The penumbra is the pale outer part of a shadow, where the light source is only partly blocked. During a solar eclipse, everyone inside the Moon's penumbra, a region thousands of kilometres across, sees a partial eclipse. When the Moon moves through Earth's penumbra, it dims so slightly that most people never notice.
Understanding Penumbra
Why a penumbra fades towards its edge
Light from a large source such as the Sun comes from a whole disc, not a single point. Just outside the umbra the blocking body hides almost all of that disc; at the outer edge of the penumbra it hides almost none. In between, the hidden share falls smoothly, which is why a partial solar eclipse is deep near the path of totality and only a nibble near the edge of the penumbra. On 2 August 2027 the Sun is 92% hidden in Lisbon and 42% in London: both are in the penumbra, at different distances from the path.
How big the shadows are
The four shadows involved in eclipses, measured across where it matters: on Earth's surface for a solar eclipse, at the Moon's distance for a lunar one.
| Shadow | Width | What it gives |
|---|---|---|
| The Moon's umbra, on Earth | About 100 to 300 km | A total solar eclipse |
| The Moon's penumbra, on Earth | About 7,000 km | A partial solar eclipse |
| Earth's umbra, at the Moon | About 9,200 km | A total or partial lunar eclipse |
| Earth's penumbra, at the Moon | About 16,000 km | A penumbral lunar eclipse |
Earth's penumbra and the lunar eclipse
Every lunar eclipse starts and ends in Earth's penumbra. For the first part of the crossing nothing seems to happen, because only a sliver of the Sun is hidden from the Moon; the dimming becomes visible only once roughly two thirds of the Moon is inside. Some eclipses get no further: three penumbral eclipses fall in 2027, on 20 February, 18 July and 17 August, before the Moon reaches Earth's umbra again with a small partial eclipse on 12 January 2028.