Glossary · Thiên văn học
Umbra
The inner, darkest part of the shadow cast by Earth or the Moon, where the Sun is completely blocked. Around it is the penumbra, the faint outer part of the shadow, where the Sun is only partly obscured.
In a total lunar eclipse, the Moon moves into Earth's umbra. Some sunlight passing through Earth's atmosphere still reaches the Moon's surface and lights it dimly. Shorter wavelengths such as blue and violet scatter more easily, while red and orange make it through, so the Moon turns reddish-orange — the "Blood Moon"; the more dust or clouds in the atmosphere, the redder the Moon appears. If the Moon passes through only part of the umbra, the eclipse is partial; if it passes only through the penumbra, it dims so slightly that it can be hard to notice.
During a solar eclipse, the Moon's shadow on Earth's surface is only about 480 km wide. People in the umbra see a total eclipse, while people in the penumbra see a partial one.