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Moon near Aldebaran on February 23

Bright star Aldebaran on the moon’s dark edge, just before the moon passed in front of the star. Photo by Michael Caruso in Morgantown, West Virginia.

Tonight – February 23. 2018 – finds the moon near Aldebaran, the brightest star in the constellation Taurus the Bull. The two will be especially close together as seen from Europe and Asia.

In fact, if you live at just the right spot on Earth, you can watch the moon occult – cover over – Aldebaran for a portion of the night tonight. The star will disappear behind the dark side of the moon and reappear on its illuminated side. For instance, if you live in Moscow, Russia, the lunar occultation of Aldebaran will happen from 20:30 (8:30 p.m.) to 21:33 (9:33 p.m.) local time. See the worldwide map below for more details.

Worldwide map via IOTA. Everyplace north (above) the curved white line sees the occultation in a nighttime sky, Places to the north of the short blue line has the occultation happening at evening dusk, and places to the north of the red line has the occultation taking place in a daytime sky. Click here to find out the Universal Time of the occultation for over a thousand localities. You must convert Universal Time to your local time. Here’s how.

Of course, the moon and Aldebaran are only close together on the sky’s dome. They aren’t close together at all in space but reside on or near the same line of sight tonight.

The moon, our closest celestial neighbor, is about 230 thousand miles (371 thousand km) from Earth tonight, whereas Aldebaran lies way, way beyond the moon, at 65 light-years. Given that one light-year is about 25 million times the distance of tonight’s moon, that places Aldebaran at about 1.6 billion times the moon’s distance. Wow!

Aldebaran only looks small next to the moon because this red giant star looms so far away. If Aldebaran replaced the sun in our sky, its diameter would span about 20o of our sky. That’s 40 times the diameter of our sun, which covers about 1/2o of sky!

Compare the size of Aldebaran with our sun. Image via Wikipedia

By the way, the sky chart below shows the moon and Aldebaran as they appear from middle North American latitudes. Here, we see the moon to the east of Aldebaran. If you live in the world’s Eastern Hemisphere – Europe, Africa, Asia, Indonesia, Australia and New Zealand – you’ll see the moon offset in the direction of Aldebaran, or to the west of Aldebaran.

At northerly latitudes in Eastern Europe and Asia, you can actually watch the moon occult (cover over) Aldebaran for a portion of the night tonight. Aldebaran will disappear behind the moon’s dark side and then reappear on the moon’s illuminated side.

The moon moves eastward through the constellation Taurus the Bull for several days each month. When the moon moves out of this region of sky, use Orion’s Belt to locate the star Aldebaran and the Pleiades star cluster.

Lunar occultations of Aldebaran have enabled astronomers to measure the angular diameter of this star and to estimate its physical diameter at around 40 solar. We’re in the midst of a series of 49 monthly occultations of Aldebaran that began on January 29, 2015, and will conclude on September 3, 2018.

When the moon is no longer there to guide you, star-hop via the three medium-bright stars of Orion’s Belt to reddish Aldebaran. Image via SolarEmpireUK

Tonight – February 23, 2018 – look for the moon near Aldebaran, the ruddy eye of the Bull.

Bruce McClure

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