The Moon · The Sun · Observing
Penumbra
Say it: pih-NUM-bruh IPA: /pɪˈnʌmbrə/ noun
Penumbra: The part of a shadow where an extended light source is only partly blocked.
The Sun looks like a disk rather than a point. In Earth's penumbra, a lunar observer would see only part of that disk hidden by Earth.
- Example
- The Moon dimmed subtly as it entered Earth's penumbra.
- Why astronomers use it
- It explains partial illumination during eclipses and why some lunar eclipses are hard to notice.
- Tiny cosmic connection
- In solar physics, penumbra also names the outer region of a sunspot; that structure is not an eclipse shadow.
Observatory The Moon The Sun Observing
Where to check it
Written for Cool Moon Facts in its own words; the sources cover the same idea in more depth.