The ever-moving lunar terminator has shifted to throw three prominent craters into contrast: Ptolemaeus, Alphonsus, and Arzachel.
Ptolemaeus, Alphonsus, and Arzachel appear near the lunar terminator just south of the Moon’s center tonight. Credit: Consolidated Lunar Atlas/UA/LPL
Already high in the sky at sunset, the Moon is some 45° high in the southwest an hour after the Sun disappears. Fix your telescope on the center of the Moon, near the terminator dividing lunar night and day. Just south of the lunar equator is a series of three craters, starting with Ptolemaeus. About 95 miles (153 kilometers) wide, its floor appears at first glance quite smooth except for one notable pockmark in the northeast. But Ptolemaeus is actually covered in craterlets that have been buried over time by ejecta from nearby impacts. Study its broad floor to see if you can find any telltale depressions.
Just south of Ptolemaeus is Alphonsus, nearly 70 miles (110 km) wide. This crater hosts a noticeable central peak as well as a north-south ridge spanning its entire width.
Southernmost of the trio is Arzachel. It is smallest (60 miles [96 km]) and youngest of the three, also hosting a central peak. Compare this crater’s walls and other features to Alphonsus — because Arzachel is younger, it is more sharply defined as it has had less time for impacts both near and far to affect its landscape.
Sunrise: 5:33 A.M.
Sunset: 8:24 P.M.
Moonrise: 1:22 P.M.
Moonset: 1:39 A.M.
Moon Phase: Waxing gibbous (56%)
*Times for sunrise, sunset, moonrise, and moonset are given in local time from 40° N 90° W. The Moon’s illumination is given at 12 P.M. local time from the same location.
For a look ahead at more upcoming sky events, check out our full Sky This Week column.
