The Moon meets Saturn while Mercury and Venus pair up at dusk.
The evening sky is host to three bright planets. Brightest of all is Jupiter, which gleams at magnitude –1.8, but lies only about 15 degrees above west horizon an hour after sunset. For telescope users, the Jupiter season is near its end—the planet is too low for detailed views for the most part. Fairing a little better is Mars. It’s still bright (magnitude –1.1) and some 20 degrees above the south horizon as darkness falls. The red planet displays a distinctly gibbous phase and has a diameter of just under 15 arc seconds. Farthest east is Saturn, which is arguably the most rewarding planet in a telescope at the moment. It transits the meridian at about 10:30 p.m., local daylight time. Saturn’s rings are favourably tilted toward Earth this apparition and show up nicely even in small telescopes.
The Moon reaches first-quarter phase at 8:52 p.m., EDT, this evening.
Tonight the distant planet Neptune lies just ½ degree south of the 3.7-magnitude star lambda (λ) Aquarii. In other words, if you can find the star, you should be able to easily spot Neptune in your telescope or binoculars. The 7.8-magnitude planet remains near the star for the next few weeks.
This evening the waxing gibbous Moon will lie near Saturn. They will be closest at around 2:13 a.m., EDT (July 16), when they are less than 2½ degrees apart. A fine binocular or naked-eye sight.
If you have an completely unobstructed west-northwest horizon and feel up for an observing challenge, try sighting Mercury and Venus very close together in the dusk sky. The planetary pair are bright (magnitudes –1.1 and –3.9 respectively), but have an altitude of barely more than 5 degrees at sunset. Binoculars or a small telescope (ideally a GoTo model) are a must. At their closest (at 6:48 p.m., EDT), the duo will be separated by just ½ degree.
With the Moon lighting up the sky over the July 15 – 17 weekend, searching out faint fuzzies isn’t really in the cards. But there are still deep-sky treasures aplenty. One of the season’s prettiest sights is the lovely double star, Albireo. This gem has three things going for it. First, it’s easy to find. Situated in the constellation Cygnus, Albireo marks the foot of the Northern Cross. Second, its component stars are both reasonably bright, at magnitudes 3.4 and 4.7. That means even in strong moonlight, both stars are easy to see. Third, and most important, Albireo looks wonderful in just about any telescope. That’s largely thanks to the contrasting colours of its two suns.
The brighter star is a lovely, golden yellow, while its companion is a pale, icy blue. These colours tell us something about the stars themselves. Yellow-orange stars tend to be relatively cool compared with bluish-white stars—in this case, 4,000 degrees Kelvin versus 12,000 degrees, respectively. Albireo is a real celestial showpiece, and for my money, looks best in small telescopes used at low power. About 25× is optimal—excess magnification puts too much space between those colourful components and robs Albireo of its visual impact.
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