This is Dudley Observatory’s Skywatch Line for Friday, July 6 through Sunday, July 8, written by Sam Salem.
On Friday, the Sun rises at 5:24am and sets at 8:37pm; the Moon rises at 12:52am and sets at 1:27pm. The Moon reached last-quarter phase on Thursday at 3:51am. Watch the waning crescent Moon, Pleiades star cluster, and bright star Aldebaran in dark eastern skies before dawn on Sunday. Aldebaran is the brightest star in the constellation Taurus the Bull. The Pleiades cluster is fainter, especially as darkness gives way to dawn.
Earth reaches its most distant point from the Sun, aphelion, on Friday at 1:46pm. The word aphelion comes from the Greek words apo meaning away and helios, for the Greek god of the Sun. Earth’s tilt causes the seasons, not Earth’s distance from the Sun. However, Earth’s varying distance from the Sun does affect the length of the seasons. Earth is always farthest from the Sun in early July during northern summer and closest in January during northern winter. That’s because, at our farthest from the Sun, Earth is traveling most slowly in its orbit. That makes summer the longest season in the Northern Hemisphere and winter the longest season on the southern half of the globe. Meanwhile, winter is the shortest season in the Northern Hemisphere, and summer is the shortest in the Southern Hemisphere.
Mercury is coming to the end of its brief evening apparition. Venus, shining at magnitude –4.1, can be spotted minutes after sundown. Mercury sets at 10:03pm and Venus sets at 10:51pm, on Friday night. Like Mercury, Jupiter is winding down its current apparition. The magnitude –2.3 planet is already at the meridian when the Sun sets. Look early to catch Jupiter at its best. Jupiter sets around 1:50am this weekend. Saturn, at magnitude zero, rises around 7:45pm, culminates slightly before 1am, and sets around 4:55am this weekend. Mars rises in the southeast around 10:15pm and reaches the meridian at 3:15am. Mars is only weeks away from its close opposition. Now it exactly matches Jupiter’s brightness and its disk has grown to 21.6 arc seconds diameter.
The darkest sky settles in at around 1am. That’s the ideal hour to enjoy the globular clusters that congregate in and around the Milky Way. The constellations Sagittarius and Ophiuchus harbor seven globulars each. Of the 14 globular clusters scattered across this region, M22, in Sagittarius is the most rewarding. This cluster is a delight in any size telescope. Glowing at magnitude 5.1, M22 is the brightest Messier globular. Locating M22 is straightforward. Using your binoculars or finderscope, you’ll find M22 about 2½ degrees northeast of 2.8-magnitude Lambda (λ) Sagittarii, the star marking the top of the Sagittarius Teapot. Also, you can catch a second globular, M28, in the same finder field one degree northwest of Lambda. At magnitude 6.8 and half the size of M22, M28 is not nearly as impressive an object.