Goodbye Jupiter, and hello Mercury and Mars.
It’s slim pickings for planet watchers this week. Saturn is pretty much the only game in town if you’re looking for a good telescopic target. And even the ringed wonder is well past its prime. It’s less than 20 degrees above the horizon at the end of astronomical twilight and sets a little before 1 a.m., local daylight time. Mercury is at the beginning of an evening apparition. The fast-moving planet shines at magnitude –0.7, but is very low in the west. It sets only 45 minutes after the Sun. Meanwhile, in the morning sky, Mars is poised at the start of a new apparition that will culminate in May, 2016, when it reaches opposition. The 1.7-magnitude planet rises in twilight, about 1½ hours ahead of the Sun.
This morning the planet Uranus will be near the waning gibbous Moon. When they’re at their closest, just before 3 a.m., PDT, they’ll be separated by 1¼ degrees. You’ll need binoculars or a small telescope to view them together.
If you’re up for a challenge, try for the tight pairing of Mercury and Jupiter this evening. The two planets will be separated by 2/3 degree in bright twilight. Indeed, your best chance at finding them is with a computer-controlled go-to telescope that will allow you to begin your search immediately after sunset. Otherwise, a keen eye, binoculars and a completely unobstructed western horizon are your only hope.
Later, at 10:02 p.m., EDT, the Moon reaches last quarter phase, though it doesn’t rise until midnight.
The dawn sky is adorned with the crescent Moon forming an attractive triangle with the Pleiades star cluster and Aldebaran, the brightest star in the constellation Taurus. A fine naked-eye sight for early risers.
Although moonless skies beckon over the August 7 – 9 weekend, you might want to take a little time to get in some late-season Saturn observing before turning your attention to the deep sky. As noted above, the ringed planet is well past its prime, which owing to its southern declination, was quite short lived this apparition. But you can pick up the planet in fading twilight when it’s still 25 degrees above the south-southwest horizon. That’s not ideal, but it’s high enough to allow good, steady seeing conditions quite often.
So what is there to see? Rings, obviously — that’s what Saturn is famous for. But don’t neglect the planet’s brightest moons. Titan is the one you’re most likely to spot first. It shines gamely at magnitude 8.4 and is usually the brightest point of light near the planet. But if you have a 8-inch scope, you could see as many as four additional Saturnian satellites, including Rhea (magnitude 9.7), Tethys (10.3), Dione (10.4) and Enceladus (11.8). But even that is just scratching the surface. Saturn is second only to Jupiter when it comes to hoarding moons. At last count, the planet lays claim to a mind boggling 62 (or 64, depending on whose list you consult) satellites ranging in size from 5,149 kilometres (Titan) to moonlets no larger than 6 km across. Unfortunately, apart from the five already mentioned, this bountiful harvest of satellites lies beyond the reach of backyard telescopes. The RASC’s Observer’s Handbook gives the positions for Saturn’s brightest moons as does this handy on-line calculator.
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