The geometry
Earth’s shadow has a dark central cone, the umbra, surrounded by a fainter outer region, the penumbra. During a lunar eclipse, the full Moon passes through this shadow.
In a partial lunar eclipse, the Moon’s path takes it only partly into the umbra. It passes slightly above or below the center of Earth’s shadow, so one edge of the Moon dips into the umbra while the rest stays in the penumbra. If the Moon passed a little closer to the center, it would fit entirely inside the umbra and the eclipse would become a total lunar eclipse. If it passed a little farther away, it would miss the umbra altogether, leaving only a penumbral lunar eclipse.
This depends on how close the full Moon is to one of the two points where its tilted orbit crosses the plane of Earth’s orbit, known as the nodes. A partial eclipse happens when the full Moon is near a node, but not as near as for a total eclipse.
What you see
A partial lunar eclipse follows these stages:
- Penumbral phase. The Moon enters the faint penumbra. Little or nothing is visible at first, though a slight shading may appear on one side shortly before the umbra arrives.
- Partial phase begins. A dark notch appears on the Moon’s edge as it touches the umbra.
- Greatest eclipse. The bite reaches its largest size. Depending on the eclipse, it may cover a thin sliver or most of the Moon.
- Partial phase ends. The bite shrinks and slides off the opposite side.
- Penumbral phase ends. The Moon returns to full brightness.
The partial phase may last from a few minutes, if the Moon only grazes the umbra, to more than three hours for a deep eclipse that comes close to being total.
The curved edge of Earth’s shadow
The most striking feature of a partial lunar eclipse is the curve of the shadow’s edge. That curve is Earth’s own outline projected onto the Moon. The Greek philosopher Aristotle noted that the shadow seen during lunar eclipses is always round, and used this as evidence that Earth is a sphere. Only a sphere casts a round shadow from every direction.
The edge of the umbra is not perfectly sharp, because Earth’s atmosphere softens it. Through binoculars you can watch the shadow creep across craters and dark lunar plains, and see a gradual fade rather than a crisp line.
Color in the shadow
The part of the Moon inside the umbra is lit by sunlight bent through Earth’s atmosphere, the same reddened light that colors a total eclipse. During a partial eclipse, however, the contrast with the brilliant uneclipsed part is so great that the eye often sees the shadow as dark gray. A short camera exposure shows the bright part, and a longer one brings out the reddish tone in the shadow. Why that light is red is explained in why is the moon red.
How often it happens
Partial lunar eclipses are about as common as each of the other two kinds of lunar eclipse, making up roughly a third of all lunar eclipses. Like all lunar eclipses, each one is visible from the night side of Earth, so they are much easier to catch than solar eclipses. Upcoming dates and visibility maps are on the eclipse calendar.
Watching a partial lunar eclipse
A partial lunar eclipse is completely safe to watch with the naked eye, binoculars or a telescope. Check the start and end times in your time zone, and find a spot with a clear view of the Moon. If the eclipse happens near moonrise or moonset, check the exact local moonrise or moonset time. More tips are in how to view a lunar eclipse.