This is Dudley Observatory’s Skywatch Line for Friday, January 26 through Sunday, January 28, written by Sam Salem.
On Friday, Sun rises at 7:16am and sets at 5:01pm; the Waxing Gibbous Moon sets at 1:55am and rises at 12:30pm. The Waxing Gibbous Moon shines to the right of Aldebaran, the brightest star in the constellation Taurus the Bull, and lower left of the Pleiades on Friday evening.
On Sunday evening, look for the Waxing Gibbous Moon and notice the stars nearby. The Moon is within the Winter Circle stars that night. All the stars of the Winter Circle, or Winter Hexagon, are first-magnitude stars. Therefore, you would be able to find them easily in the drenching moonlight. Start at Capella and move clockwise to Aldebaran, Rigel, Sirius, Procyon, Pollux, and Castor. This asterism consists of separate stars in different constellations. These same bright stars can be seen before dawn every late summer and early fall. They can be seen in the evening every winter, hence, the name Winter Circle.
Arrayed across the southeast sky one hour before sunrise you’ll find Jupiter, Mars and Saturn. Jupiter, gleaming at magnitude –1.9, is almost due south and has an altitude of nearly 30 degrees. It is high enough above atmosphere’s turbulence to be a good telescopic target this weekend. Saturn sits only 6 degrees high in the southeast. Saturn will gradually climb higher over the coming months. Mars is positioned 8½ degrees east-southeast of Jupiter. Like Saturn, Mars position will steadily improve over the coming months.
The lunar disk is well positioned high on the ecliptic as it passes through Taurus to Gemini. There’s plenty to see along the lunar terminator each night. Sinus Iridum, the “Bay of Rainbows” is particularly well placed on Friday and Saturday. This “bay” is the ancient remains of a huge crater. Although the original impact produced an impressive 260-kilometers-wide depression, all that’s left today is the arc of the Jura Mountain range, the northern part of the crater’s rim. It’s likely that the missing southern section is buried under Imbrium lavas. On Friday evening, when the terminator crosses Sinus Iridum, you could have a fine view of the Golden Handle, the curving arc of the Jura Mountains extending past the terminator into darkness. This is an astounding sight in any telescope. Look for the subtle wrinkles that delineate Iridum’s submerged southern rim. Look again on Saturday evening, by then the alluring “Bay of Rainbows” will be fully illuminated.
On January 28, 1613, Galileo may have unknowingly viewed the, then, undiscovered planet Neptune. His observing records show a “star” that doesn’t appear in modern star catalogues. When it happened to be very near Jupiter, he thought it was just a star. On two successive nights he noticed that it moved slightly with respect to another nearby star. But on the subsequent nights it was out of his field of view. Had he seen it on the previous few nights Neptune’s motion would have been obvious to him. Unfortunately, cloudy skies prevented observations on those few critical days. Since Galle made his acknowledged discovery of Neptune on 23 Sep 1846, the planet has only completed about one orbit around the Sun, which takes 164 years and 280 days.