This is Dudley Observatory’s Skywatch Line for Friday, June 16, through Sunday, June 18, written by Sam Salem.
On Friday, Sun rises at 5:17am and sets at 8:36pm; Moon rises at 12:39am and sets at 11:58am; the last quarter Moon occurs on Saturday at 7:33am. One half of the lunar disk is always illuminated by sunlight, while the nighttime half is submerged in the Moon’s own shadow. At last quarter Moon, we see half the Moon’s day side, and half its night side. The lunar terminator is the division between the illuminated and dark parts of the Moon. It’s along the terminator that you have your best three-dimensional views of the lunar terrain through binoculars or the telescope. Try looking in the morning twilight, when the sky isn’t quite so dark, to eliminate glare from the Moon itself.
The first celestial objects you will notice in the night sky are Jupiter and Mars. These planets light up the sky as soon as the Sun goes down. Look nearly due south, roughly midway between these two planets, for sparkling blue-white Spica, in Virgo. As nightfall deepens into later evening, watch for some fainter stars to become visible. That’s when the constellations of Corvus the Crow, Crater the Cup, and Hydra the Water Snake will come into view. None of these constellations has any bright stars. Hydra is the longest constellation in the sky measuring 1303 square degrees.
Saturn reached opposition on Thursday. That means it’s up from dusk to dawn and culminates at around 1 am.
The Summer Triangle asterism marks the seasons. When the stars of the Summer Triangle light up the eastern twilight dusk in middle to late June, it’s a sign of the spring season giving way to summer. When the Summer Triangle is seen high in the south to overhead at dusk and early evening, the Summer Triangle’s change of position indicates that summer has changed into fall. As night falls in June and July, look east for a sparkling blue-white star, Vega, the brightest star in the constellation Lyra, the Harp. Vega is the brightest of the Summer Triangle’s three stars. Look to the lower left of Vega for another bright star, Deneb, the brightest in the constellation Cygnus, the Swan and the third brightest in the Summer Triangle. An outstretched hand at an arm length approximates the distance from Vega to Deneb. Look to the lower right of Vega to locate the Summer Triangle’s second brightest star, Altair, the brightest star in the constellation Aquila, the Eagle. A ruler held at an arm length fills the gap between these two stars. Under a dark sky on a moonless night, you’ll see the great swath of stars of the Milky Way passing in between the Summer Triangle stars Vega and Altair. The star Deneb bobs in the middle of this river of stars that passes through the Summer Triangle, and arcs across the sky.
With the Milky Way high overhead in the middle of the night and the absent Moon during evening hours this weekend, our home galaxy’s globular clusters are on display. M13, in Hercules, is one of the finest examples. M13 is faintly visible to the unaided eye at a dark rural site. In binoculars it looks like a fuzzy “star,” but a telescope reveals the details of the globular. Home to almost a half million stars, uncountable stellar points are arrayed against a glowing core of densely packed starlight. Try different telescope magnifications. High power reveals individual stars best, but lower magnification presents M13 nicely situated within its star field.