Stars over Sisters 11/24/2020

The late fall constellation of Aries is a particularly appropriate celestial feature to highlight in a month that takes us from autumn into the winter season.

Though Aries (Latin for ram) is not very recognizable as its namesake, this midsize constellation does contain three moderately bright stars, of which Hamal is the most luminous at a magnitude of 2.0. Hamel is officially classified a red giant star and its light has a definite orange hue. It is estimated to be about 3.4 billion years old and is 66 light-years away. The other two stars, Sheratan and Mesarthim, together with Hamal, denote the ram’s head.

Aries is sandwiched between Pisces to the west and Taurus to the east. Look for it nearly overhead at around 8 p.m. during December.

Aries enjoys special status among the other members of the Zodiac. In ancient antiquity, when many of the constellations were being established, the vernal equinox (intersection of the celestial equator and the ecliptic at the ascending node) resided in Aries. When the sun arrived at this location on its annual journey across the sky, marking the beginning of spring in the Northern Hemisphere, there was reason for celebration. However, because the earth’s axis of rotation precesses over time, that point is not fixed in space but slowly moves westward along the ecliptic. While today the vernal equinox is in the constellation of Pisces, many still refer to it as the First Point of Aries.

Since Aries lies well away from the plane of the Milky Way it’s not surprising that deep sky objects are hard to come by. There are no star clusters and only a handful of galaxies, though none of them are Messier objects. Still, there is one galaxy worthy of mention.

NGC 772 is an unbarred spiral galaxy that is around 200,000 light-years in diameter, about twice the size of our own Milky Way galaxy. Surrounded by several satellite galaxies, it is about 130 million light-years away. Two supernovae have been observed in NGC 772, both discovered within 49 days of each other in 2003.

In Greek mythology, Aries was the golden ram that Phrixus, son of Athamas, king of Boeotia, and Nephele, rescued and took to Colchis on the eastern coast of the Black Sea. There he sacrificed the ram to the gods then placed its skin, the golden fleece, inside the Colchis temple.

The annual Geminid meteor shower is expected to peak on the evening of December 13-14. Because moonlight will not interfere this year, counts could exceed 50 meteors per hour! The source of the meteors seems to be associated with debris from an object call 3200 Phaethon, which is probably an asteroid or a burnt-out comet orbiting the sun. The Geminids and Perseids in August, are the two most prolific meteor showers of the year.

This month officially marks the beginning of winter in the Northern Hemisphere. On Monday December 21, at 2:02 a.m. PST the sun will reach its southernmost latitude and stand directly above the Tropic of Capricorn, resulting in the least amount of daylight in a 24-hour period for the year.

As the month begins, Jupiter and Saturn can be found in the low southwestern sky at dusk. On December 21, the two gas giants come into conjunction and are separated by just 0.1 degrees. Near the end of the month both planets will set at about 6:30 p.m. Mars is still shining brightly in the southeastern sky, in the southern sky at month’s end. Brilliant Venus rises at about 5 a.m. early in the month. Mercury is too close to the sun to be seen this month but will emerge as an evening object early in 2021.

As the month begins, the moon’s degree of illumination decreases over time, until new moon arrives on December 14. The moon will then progressively wax until the Full Cold Moon appears on December 29.