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Capricornus, the Goat

Plus the planets for October.

 

Last month the Earth passed through the September Equinox, marking the beginning of autumn. When the Earth has moved through another quarter of its orbit, the Sun will be in Sagittarius at the December Solstice, marking the beginning of winter in the Northern Hemisphere. But 3,000 years ago, the December Solstice occurred when the Sun was in the constellation of Capricornus (sometimes called Capricorn).

starry dome

(Click the image for a larger map)

 

Capriornus is about halfway up in the southern sky, just east of the meridian, shortly after it gets dark. This constellation is a member of the zodiac, so the ecliptic passes through it. Precession of the equinoxes has moved the December Solstice out of this constellation into Sagittarius. There are several galaxies and star clusters in Capricornus, including the globular star cluster M30.

On a globe of the Earth, the northern and southern limits that mark off the areas along the equator where the Sun can be overhead (the Tropics) are the Tropic of Cancer (north) and the Tropic of Capricornus (south). They were so named because at the time, the Solstices occurred in the constellations of Cancer (June) and Capricornus (December). Like a top that starts to wobble as it is slowly spinning down, the Earth also has a slow wobble with a period of 25,772 years. This wobble gradually moves the position of the equinoxes and solstices eastward along the ecliptic, so the December Solstice has moved from Capricornus to Sagittarius over the last 3,000 years.

In mythology, Capricornus represents Pricus, father of the sea-goats, honorable and intelligent creatures with a goat's head and body connected to the tail of a fish. They lived in the sea, but could crawl up on land. The longer they stayed on land, the more they transformed into normal goats and lost their intelligence and ability to talk.

Pricus was the offspring of Chronos, the god of time, and shared his immortality as well as his ability to change time. Pricus saw all his offspring leave the sea and become normal goats, leaving him alone. This greatly saddened him, and he used his power to reverse time until all his children were back in the sea. He tried to convince them that they should stay in the sea, but one by one, they crawled up on the shore again and became goats.

Pricus reversed time again and again, each instance failing to convince his children to stay in the sea. He finally realized that he could not control the destiny of his children. He despaired of being the only sea-goat and begged Chronos to let him die. Chronos instead placed him in the sky to live out his immortality.

Located about 400 million light-years out in Capricornus is the galaxy group known as HGC 87. Composed of at least three galaxies, this group is listed in Paul Hickson's catalog of compact galaxy groups. This catalog, HCG (Hickson Compact Group), lists 100 groups of galaxies that are in close proximity. In HGC 87, there are four galaxies within a 120 seconds-of-arc diameter circle. Three of the galaxies are actually close together in space as well, but a small spiral galaxy (called HGC 87D) near the center of the group is probably a more distant background galaxy.

The remaining three galaxies are gravitationally tugging on each other. The largest galaxy in the group, HGC 87A, is an almost edge-on spiral galaxy with an obvious dust ring blocking the light from the inner part of the galaxy. This ring should be in the same plane as the rest of the galaxy, but the nearby elliptical galaxy HGC 87B has been tugging on 87A and distorting its shape. The dust ring is being pulled up on the west side (near HGC 87B), dropping down on the east side relative to the rest of the galaxy.

Over time, these galaxies will pull themselves together and merge. The process will take millions and millions of years as HGC 87A and HGC 87B merge first, and then pull in the spiral galaxy HGC 87C. At first, the nuclei of the galaxies will hold together and the combined galaxy will have multiple nuclei orbiting around each other. Eventually, they will merge to form the nucleus of a giant elliptical galaxy. The individual stars will take up orbits around the nucleus, traveling in vastly different planes, forming the outer galaxy. Since the stars are no longer in the same plane, the beautiful spiral structure of these galaxies will be destroyed to form a giant blob of stars — an elliptical galaxy.

 

 

The Planets for October


Mars starts the month in Libra, but rapidly moves into and through Scorpius, ending the month in Ophiuchus. As it gets dark, Mars will be 18 degrees up in the southwest. At magnitude +1.2, the God of War sets at 8:45 p.m. at midmonth. Its disc is just 4.7 seconds-of-arc across, making it a poor observing target.

Watch the Skies
(times MDT)


Oct. 3, 2 a.m. — Venus 0.1° S of Regulus
Oct. 8, 1:33 a.m. — Last Quarter Moon
Oct. 15, 6:02 a.m. — New Moon
Oct. 20, 10 p.m. — Orionid Meteor Shower peaks
Oct. 21, 9:32 p.m. — First Quarter Moon
Oct. 26, 4 p.m. — Mercury farthest E of Sun (24°)
Oct. 29, 1:49 p.m. — Full Moon

Shining at magnitude -2.7, Jupiter is moving westward in Taurus. The King of the Gods rises at 9:30 p.m. and is visible the rest of the night. It has a disc that is 45.0 seconds-of-arc across.

With Saturn too close to the Sun, the last planet to be visible during October early mornings is Venus. The Goddess of Love zips eastward through most of Leo and ends up in Virgo by the end of the month. The unmistakable second planet from the Sun rises around 4:15 a.m., glowing at magnitude -4.0. It is 31 degrees above the eastern horizon as the sky gets light. The disc is 15.5 seconds-of-arc across and 75% illuminated, becoming fuller as Venus starts to swing around behind the Sun.

The Orionid Meteor Shower peaks on the morning of Oct. 21. The radiant for this shower is just northeast of Orion's upraised arm. The Moon will be completely out of the way for this shower, making this a good viewing opportunity. Best viewing is from 1 a.m. until dawn breaks; you may be able to see up to 20 fragments from Halley's Comet per hour. So put on your coat and get out your chaise lounge. Lie back and remember to "keep watching the sky"!

 

 

An amateur astronomer for more than 40 years, Bert Stevens is
co-director of Desert Moon Observatory in Las Cruces.

 

 



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