Stars over Sisters

The featured constellation for December is Eridanus, the celestial river. It is the sixth largest, and second longest, constellation in the entire sky. Beginning with the star Cursa, near the left foot of Orion, the river flows westward to the eastern border of Cetus. From here the flow turns eastward before plunging deep into the celestial southern hemisphere, ending at Achemar, the constellation's brightest star.

This month Eridanus lies in the southeastern sky at about 9 p.m. local time. To find it, first locate Rigel in Orion. Cursa lies about 4 degrees to the north northwest of Rigel. Follow an irregular string of dim stars to the west that turns back in an easterly direction. At this point the river turns sharply south, preventing the constellation's southernmost stars from rising above the horizon at our latitude here in Central Oregon. Unfortunately, this includes Eridanus' brightest star, Achemar.

Although there are varying stories associated with the mythology of Eridanus, most of them center around Phaethon, son of Helios the sun god. It was Helios' job to drive his golden chariot across the sky every morning and evening, driving the sun to rise and set. One day a youthful Phaethon restlessly urged his father to let him drive the chariot himself. Reluctantly, Helios agreed, but told him to be cautious, as he didn't think that Phaethon would have the strength to manage the horses.

As dawn drew near in the eastern heavens, Phaethon mounted the chariot. However, he immediately lost control of the reins, and the chariot veered in all different directions of the sky. Zeus, the sky and thunder god, was not amused by this, and shot Phaethon down with a thunderbolt. The Greeks regarded the blazing trail left behind from Phaethon's journey as a river, now known as Eridanus. In another tale, it is the water that flows from Aquarius.

While there are no Messier objects in Eridanus, the constellation does contain many very fine galaxies that are listed in the New General Catalogue (NGC) of deep-sky objects. One of the most visually striking of these is NGC 1300, considered to be prototypical of a barred spiral galaxy. Barred spirals differ from normal spiral galaxies in that the arms of the galaxy do not spiral all the way into the center but are connected to the two ends of a straight bar of stars that contains the nucleus.

Observational evidence suggests that there are more barred spiral galaxies today than existed billions of years earlier. This implies that bars are a sign of maturity among spiral galaxies. NGC 1300 is approximately 110,000 light-years in diameter, slightly larger than our Milky Way galaxy, and is about 69 million light-years away.

Winter in the Northern Hemisphere begins on Thursday December 21, 2023, at 7:27 p.m., when the sun lies directly above the Tropic of Capricorn. The event will result in the fewest hours of daylight of the year.

The peak of the Geminid Meteor Shower occurs on the evening of December 13 until dawn the next morning. A two-day-old moon will set shortly after the sun, ensuring ideal meteor watching without the interference of moonlight. More than 50 meteors per hour are expected to fall. The meteors are caused by Earth intersecting streams of small dust and ice particles shed from asteroid 3200 Phaethon.

Venus will be a dazzling morning "star" during December but will fade slightly by month's end as the planet drops lower in the predawn sky. The remainder of the planetary action takes place on the evening stage.

Appearing high in the east by nightfall, Jupiter will shine brightly nearly all night long. Saturn is there too, only a bit lower in the southwest sky and not as radiant. Mercury shows up during the first half of December standing about 6 degrees above the southwestern horizon before dropping into the sun's glare by month's end. Mars is still too close to the sun to be observed.

Dark sky tip: To help reduce light pollution during the holidays, set a timer on outdoor string lights or arrange to shut them off before 10 p.m. Choose warmer-colored lights, if possible.