Constellation of the Month: Aries, the Ram

Most people have heard of Jason and the Argonauts, those Greek adventurers who set off to find the Golden Fleece, but have you ever wondered where the Golden Fleece came from? The answer is to be found in the mythology of our December constellation of the month, Aries, the Ram.

Click the map above to enlarge it. On December evenings the constellation Aries is about halfway up in the east-northeastern sky is. Four thousand years ago, the equator crossed the ecliptic in this constellation. That crossing point is called the First Point of Aries. Even though it retains that name, the crossing point has moved along the ecliptic and is now in the constellation Pisces as marked on the chart. Over the decades, this point will continue to move westward along the ecliptic.

Nephele, the goddess of the clouds, married King Athamus of Boeotia and they had a pair of twins — a son, Phrixus, and a daughter, Helle. King Athamus divorced Nephele for Ino, who became the twins' stepmother. Ino hated the twins and decided to do away with them, but could not kill them herself without incurring the wrath of her husband. She concocted a devious plot against them, starting by having the town's entire seed crop roasted so they would not grow. The local farmers, knowing they had no seed to plant for the next season, decided to ask a nearby oracle what they should do. Ino bribed the men sent to consult the oracle so they would return saying that the oracle had said to sacrifice the twins to Zeus.

Nephele got wind of Ino's plot, and sent a flying golden ram to rescue her children. They climbed on the ram's back and it flew off to the east. Flying over what is now the Dardanelles, which connect the Aegean Sea and the Sea of Marmara, Helle slipped off and fell into the strait and was lost. The strait was originally named for her, the Sea of Helle, or the Hellespont.

Phrixus hung on, and the ram got him to Colchis on the eastern shore of the Black Sea. Having completed his mission, the ram told Phrixus to sacrifice it to the gods and remove its golden fleece. Phrixus took the Golden Fleece and presented it to the king of Colchis, Aeetes, who was impressed with the gift and gave Phrixus shelter. The fleece was hung in the sacred Grove of Ares, where it was guarded day and night by a dragon that never slept. Eventually, Jason and the Argonauts came to liberate the Golden Fleece and return it to Thessaly, but that is another story.

For its great deed of saving Phrixus, the golden ram was put in the sky as the constellation Aries. This constellation is made of faint stars, the brightest of which, Hamal, is only second magnitude. It is said that this is a faint constellation because Aries no longer has his glowing golden fleece to light up that part of the sky.

In the pre-Christian era, the Sun would just enter Aries as spring began with the March Equinox, when the center of the Sun is exactly on the celestial equator as it heads north for the summer. This point was called the First Point of Aries. Through precession, Earth's gyroscopic wobble, the First Point of Aries has moved west along the ecliptic into Pisces, but it has retained the name. It marks the origin of the Right Ascension (longitude) coordinate system in the sky.

Right Ascension is measured along the celestial equator. When astronomers first started measuring the positions of objects in the sky, they did it by timing when objects would cross a particular point in the sky, usually the line connecting the south point on the horizon through the zenith to the north point on the horizon. This line is called the meridian. Since the measurements were done using time, Right Ascension is measured in hours, minutes and seconds. The highest value is 23 hours, 59 minutes, 59.9999 seconds before returning to zero hours.

The other coordinate is called Declination and is similar to latitude on the Earth. Declination is measured from the celestial equator at zero degrees. It increases going north to plus-90 degrees at the north celestial pole and decreases from the celestial equator to minus-90 degrees at the south celestial pole. These two coordinates can describe the position of any object in the sky. Many larger telescopes will tell the user where it is pointing using these two coordinates. Astronomers use these coordinates to communicate the location of objects in the sky, just as a navigator tells the captain of a ship their location in latitude and longitude.

The Planets for December 2007

December is Mars Month. Mars will be the only planet in our evening sky, rising just after sunset in the east-northeast. Mars will actually be closest to the Earth on Dec. 19, when it will be 15.9 seconds-of-arc across at magnitude -1.6. The God of War will be in opposition (on the opposite side of the Earth from the Sun) on Dec. 24. Next year, Mars will spend the entire year shrinking, so this is the best time to look at it for quite a while. Mars will be in Gemini until just before the end of the month, when it moves into Taurus.

Saturn will remain in almost the same place in Leo all month as it goes through its stationary point and switches from direct (eastward) motion to retrograde (westward) motion. The Ringed Planet is 18.5 seconds-of-arc across at magnitude 0.6. The rings are tipped 6.6 degrees to our line-of-sight with the southern face showing, 41.9 seconds-of-arc across. Saturn comes up in the east around 11 p.m.

The only other planet in our December sky is the Morning Star, Venus. Look to the east-southeast around 4 a.m., and this magnitude -4.14 gem will be glowing brightly as it comes over the horizon. At midmonth, its waning gibbous globe will be 71-percent illuminated and 16.0 seconds-of-arc across. Venus will start the month in Virgo and slip into Libra on Dec. 11. It will continue to move eastward across the constellation for the rest of the month. Mercury and Jupiter are too near the Sun for observing this month.

The December Solstice occurs on the 21st at 11:08 p.m., marking the beginning of winter in the northern hemisphere. So enjoy the holidays and "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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