Skywatch Line for Friday, December 26, through Sunday, December 28, written by Sam Salem
This is Dudley Observatory’s Skywatch Line for Friday, December 26, through Sunday, December 28, written by Sam Salem.
On Friday, Sun rises at 7:25am and sets at 4:27pm; Moon rises at 11:11am and sets at 11:23pm.
On Friday evening, the waxing crescent Moon will stand roughly 40° high in the southwest, with Saturn to the lower left of the Moon. Later that night, the Moon passes 3 or 4 degrees north of Saturn at 11pm. They will set a little before midnight. The Moon will also be near Saturn on Saturday night.
First-quarter Moon occurs at 2:10pm on Saturday. First quarter Moon rises around noon and sets around midnight. At dusk the Moon shines a fist or a little more to the upper left of Saturn. Fomalhaut sits nearly three fists below Saturn. They are almost exactly equal in brightness. By 8pm, the Moon hangs straight above Saturn, lower in the southwest.
On Sunday, a day past first quarter Moon, use a telescope to view the sunrise terminator (day/night line) crossing the middle of big, round, Mare Imbrium ringed by mountains and exhibiting flat-floored Plato just off its northern rim. Near the Moon’s south limb, the terminator has unveiled much bigger Clavius with its unique curving arc of four internal craters. Also, you can watch the Lunar X and V on the Moon using binoculars or a telescope when the sunlight hits the rims of specific craters at the lunar terminator creating temporary bright shapes, typically around the First Quarter Moon phase, These effects are due to long shadows from crater rims and are most striking as the Sun rises over that part of the Moon. Use a Moon map to help you locate the lunar mountains and craters and check specific time for best few hours of viewing the Lunar X and V.
Jupiter, magnitude –2.6 in eastern Gemini, rises in the east-northeast soon after full dark. It dominates the eastern sky as the evening proceeds, then the high southeast. Castor and Pollux shine nearby.
Saturn shines at magnitude 0.7, still easy to pick out with the naked eye. Through a telescope, its disk stretches 17” across, its rings some 39” from end to end. Those rings are tilted just 0.8° to our line of sight, appearing close to edge on. The planet’s brightest moon, Titan, lies just over 2’ west of the center of the planet. Far to the east, Lapetus, Saturn’s unique two-toned moon named after Greek myth, is at its faintest (12th magnitude) nearly 8.5’ away. Closer to the ringed planet are 10th-magnitude Tethys, Rhea, and Dione. These three moons cluster close to each other on Saturn’s western side tonight, inside the orbit of Titan.
Drop your gaze far below Saturn toward the horizon to locate a similarly bright light, only about 13° high. This is magnitude 1.2 Fomalhaut, the brightest star in Piscis Austrinus the Southern Fish. This star is famous for the large, dusty disk of planet-forming material around it. Several years ago, astronomers found what they believed was a young planet orbiting within the disk, which they named Fomalhaut b. In 2014, after 10
years of observations, it disappeared from view. Astronomers ultimately determined Fomalhaut b likely hadn’t been a planet at all, but a collection of debris from the previous smash-up of protoplanetary pieces within the disk. As that debris cloud expanded and spread out, it faded until it was no longer detectable from Earth.