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May 25: Three planets after sunset
Tonight, let’s compare the appearance of the Moon to that of a week ago to see the effects of lunar libration, the “nodding” motion caused by the tilt of the Moon’s orbit around Earth.
High in the south shortly after sunset, the waxing Moon is in Virgo, not far from the bright star Spica. With much of its face now illuminated, look in the center of the lunar northern hemisphere for Mare Serenitatis. North and just slightly east of this ancient lava plain is a crater normally situated much closer to the limb: Endymion. Shallow and flat-bottomed, Endymion typically appears oval-shaped due to foreshortening, but tonight it should look more circular, as it appears farther from the limb thanks to libration. Stretching some 76 miles (122 km) across, Endymion is some 3.9 billion years old.
Compare the images below, showing the location of Endymion tonight and a week ago, on the 19th. Together, they illustrate the magnitude of change libration can bring.
Sunrise: 5:37 A.M.
Sunset: 8:18 P.M.
Moonrise: 4:08 P.M.
Moonset: 2:51 A.M.
Moon Phase: Waxing gibbous (84%)
*Times for sunrise, sunset, moonrise, and moonset are given in local time from 40° N 90° W. The Moon’s illumination is given at 10 P.M. local time from the same location.
