Constellation of the Month: Pisces, the Fish

Pisces, the Fish, graces our cold January skies this year as it does every year. Pisces is one of the zodiacal constellations, so named because they harbor the Sun and most of the planets. The ecliptic is the projection of the Earth's orbit around the Sun into the sky. Since most of the planets orbit the Sun in almost the same plane, you can always find them somewhere along the ecliptic. The exceptions are the dwarf planet Pluto and many minor planets, which can get quite far from the ecliptic. It is the ecliptic, passing through Pisces and the other 11 zodiacal constellations, that sets them apart from all the others.

The Sun always stays right on the ecliptic. Around March 21 of every year, the Sun, traveling eastward along the ecliptic, crosses from the southern celestial hemisphere to the northern celestial hemisphere, marking the beginning of spring here in the desert Southwest. This point where the Sun is as it crosses the equator traveling north is called the First Point of Aries and is the origin of the system of Right Ascension and Declination in the sky (equivalent to longitude and latitude on Earth). You might ask why this point is called the First Point of Aries? When astronomers first discovered it thousands of years ago, the place where the Sun crossed the equator was in the constellation Aries. Due to the wobble of the Earth's axis, called precession, the First Point of Aries moves westward about a degree every 70 years. It crossed into Pisces in 70 BCE, and will finally complete its passage through Pisces and cross into Aquarius somewhere around 2600 AD.

Pisces consists of two fishes tied together with a string. One could view this as the two connected fishes jumping out of the water at the same time, with the waterline being marked by the ecliptic. The western fish is now falling back into the water, while the northern one is managing to jump free of the water (away from the ecliptic).

The mythology story of Pisces is rather short. One day the Greek gods were having a good time when the horrible giant Typhoeus appeared on Earth. He frightened the gods so badly that they all transformed themselves into animals to escape. Jupiter became a ram, Diana a cat, and Mercury an ibis. Venus and her son Cupid were bathing along the banks of the Euphrates River and sensed Typhoeus. They dove into the waters of the river and transformed themselves into fish. To memorialize this event, Minerva placed the figures of the Two Fishes into the sky.

The Planets for January 2007

The evening sky, which has pretty much been devoid of planets for the last few months, is getting crowded again this month. Creeping eastward from the Sun in the western sky are Mercury and Venus, while Saturn sneaks in from the east. Jupiter and Mars are still in the morning sky.

Venus begins January low in the west-southwest shortly after sunset, in Sagittarius. Venus quickly moves into Capricornus, then dashes into Aquarius by month's end. Having passed behind the Sun two month ago, Venus is still far from the Earth on the other side of the Sun. This makes it relatively small at only 10.6 seconds-of-arc across, and almost-full at 95-percent illuminated. Venus will slowly get larger, become less full, and brighten from its current -3.9 magnitude during the first part of this year.

Mercury pops out of the morning twilight during the last part of January. Appearing in Capricornus just below and to the right of Venus around Jan. 20, Mercury moves into Aquarius on the last day of the month, when it will be shining at magnitude -0.9, with its 6.0 second-of-arc disc 77-percent sunlit. Mercury will still be visible in early February.

Saturn rises in the early evening, around 7:30 p.m. Still in Leo, Saturn is well placed for telescopic observation later in the evening when it is higher in the sky. Shining at magnitude 0.0, Saturn will be closest to the Earth next month. This is a good time to take a look at the Ringed Planet's 19.9 second-of-arc disc and its 45.2 second-of-arc-across rings. The rings are tilted up 13.0 degrees with the southern face showing, but will slowly close during the year.

Jupiter and Mars are in the morning sky, rising shortly before the Sun comes up. First up is Jupiter, rising around 4 a.m. The King of the Planets is magnitude -1.9 and stays in Ophiuchus all month. The planet's disc is 32.7 seconds-of-arc across.

Following Jupiter an hour and a half later, Mars comes up around 5:30 a.m. Like Venus, Mars is still far from the Earth, so its disc is only a tiny 4.0 seconds-of-arc across. Shining at magnitude 1.5, Mars starts the month in Ophiuchus and moves quickly into Sagittarius where it ends the month.

The Earth is nearest the Sun on Jan. 3, but the effects of the low winter Sun far outweigh the slightly closer distance. Bundle up and stay warm so you can "keep watching the sky"!

An amateur astronomer for more than 35 years, Bert Stevens is co-director of Desert Moon Observatory in Las Cruces.

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