars, but I’m not committing to that. Just point your scope at the moon about 15 minutes before the occultation start and identify Rho Leonis, the only brightish star close to the left or east edge of the moon.

I hope it’s clear for you. Have at it!