The Moon is lining up for a pass through Earth’s long shadow tomorrow afternoon and evening. Unfortunately, though, it’s a passage through the bright outer part of the shadow, so few moonwatchers are likely to notice much difference.

That part of the shadow is known as the penumbra, so the Moon’s passage through it is a penumbral lunar eclipse.

Lunar eclipses occur only at full Moon because that’s when the Moon lines up opposite the Sun, which is where Earth’s shadow is. Most full Moons don’t produce eclipses, though. The Moon’s orbit around Earth is tilted slightly with respect to Earth’s orbit around the Sun. So most months, the Moon passes a little above or below the shadow, leaving the Moon uneclipsed.

When the geometry is just right, though, the Moon passes through some portion of the shadow. If the Moon is totally immersed in the dark inner portion of the shadow, then the eclipse is total, so the entire Moon looks dark orange or red. If only part of the Moon passes through the shadow, it’s a partial eclipse, so it looks like something took a “bite” out of the Moon.

And if the Moon stays in the penumbra, there’s hardly anything to notice — only the slightest darkening of the lunar disk. And that’s what happens tomorrow, with the moment of greatest eclipse at about 6:45 p.m. Central Time.

If you don’t notice the eclipse, at least you can enjoy the bright moonlight — and a bright star near the Moon, which we’ll talk about tomorrow.

Script by Damond Benningfield