This is Dudley Observatory’s Skywatch Line for Friday, July 20 through Sunday, July 22, written by Sam Salem.
On Friday, the Sun rises at 5:35am and sets at 8:28pm; the waxing gibbous Moon sets at 12:43am and rises at 2:12pm. The first quarter Moon occurred on Thursday at 3:52pm. On Friday evening, the waxing gibbous Moon is positioned just three degrees north of Jupiter. The Moon will be closest to Jupiter at around 11:15pm. Lunar Straight Wall, or Rupes Recta, can be seen on Friday evening using a telescope. The Straight wall is a linear fault on the Moon in the southern part of the Mare Nubium. The Rupes Recta casts a wide shadow that gives it the appearance of a steep cliff when the Sun illuminates the feature at an oblique angle. This takes place about day 8 of the Moon’s orbit.
The 0.8-magnitude Mercury is wrapping up its current apparition and sets one hour after the Sun. Venus gleams at magnitude –4.2 due west at dusk. Jupiter, at magnitude –2.2, is already past the meridian at sundown. Aim your scope at Jupiter early to spot it higher in the sky to increase your chances of experiencing steady seeing conditions. You don’t have to wait until dark to observe Jupiter as the planet shows up well at dusk, soon after sunset. Jupiter belts and the Great Red Spot are more pronounced when contrasted against a deep blue, twilight sky.
Saturn, at magnitude 0.1, culminates around 11:15pm this weekend. Saturn hangs low in the south. Shift your scope to catch Saturn rings on full display, just one degree shy of their maximum tilt. This makes the Cassini division easier to sight.
The excitement is mounting as we count down to Mars’ most favorable opposition since 2003. Mars reaches opposition next week, and, at magnitude –2.7, noticeably outshines Jupiter. Mars rises around 9:15pm and hits the meridian a little after 1:30am. Mars’ disk this weekend spans 23.7 arc seconds, which is only a hair shy of the maximum it will attain during closest approach on July 31. However, Mars hangs low in the sky and the dust storm obscure virtually all Mars’ surface markings, no matter how powerful the telescope you use. Watch and hope that the dust settles soon.
This week, the International Astronomical Union’s Minor Planet Center published the orbits for 12 newfound Jovian moons, bringing Jupiter’s total to 79 moons. The team at Carnegie Institution for Science, along with collaborators at the University of Hawaii and Northern Arizona University were able to spot a bit fainter objects than anyone has been able to go in the past. This led the team to the discovery of Jupiter’s 12 new moons. Three of the new moons travel in sync with Jupiter’s rotation, or in prograde orbit. The other 9 have backward, or retrograde, orbits. The smallest newly discovered moon, Valentudo, has a prograde orbit that crosses the retrograde moon orbits, setting the stage for possible moon-moon collision.