This weekend, August 16 - 18, look for the lunar craters Ptolemaeus, Hell, Bonpland & Parry, and the Open Clusters NGC 7150, NGC 7235, and NGC 6811
Friday, August 16
Tonight the waxing Moon's most notable features will be the vast area of craters dominating the south-central portion near and along the terminator. Now emerging is Ptolemaeus - just north-northeast of Albategnius. This large round crater is a mountain-walled plain filled with lava flow. With the exception of interior crater Ptolemaeus A, binoculars will see it as very smooth. Telescopes, however, can reveal faint mottling in the surface of the crater's interior, along with a single elongated craterlet to the northeast.
Despite its apparent uniformity, close inspection has revealed as many as 195 interior craterlets within Ptolemaeus! Look for a variety of interior ridges and shallow depressions.
When the Moon is low to the southwest, we have a chance to go northeast to Cepheus for a new study - NGC 7160 (Right Ascension: 21:53.7 - Declination: +62:36). At magnitude 6.1, this small open cluster is easily identified in scopes and may be seen as a faint starfield in binoculars. You'll find it about a finger-width north of Nu Cephei.
Saturday, August 17
On the lunar surface tonight, we'll start by following the southward descent of the large crater rings Ptolemaeus, Alphonsus, and Arzachel, to a smaller, bright one to the southwest named Thebit. We're going to have a look at Hell...
Just west of Thebit and its prominent A crater to the northwest, you see the Straight Wall - Rupes Recta - appearing as a thin, white line. Continue south until you see large, eroded crater Deslandres. On its western shore, is a bright ring that marks the boundary of Hell. While this might seem like an unusual name for a crater, it was named for an astronomer - and clergyman!
Once you've been to Hell, let's go to the heavens for NGC 7235 (Right A scension: 22:12.6 - Declination: +57:17). Locate the star crowded area of Epsilon Cephei which will also include this 7.7 magnitude open cluster in the same low power field. Give it a try. Look for a small, rectangular assortment of 10th magnitude and fainter stars, including a beautiful ruby red, west-northwest of Epsilon.
Sunday, August 18
Due south of mighty Copernicus on the eastern edge of Mare Cognitum, you will see a ruined pair of flattened craters. They are Bonpland and Parry - with Frau Mauro just above them. The smallest and brightest of these ancient twins is the eastern Parry. Have a look at its south wall where a huge section is entirely lost. It was near this location that Ranger 7 ended its successful flight in 1964. Just south of Parry is another example of a well-worn Class V crater. See if you can distinguish the ruins of Guericke. Not much is left except for a slight U-shape to its battered walls. These are some of the oldest visible features on the Moon!
If you'd like to head for something very young, have a look at 6.8 magnitude open cluster NGC 6811 (Right Ascension: 19:37.3 - Declination: +46:23) in Cygnus. This mid-sized, unusually dense open cluster is found less than finger-width north-northwest of Delta - the westernmost star of the Northern Cross. Like most open clusters, the age of NGC 6811 is measured in millions, rather than billions, of years. Visible in binoculars on most nights, telescopes should show a half dozen or so broadly-spaced resolvable stars overlaying a fainter field. Be sure to return again on a moonless night, and have another look a disparate double Delta!
Until next week. Wishing you clear skies!
About Tammy Plotner - Tammy is a professional astronomy author, President Emeritus of Warren Rupp Observatory and retired Astronomical League Executive Secretary. She's received a vast number of astronomy achievement and observing awards, including the Great Lakes Astronomy Achievement Award, RG Wright Service Award and the first woman astronomer to achieve Comet Hunter's Gold Status.