The geometry
Earth’s shadow has two parts. In the dark central umbra, Earth blocks the Sun completely. In the surrounding penumbra, Earth blocks only part of the Sun’s disk. If you stood on the Moon in the penumbra, you would see a partial solar eclipse, with Earth covering part of the Sun.
In a penumbral lunar eclipse, the full Moon crosses this outer zone without entering the umbra. This happens when the full Moon falls near, but not very near, one of the nodes where its tilted orbit crosses the plane of Earth’s orbit. A little closer and the eclipse would be a partial lunar eclipse; much closer and it would be a total lunar eclipse.
Why it is so hard to see
In the penumbra, the Moon still receives sunlight from part of the Sun. The dimming is gradual: barely anything near the outer edge of the penumbra, and stronger close to the umbra. The human eye adapts easily to changes in brightness, so a slight overall dimming of an already bright full Moon is very hard to notice.
What observers usually report is a soft gray or smoky shading on the side of the Moon nearest the umbra. The shading has no sharp edge. As a rule of thumb, it becomes noticeable only when a large part of the Moon’s diameter is inside the penumbra, often quoted as around two-thirds or more, and when the Moon passes close to the edge of the umbra.
Penumbral magnitude
Astronomers describe a penumbral eclipse with its penumbral magnitude: the fraction of the Moon’s diameter inside the penumbra at greatest eclipse.
- Below about 0.5, the eclipse is usually invisible to the eye.
- Higher values bring a more noticeable shading near one edge.
- A value of 1 or more means the whole Moon is inside the penumbra. This rarer variety is called a total penumbral eclipse, and the side closest to the umbra can look distinctly darker.
The umbral magnitude of a penumbral eclipse is always below zero, since the Moon never reaches the umbra. Related measures are explained in eclipse magnitude.
How often it happens
Penumbral eclipses make up roughly a third of all lunar eclipses. They often appear in a sequence at the start or end of a series of eclipses, when the Moon’s path through the shadow is drifting toward or away from the umbra over many years. Upcoming eclipses of every kind, including penumbral ones, are listed on the eclipse calendar.
How to spot one
If you want to try to see a penumbral eclipse:
- Look around greatest eclipse, when the shading is strongest, rather than at the listed start, when nothing is visible.
- Compare both sides of the Moon. The edge closer to the umbra should look a bit grayer than the opposite edge.
- Use binoculars, which make the contrast easier to judge.
- Take photos with the same fixed exposure before the eclipse and at its peak. Side by side, the dimming is much easier to see than with the eye.
Like all lunar eclipses, a penumbral eclipse is safe to look at, with or without optics. It is visible from anywhere the Moon is above the horizon during the event, so check the local moonrise and moonset times for that night. For general planning advice, see how to view a lunar eclipse.
Penumbral stages of every lunar eclipse
Every lunar eclipse, including total and partial ones, starts and ends with a penumbral stage. Those stages are just as subtle as a penumbral eclipse, which is why most people begin watching only when the dark umbra first touches the Moon.