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April 22, 2024

Backyard Astronomy: Orion's Prey (2004-01-24)

By Cheryl Haimann

The great hunter, Orion, rules the winter sky. In one hand he holds a pelt; with the other he raises a club high overhead. His two faithful dogs, one large and one small, are nearby. But what are they hunting? There are two animals nearby, and clever prey that they are, they keep themselves well hidden.

Lepus, the Hare, sits at Orion's feet. Ancients called this dim grouping of stars Orion's chair, which did nothing to explain why a hunter holding a bloody animal skin would be allowed into the house, much less offered a chair. Later, and more rational, Greeks and Romans reassigned the constellation to represent a hare.

Lepus (LEE-pus) contains the only globular cluster visible in winter. 8th magnitude M79 is somewhat challenging to find with binoculars, and nicely resolved in a mid-sized telescope. The two brightest stars in the middle of Lepus point to M79.

Even more elusive is that rarest of beasts, Monocerus, the Unicorn. This constellation is very faint, and probably impossible to see from lighted locations. Even from a dark site, Monocerus (mon-OSS-err-us) is a challenge. The unicorn's brightest star is dimmer than the faintest of the major stars in Lepus.

Why would the ancients even bother with such a faint group of stars? Short answer: they didn't. Monocerus was not declared a constellation until the 1600s. The name is a Latin and Greek hybrid meaning "one horn." Some speculate that the idea of the unicorn came from a misunderstood reference to rhinoceros.

This unicorn manages to hide in plain sight. Besides being in the Milky Way, it is also right in the middle of the Winter Triangle, formed by Betelgeuse, Sirius, and Procyon. Orion and his dogs are circling Monocerus. In fact, the smaller dog, Canis Minor, is sometimes said to be riding on the unicorn's back.

The constellation may not be much to look at, but there are several deep sky objects in this area that are visible with binoculars. M50 is an open cluster not quite halfway between Sirius and Procyon. M46 and M47 are also open clusters. With Sirius and the hindquarters of Canis Major, these two clusters form the third point of a more-or-less equilateral triangle.

Planet watch: On Saturday, January 24, the crescent moon will be just left of Venus at sunset. By Tuesday, the moon will be almost at first quarter and just below Mars. The visible size of the orange planet is now only one third of its size last summer, and it is continuing to fade.
Article © Cheryl Haimann. All rights reserved.
Published on 2004-01-24
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