Skywatch Line for Friday, May 10 through Sunday, May 12, 2019

This is Dudley Observatory’s Skywatch Line for Friday, May 10 through Sunday, May 12, written by Sam Salem.

On Friday, Sun rises at 5:39am and sets at 8:06pm; the waxing crescent Moon rises at 10:34am and sets at 12:59am. The Moon is at first-quarter phase on Saturday at 9:12pm. On Friday evening, the Moon encounters the Beehive Cluster (M44) in constellation Cancer. In a small telescope, watch the dark limb of the Moon occult several cluster stars. The Moon begins to cover the Beehive at roughly 10pm, and takes a little less than two hours to completely traverse the cluster. On Saturday, the Moon shines almost 10 degrees to the right of Regulus. Find Algieba, or Gamma Leonis, above Regulus, only a little fainter. They’re the two brightest stars of the Sickle of Leo, the lion.

Mars, at magnitude 1.7, is the early evening single planet. Mars continues to hang around low in the west as darkness falls, accompanying the lingering bright stars of winter. Mars is passing between the horn-tips of Taurus, Beta and Zeta Tauri this week. Find this lineup far lower left of bright Capella, and farther lower right of Pollux and Castor. Jupiter rises in the southeast at the eastern foot of Ophiuchus a little after 10:30pm. Gleaming at magnitude –2.5, Jupiter culminates before 4am, well before dawn begins, with orange Antares twinkling 14 degrees to its lower right. Saturn clears the southeast horizon around 12:30am. At magnitude 0.4, Saturn is the brightest dot in eastern constellation Sagittarius. Venus is now visible only during bright twilight, very low in the brightening dawn. Just 20 minutes before sunrise, look for it a little above the horizon exactly due east.

First-quarter Moon is high in the evening sky this weekend. On Saturday evening the terminator lies across the crater trio of Ptolemaeus, Alphonsus, and Arzachel. Each one has a distinctive appearance. Northernmost is 164-kilometer-wide Ptolemaeus, the biggest of the three. Notice its smooth mare-covered floor. Immediately south of Ptoelmaeus is 108-km-diameter Alphonsus, which features a curious north-south ridge. Last is Arzachel, arguably the most conventional of the three. Spanning 96 km, Arzachel has a classic crater appearance with its terraced rim and central mountain peak. The 40-km-wide Alpetragius is nestled between Alphonsus and Arzachel. Alpetragius has a large, rounded, central-mountain. It looks like a nest with a single egg inside. This weekend, take a moment to look in on these craters and notice how dramatically their appearance changes with increasing illumination. Use a lunar map to help locating these craters.

Three zero-magnitude stars shine after dark in May. Arcturus shines high in the southeast, Vega sits much lower in the northeast, and Capella lies in the northwest. They appear so bright because each is at least 60 times as luminous as the Sun, and because they’re all relatively nearby. Vega is the brightest star in the northeast late these evenings. Look 14 degrees, about a fist and a half at arm’s length, to Vega’s upper left for Eltanin, the nose of Draco the Dragon. Closer above and upper left of Eltanin are the three fainter stars of Draco’s stick-figure head, also called the Lozenge. Draco always points his nose to Vega no matter how we see them. The faintest star of Draco’s head, opposite Eltanin, is 4th-magnitude Nu Draconis. It’s an equal-brightness double star.

Celebrate National Astronomy Day on Saturday. Join Dudley Observatory celebration at miSci on Saturday from 1-5pm with family-friendly and hands-on activities. Visit the Dudley Observatory website for more information.

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