Tonight – April 18, 2017 – the moon won’t rise over your eastern horizon until late, likely after midnight. Given clear skies, you should easily see the moon in the morning sky, even after sunrise, on April 19. The moon will be at or near its half-lit last quarter phase, when the moon’s disk appears half-illuminated in sunlight and half-immersed in its own shadow. It’ll look like the image at the top of this post, which is by Lilliana Mendez of North Bergen, New Jersey.
In the space-based view, of course, the moon is always half-illuminated. Now we’re seeing half the moon’s day side, and half its night side.
The terminator – shadow line dividing the lunar day and night – shows you line of sunset on the waning moon. It’s along the terminator that you have your best views of the lunar terrain through binoculars or the telescope.
The exact last quarter phase for this April, 2017 moon occurs on April 19 at 9:57 UTC. At North American latitudes, that translates to 6:57 a.m. ADT, 5:57 a.m. EDT, 4:47 a.m. CDT, 3:47 a.m. MDT and 2:47 PDT.
You might think the half-illuminated quarter moon should be about half as bright as the full moon. But that’s not the case. The last quarter moon is about 1/12th as bright as a full moon. Astronomers say the moon’s rough, sphere-shaped surface accounts for the surprising difference in the amount of moonlight cast by the quarter moon versus the full moon.
Meanwhile – and this might surprise you – the last quarter moon is slightly fainter than the first quarter moon. The last quarter moon shines at about 1/12th a full moon’s brightness (in contrast to 1/12th for the first quarter moon).
The last quarter moon is slightly less bright than the first quarter moon because the illuminated side of the last quarter moon is more covered over by maria – low lying plains of hardened volcanic basalt. The dark maria reflect sunlight less effectively than do the lighter-colored lunar highlands.
Bottom line: Watch for the moon at or near its last quarter phase. It’ll likely rise after midnight on the morning of April 19, 2017.
Bruce McClure has served as lead writer for EarthSky's popular Tonight pages since 2004. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.