English: Lunar craters Theophilus & Cyrillus

Imaged 2013 SEP 24, 10:08 UT

The Moon returns to the evening skies this week, wending her way through the dim autumnal constellations as she waxes to First Quarter phase, which occurs on the 10th at 12:57 am Eastern Standard Time. Look for Luna’s crescent above the dazzling disc of Venus in the southwestern sky at dusk on the 6th. For the rest of the week she’s pretty much on her own. With little competition from other bright attractions this will be another good week to get to know the Moon’s many interesting attractions. Virtually any optical aid will transform her from a silver-flecked light in the sky to a place with stark, sun-drenched landforms. On the evening of the 8th point your telescope at the Moon to see one of my favorite lunar landmarks, the prominent "double crater" pair of Theophilus and Cyrillus. They will be very prominent, about halfway between the cusps along the terminator line. Theophilus is the fresher of the two craters, with much of its ejecta blanket filling the floor of Cyrillus. Theophilus is about 100 kilometers (61 miles) in diameter and 3.3 kilometers (11,000 feet) deep. Its prominent central peak rises nearly 1.5 kilometers (one mile) above its floor. As impressive as this crater may appear, looks can be deceiving. If you were able to stand on the floor of the crater about halfway between the terraced walls and central peak, you would think you were on a vast, flat plain. Since the Moon is a much smaller sphere than the Earth, your apparent horizon is much closer, on the order of 1.5 kilometers (one mile), and the curvature of the surface drops away very quickly.