Aldebaran greets the Moon while Mercury and Venus hang low.
There’s a veritable planetfest in this week’s evening sky—but you can enjoy it only if you have an unobstructed western horizon. Jupiter, Mars and Saturn are easy to identify in fading twilight, as they form an attractive string of lights along the horizon, from south to west. All three are bright: Jupiter is magnitude –1.8; Mars is magnitude –0.9; and Saturn is magnitude 0.3. And you can add two more targets to your tally if you to start early and scan the west horizon with your binoculars. With a little luck you might sweep up Mercury and Venus. At magnitude –3.9, Venus will be the easier catch of the two. You should be able to spot it moments after the sun goes down, when the planet has an elevation of about 5 degrees. Venus sets a bit north of due west, 45 minutes after the Sun. Somewhat higher, but a lot fainter, is Mercury. It shines at magnitude –0.3 and dips below the horizon one hour after the Sun. Both inner planets are at the start of their evening apparitions, so if you don’t pull off a sighting under this week’s challenging conditions, don’t despair—the situation will gradually improve.
The Moon reaches last-quarter phase today at 5 p.m., EDT.
The Southern Delta Aquarid meteor shower peaks tonight. It’s a modest display that, under ideal conditions, produces some 20 meteors per hour, radiating from a point roughly 15 degrees north of the first-magnitude star Fomalhaut, which lies due south a little before 4 a.m., local daylight time.
This morning, the Moon and Aldebaran, the brightest star in Taurus, have a very close encounter. How close depends on where you live. Observers in the prairie provinces are best situated as the Moon is nearest Aldebaran at 4:33 a.m., MDT (6:33 a.m., EDT). From Calgary, the lunar disc will pass only ¼ degree south of the distinctly reddish, 1st-magnitude star. The close encounter should be a lovely sight in binoculars, a small telescope or even with your bare eyes.
The Moon is largely out of the picture over the July 29 – 31 weekend, so deep-sky observers can enjoy one of only a handful of moonless summer weekends. For observers at the latitude of Edmonton, Alberta (54 degrees north), this is the first summer weekend that features any astronomical darkness! One class of deep-sky object demanding moonless country skies is the so-called dark nebulas. Indeed, if you can’t see the Milky Way really well from your observing site, these opaque interstellar clouds of dust and gas are impossible to spot.
When you look along the band of the Milky Way, you’ll notice numerous dark rifts and patches. One conspicuous area, known as the Northern Coal Sack, hides in the celestial weeds southeast of 1.2-magnitude Deneb, the brightest star in Cygnus. This feature is part of a much larger dark assemblage known as the Great Rift, which bisects the summer Milky Way for much of its length.
Another well-known dark nebula is Barnard’s E, located a few degrees northwest of 0.9-magnitude Altair, the brightest star in Aquila. Use your binoculars (or a small, wide-field telescope) to detect a ragged, E-shaped blackish blob about one degree due west of 2.7-magnitude gamma (γ) Aquilae. Observing the ghostly “E” takes some getting used to, but once you train your eye to look for an absence of starlight, you’ll be surprised at the number of other dark nebulas you can pick up with binoculars.
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