This is Dudley Observatory’s Skywatch Line for Friday, December 27, through Sunday, December 29, written by Sam Salem.
On Friday, Sun rises at 7:25am and sets at 4:28pm; the waxing crescent Moon rises at 8:40am and sets at 5:56pm. On Saturday evening, brilliant Venus appears just 2 degrees above the Moon’s slender crescent. The two brightest objects in the night sky make a stunning pair from early twilight until they set shortly after 7pm.
You will need binoculars to spot Saturn in the glare of evening twilight. Saturn, at magnitude 0.6, is sinking away into the sunset, farther and farther to the lower right of Venus. Saturn sets few minutes after 5:30pm this weekend.
Mars, at magnitude 1.6, is low in the east-southeast, in constellation Libra, in early dawn. It’s on the far side of the Sun and is a very tiny 4 arc-seconds in apparent diameter.
On Friday, Jupiter passes behind the Sun from our perspective. This conjunction occurs at 1pm. The Sun’s glare makes it impossible to see the planet. Jupiter will return to view before dawn in mid-January.
The variable star Algol in constellation Perseus reaches minimum brightness, of magnitude 3.4, around 8:51pm, on Friday. If you start watching it after darkness falls, you can see it more than triple in brightness, to magnitude 2.1, over the course of about five hours. This eclipsing binary star runs through a cycle from minimum to maximum and back every 2.87 days. Algol appears in the eastern sky after sunset and passes nearly overhead around 9pm.
The final week of the year offers a good chance to spot the 10th-largest member of the asteroid belt. The belt is a collection of at least several hundred thousand objects that lies between the orbits of Mars and Jupiter. Asteroid 15 Eunomia is sliding through the Water Jar asterism in the constellation Aquarius. This group of 4th- and 5th-magnitude stars, which comprises Gamma (γ), Pi (π), Zeta (ζ), and Eta (η) Aquarii, lies nearly 20 degrees southwest of the south-western corner of the Great Square of Pegasus. Eunomia currently glows at 10th magnitude. You’ll need a small telescope to pick it up.
Deneb marks the top of the Northern Cross, and the star Albireo marks the bottom. Find the Northern Cross shining fairly high in the west at nightfall. It sinks downward during the evening hours, and stands over the west-northwest horizon around mid-evening.
Friday marks the 448th birthday of Johannes Kepler. The German astronomer is best known for formulating laws of planetary motion which enabled Isaac Newton to devise the law of gravitation. Working from the accurately measured positions of the planets recorded by Tycho Brahe, Kepler mathematically deduced that planets move in elliptical orbits with the Sun at one focus. Kepler suggested that the tides were caused by the attraction of the Moon.