The gibbous Moon soars across the sky tonight. It’s about three days past full, so the Sun lights up about 90 percent of the lunar hemisphere that faces our way. That makes the Moon nice and bright. But it’s not as bright as you might expect. In fact, it’s only about half as bright as the full Moon.

There are a couple of reasons for that. One is our viewing angle. The full Moon stands opposite the Sun in our sky, so the sunlight that strikes it is reflected straight back toward Earth. That makes the Moon a more efficient mirror.

But the main reason is the shadows. At full Moon, the shadows on most of the visible surface are short. In fact, there are almost no shadows at all across the center of the lunar disk. But as the Moon moves in its orbit around Earth, the angle between the Sun and Moon changes. The Sun drops lower in the lunar sky, so the shadows grow longer as seen from Earth. More shadows mean a darker surface.

Despite appearances, none of the Moon is especially bright. It reflects only a bit more than one-tenth of the sunlight. It looks so bright only because it’s a close, big presence – lighting up the night sky.

A bright star joins the Moon tonight: Regulus, the heart of the lion. It’s below the Moon as they climb into good view, about 9 or 9:30. The Moon will slide toward the star during the night, and they’ll be especially close as the dawn twilight begins to erase the star from view.

Script by Damond Benningfield