Long nights at the end of the year have most of us rising well before dawn. Though we may long for light, the dark morning sky presents its own beauty worth exploring before the rush of the day.
Venus began rising as the morning star at the end of October as it moved ahead of Earth on the great racetrack of the solar system shown in Figure 1. Since Venus is 30 percent closer to the sun, it moves about 10,000 mph faster in its orbit than Earth does. Every 1.6 years, Venus passes Earth, moving between it and the sun in an event known as inferior conjunction. Mercury, zipping along 30,000 mph faster than Venus, does this almost three times a year, most recently on Dec. 20.
Since Oct. 29, Venus has been moving westward away from the sun, so it is in the morning sky. (Objects east of the sun are in the evening sky.) On Jan. 8, it will be as far west of the sun as it can get, a position known as greatest western elongation. After that, its orbital motion will carry it back toward the sun to pass behind it on Aug. 16, 2011. Meanwhile, it will provide a bright beacon to greet us in these dark mornings of winter.
The sky at 6:30 a.m. on Jan. 9, when Mercury is at greatest western elongation is shown in Figure 2. Venus is the brightest object, so it will be no challenge to find in the southeast, and provides a guide to two of my favorite stars: Zubeneschamali and Zubenelgenubi.
Though now in the constellation of Libra, these stars, with names meaning "northern claw" and "southern claw," used to be part of the constellation of Scorpius whose tail is still below the horizon as the sun rises. Mercury rises just after the bright red star, Antares, the heart of the scorpion. It will not get more than about 15 degrees above the horizon before getting lost in the glare of dawn. Look for it as part of a triangle with Venus and Antares. It may be easiest to spot it first with binoculars, then pick it out with your eye once you know where it is.
Venus and Mercury aren't the only planets in the morning sky, Saturn rises at 1 a.m., just south of Arcturus (Ark-TOUR-Us), the second brightest star in the northern sky. By 6:30 a.m., as shown, Arcturus is high above Venus and Saturn can be found above Spica (SPIKE-uh, "the ear of grain") in Virgo. To find these stars without the aid of planets, start at the Big Dipper, follow the arc of the handle in an "arc to Arcturus," then make a "spike to Spica." Below and right of Spica is the kite-shaped constellation of Corvus, the crow, that stands out in a region of fairly dim stars.
In the course of the winter, Venus will seem to hover in the southeast, slowly dropping back toward the horizon and diming as it grows smaller with increasing distance from Earth. Meanwhile, as Earth moves about one degree along its orbit each day (moving 360 degrees in 365 days), the wedge of sky we see at night shifts by about one degree each night. Thus the stars seen at 6:30 tomorrow morning will be one degree farther west the following morning, having risen four minutes earlier (Earth rotates 360 degrees in 24 hours, 15 degrees in one hour or 1 degree in four minutes). This means the stars will rise past Venus as it lingers just above the horizon at dawn. Saturn moves much more slowly in its orbit, so it will move only about 3 degrees away from Spica toward Porrima (Pour-EE-Ma, the goddess of prophecy) as it rises earlier and earlier with the stars. In early April, Saturn will rise at sunset and be bright in the evenings of spring. It will have brightened due to being 1.3 billion miles closer to us by then as Earth will have caught up to the ringed world, lumbering 44,000 mph slower in its orbit.
With Christmas behind us, Epiphany, the Feast of the Magi, approaches, and questions about the Star of Bethlehem arise. If you're interested in this topic, I invite you to The Wild Center in Tupper Lake for presentations and discussion by myself and Jan Wojcik (director of the Clarkson Reynolds Observatory) tomorrow night. The lecture will begin at 7 p.m., but we invite you to join us at 6:30 p.m. for eggnog, cookies and homemade fudge.
If you have any questions about astronomy, please visit the Adirondack Public Observatory website at apobservatory.org or e-mail Aileen at firstname.lastname@example.org.