Moon and Regulus

The Moon and a bright star huddle close late tonight. Regulus, brightest star of Leo, the lion, rises below the Moon. They climb into good view by 1 or 2 a.m. The Moon will scoot toward Regulus during the night, so they’ll be at their closest at dawn.

The Moon is a bit more than a week past full. Tonight, sunlight will illuminate roughly 40 percent of the lunar hemisphere that faces our way. So it seems logical to assume that tonight’s Moon would appear about 40 percent as bright as a full Moon. But that’s not the case at all. Instead, it’s only about 12 percent as bright.

One reason for the discrepancy is the viewing angle. When the Moon is full, it lines up opposite the Sun in our sky. So when we see it, a lot of moonlight is reflected directly toward us. As the Moon moves through its cycle of phases, though, the viewing angle changes. Less sunlight is bounced back in our direction. Instead, most of it scatters into space.

Another reason is the shadows. When the Moon is full, it’s high noon on the central portion of the disk as seen from Earth. At that angle, there are few shadows to darken the surface.

At the Moon’s current angle, though, it’s getting to be late afternoon on the portion of the Moon we see in sunlight. At the lower angle, the Sun casts long shadows over the landscape. That darkens the Moon by quite a bit — making it fainter than we might expect.

Tomorrow: life inside the “soot line.”
 

Script by Damond Benningfield