This weekend we head toward a full moon, which occurs on Monday, July 22nd. With such a big and bright object in the sky, observing is limited more toward lunar targets and multiple star systems. So let's try a few new destinations, and enjoy some views that are generally within the reach of smaller telescopes. Remember, when it comes to smaller telescopes, size isn't everything - there's absolute truth to the saying "the best telescope is the one that's used most" - so get yours out!
Friday, July 19, 10:30 PM PDT
Aristarchus, Longitude: 47.4° West, Latitude: 23.7° North.
This 24 mile diameter crater is a showpiece object, paired nicely with its neighbor crater Herodotus and the rough terrain of surrounding region. Aristarchus is a relatively young feature at only 450 million years old. Its walls are especially steep, most notably along its northern edge, and contain excellent terraces. You should also be able to see its very flat floor and elongated central peak. Bump up the magnification to see the numerous craterlets that fan out from and surround this great crater. This target is great in as little as a 50mm telescope.
Vallis Schroter, Longitude: 51.0° West, Latitude: 26.0° North.
Situated very close to Aristarchus and Herodotus, this object called "Schroter's Valley" is a real treat. The valley is a rille, and is quite dramatic. Look just north of Herodotus, you'll find a crater called the "Cobra Head" defining the valley's beginning. The rille snakes to the north, then turns west, a length of 96 miles and varying in width from 10 km down to 500 m at its western end. If you have 100mm telescope, you should have fine views.
Eta Ophiuchi, RA: 17h 10m 22s Dec: -15°43'29.6" (Epoch 2000).
This visual double, also known as Sabik, is easy to see even with tonight's bright moon. And it's close by the moon tonight, only six degrees north-northwest, shining at magnitude 2.4 and 84 light-years distant. This double star forms a very close pair with only 0.6" separation. The components are magnitude 2.9 and 3.4. Can you split this challenging close double in your telescope?
Saturday, July 20, 10:30 PM PDT
Tonight we continue along the Terminator - the shadow line between dark and light on the moon. Objects along this line offer great contrast, making fine features really stand out.
Cavalerius, Longitude: 66.8° West, Latitude: 5.1° North.
Paired with Hevelius, these two craters are a well placed pair under tonight's lighting conditions. Note the steep slopes around Cavalerius and the high terraced inner walls. Cavalerius contains a small elongated central peak, and lots of craterlets and striations across its floor. When viewing Hevelius, check out Rimae Hevelius running mostly north/south on its flat cratered floor. 50 mm telescope gets you these targets.
Grimaldi, Longitude: 68.6° West, Latitude: 5.2° South.
Located near the lunar equator Grimaldi is a large walled plain, 134 miles in diameter. The crater Grimaldi B is embedded in its northern perimeter. Grimaldi is easy to pick out with its smooth dark floor. Also notice striations the many striations across the floor. It is an unusual sight - something of a lone lunar sea isolated surrounded by some very rough terrain. Of course, this sea is really a large lava filled feature.
How about another double star? Move about 8-1/2 degrees south-southeast of the moon, and you'll find Nunki, or Sigma Sagittari, the top star in the handle of the Teapot asterism in Sagittarius. At magnitude 2.04 it is still easy to pick out, even with a big bright moon nearby. The trick though, is seeing its companion - at a dim magnitude 9.5, but a wide enough split at 5.2 arc minutes to make it possible. Nunki is also interesting as it sits close to the ecliptic and can be occulted by the Moon and planets. Amazingly, Nunki spins at 200 km per second, 100 times faster than the Sun!
Next week, back to the deep sky, but for now, enjoy these bright targets!
Lunar images courtesy the Lunar and Planetary Institute.
Mark Wagner is a life-long astronomy enthusiast and deep sky observer. He has spent the past twenty years popularizing amateur astronomy in the San Francisco bay area through his writing and community building. A past president of the San Jose Astronomical Association, he founded what is now the annual Golden State Star Party in California. Please post if you have comments, questions, sketches or images you've taken of the targets mentioned above.