This is Dudley Observatory’s Skywatch Line for Friday, September 6, through Sunday, September 8, written by Sam Salem.
On Friday, Sun rises at 6:26am and sets at 7:22pm; the waxing gibbous Moon rises at 2:42pm and sets at midnight. Look for planet Saturn in the vicinity of the Moon as darkness falls on Saturday and Sunday. See the Moon to the west of Saturn as darkness falls on Saturday, and then to the east of the Moon as darkness falls on Sunday. Saturn is a bit brighter than a 1st-magnitude star, but it still might be hard to see in the Moon’s glare. Place your finger over the obtrusive waxing gibbous Moon for a better view of Saturn, the most distant planet that you can easily see with the eye alone.
The early evening sky features five stars of first magnitude plus a pair of conspicuous planets. Arcturus, Antares, Altair, Deneb, and planets Jupiter and Saturn are visible in the early evening sky, in addition to the waxing Moon. Jupiter, at magnitude –2.2, begins losing altitude before sundown, and sets before midnight. The observing window for Saturn is open a little wider than for Jupiter. The ringed planet is at its highest and due south at roughly 9:00pm. Saturn never strays far from the horizon as it resides in eastern Sagittarius. Saturn usually looks good in just about any telescope. Try to spot Saturn’s brightest moon, 8.6-magnitude Titan. Most of this week, Titan is located about three ring diameters east of Saturn. Titan is the brightest star-like dot nearest the planet.
On Friday night, Neptune has its closest approach to 4.2-magnitude star, Phi (φ) Aquarii, in eastern constellation Aquarius. When you center Phi Aquarii in your scope, you should easily be able to spot the 7.8-magnitude Neptune too. Neptune moves so slowly in our night sky. It’ll be near Phi for several nights before and after this date. This conjunction is a good opportunity to observe Neptune If you haven’t managed to locate this faraway planet before.
The bright moonlight this weekend puts faint deep-sky targets out of reach for the most part. However, bright double stars are relatively unaffected by moonlight or light pollution. One of the finest and best-known examples is Epsilon (ε) Lyrae, known as the Double-Double. Even steadily held binoculars will readily show the two main component stars, Epsilon1 and Epsilon2. They shine at magnitude 5.0 and 5.3, respectively, and are separated by a generous 210.5 arc-seconds. The challenge lies in seeing each of those stars as double. Epsilon1 is the more northerly of the pair. It has a 6.1-magnitude companion sits just 2.1 arc-seconds away. Epsilon2’s partner shines at magnitude 5.4 and is separated by 2.4 arc-seconds. The nearly equal magnitudes and marginally wider gap of Epsilon2 makes it slightly easier to split than Epsilon1. The key to resolving all four component stars is using sufficient magnification on a night with reasonably steady seeing conditions so that the individual stars in each pair don’t blur together. Individually, each pair in the Epsilon set is a lovely sight. However, the very best view is in an eyepiece that provides enough magnification for a clean split, yet retains a wide enough field of view to easily contain all four stars.
Observe the Lunar Straight Wall on Saturday evening. The Straight Wall, or Rupes Recta in Latin, is a linear fault on the Moon, in the southeastern part of the Mare Nubium. When the Sun illuminates the feature at an oblique angle at about day 8 of the Moon’s orbit, the Straight Wall casts a wide shadow that gives it the appearance of a steep cliff. The fault has a length of 110 km, a typical width of 2–3 km, and a height of 240–300 meter. This is the most well known steep slope on the Moon. Use a lunar map to help you locate this most popular lunar target for amateur astronomers.