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About the cover


Aries, the Ram

Plus the planets for January.

This month we are looking at the constellation Aries, the Ram, which is high up in our southern sky in January. Unlike some bold constellations such as Orion, this constellation is a little hard to find, with mostly faint stars that do not seem to form much of a constellation figure. The brightest star in this constellation, Hamal (Alpha Arietis), is barely second magnitude.


Click the image for a larger map.


Aries, the Ram, is high in our southern sky on January evenings. Most of the stars in this constellation are rather faint, so this is a difficult constellation to locate in the sky. Two millennia ago, this constellation contained the First Point of Aries, where zero degrees declination and zero hours of right ascension is defined by the intersection of the ecliptic and the celestial equator. Precession has moved the First Point eastward into Pisces, but it still retains that name.

The Greek myth behind this constellations starts with the sea god Poseidon pursuing the beautiful maiden Theophane. After she rejects him, Poseidon changes her into an ewe and himself into a ram. The product of their union is a magical ram with a golden fleece.

Meanwhile, in Boeotia, King Athamus married Nephele, the goddess of the clouds, and they had a pair of twins, a son, Phrixus, and a daughter, Helle. King Athamus later divorced Nephele for Ino. Ino hated the twins and decided to do away with them, but Nephele got wind of Ino's plot, and sent the magical golden ram to rescue them. Phrixus and Helle climbed on the ram's back and flew east into Turkey. Helle could not hang on and fell into the sea. When the ram landed safely, he told Phrixus to sacrifice him to the gods and remove his golden fleece. Eventually, Jason and the Argonauts came to procure the fleece and take it to Thessaly.

For saving Phrixus, the golden ram was put in the sky as the constellation Aries. Without his golden fleece to provide illumination, this constellation has only faint stars to mark the ram's position in the heavens.

Not only are the stars faint in Aries, but so are the galaxies. Not far from the star Sheratan (Beta Arietis) in northeastern Aries is the NGC 691 galaxy group. This group contains seven galaxies (NGC 691, NGC 694, NGC 680, NGC 678, NGC 697, IC 167, and IC 1730). These galaxies are 117 to 130 million light-years away from us. They all reside in a 45-minute-of-arc circle in the sky.

These galaxies are kept together in space by their mutual gravity. But if you measure the masses of these galaxies, you will discover that there is not enough mass in the visible galaxies to generate enough gravity to keep them together. There must be some other mass in the cluster holding them together. X-rays show a very hot gas between them. But the total mass of the hot gas and the galaxies still produces only one-sixth of the gravity needed to hold the cluster together. The typical explanation for the missing mass is dark matter. This strange material has mass but does not interact with normal matter, so we cannot see it or measure it.

A more novel theory was proposed by Joel Brownstein and John Moffat, researchers at the Perimeter Institute for Theoretical Physics and the University of Waterloo in Ontario. A theoretical particle called a graviton, which carries the force of gravity from one mass to another, appears in large numbers in regions crowded with massive objects like stars. This appearance out of the vacuum of space is called a "quantum fluctuation," where particles just come into existence (or go out of existence) without any specific cause. The increased number of gravitons makes gravity much stronger in these galaxy groups, making dark matter unnecessary. Which of the two theories represents reality will need to be decided by astronomers in the future based a better understanding of our universe.



The Planets for January


Mars is still in our evening sky, just 12 degrees above our west-southwestern horizon as it gets dark. With a diameter of only 4.2 seconds-of-arc across, Mars is a difficult target for your telescope. Shining at magnitude 1.2, the God of War starts the month in western Capricornus, moving eastward into Aquarius as the month ends. Mars sets around 7 p.m.

Jupiter is moving westward in Taurus, just north of the Hyades star cluster. Jupiter will be 50 degrees up in the east as it gets dark, and sets at 3:45 a.m. The King of the Gods glows at magnitude -2.7 and its disc is 45.0 seconds-of-arc across. Jupiter is still a good view in a telescope, especially when high in the sky around 9 p.m.

Watch the Skies

(all times MST)


Jan. 1, 10 p.m. — Earth closest to the Sun for 2013

Jan. 4, 8:58 p.m. — Last Quarter Moon

Jan. 10, 5 a.m. — Venus 3 degrees south of the Moon

Jan. 11, 12:44 p.m. — New Moon

Jan. 18, 4:45 p.m. — First Quarter Moon

Jan. 21, 7:30 p.m. — Jupiter 20 minutes-of-arc north of Moon's disc

Jan. 26, 9:38 p.m. — Full Moon

Saturn rises around 1:15 a.m. in Libra, where it is moving slowly eastward. The Ringed Planet's disc is 16.7 seconds-of-arc across, while the Rings are tilted down 19.2 degrees with the northern face showing. They are 37.9 seconds-of-arc across. Saturn shines at magnitude +0.6.

Venus is ending its run in the morning sky, rising around 6 a.m. in the east-southeast, barely six degrees above the horizon as it gets light. Venus starts the month in south-central Ophiuchus, moving rapidly eastward into Sagittarius, traveling almost all the way across that constellation by month's end. The Goddess of Love is a beacon glowing at magnitude -3.9. Venus' disc is almost full, 95% illuminated, and 10.4 seconds-of-arc across at midmonth.

Mercury is too close to the Sun to be visible this month, but will be back in the evening sky in February.

On Jan. 21 at 7:30 p.m., the northern edge of the Moon's disc will pass just 20 minutes-of-arc south of Jupiter. The pair will be high in the southeastern sky and should be visible with or without a telescope. Enjoy this pairing and "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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