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Lunar Eclipse. Image credit: ESA
What is a lunar eclipse? It occurs when the moon passes behind the Earth and the Earth blocks the Sun from the Moon. This only when the Moon, Earth, and Sun are nearly perfectly aligned. The Earth must be in the middle. There is always a full moon on the night of a lunar eclipse. The length and type of the eclipse depends on the Moon’s location relative to its orbital nodes. A lunar eclipse can be viewed from anywhere on the night side of the Earth. A lunar eclipse usually lasts for a few hours.
The shadow of the Earth can be divided into two distinctive parts: the umbra and penumbra. In the umbra, there is no direct solar radiation; however, as a result of the Sun’s large angular size, solar illumination is only partially blocked in the outer portion of the Earth’s shadow. This ares is called the penumbra. This will help you to understand the different types of lunar eclipse.
A penumbral eclipse occurs when the Moon passes through the Earth’s penumbra. The penumbra causes a subtle darkening of the Moon’s surface. On rare occasions a type of penumbral eclipse, called a total penumbral eclipse, occurs. During one of these eclipses the Moon lies exclusively within the Earth’s penumbra. When these occur, that portion of the Moon which is closest to the umbra can appear somewhat darker than the rest of the Moon.
Partial lunar eclipses occur when a portion of the Moon enters the umbra. A total lunar eclipse occurs when the Moon travels completely into the Earth’s umbra. The Moon’s speed through the shadow is about one km/s, and the eclipse can last up to nearly 107 minutes. The total time between when the Moon enters and leaves the shadow can last up to 3.8 hours. The distance of the Moon from the Earth affects the length of the eclipse. If the Moon is near its apogee its orbital speed is the slowest and eclipse is longest.
A selenelion(selenehelion also referred to as horizontal eclipse) occurs when both the Sun and the eclipsed Moon can be observed at the same time. This only happens just before sunset or just after sunrise. Both will appear just above the horizon at nearly opposite points in the sky. Although the Moon is in the Earth’s shadow, the Sun and the eclipsed Moon can appear in the sky at the same time because the refraction of light through the Earth’s atmosphere causes objects near the horizon to appear higher in the sky than their true geometric position.
Every year there are at least two lunar eclipses, although total lunar eclipses are significantly less common.
We have written many articles about lunar eclipse for Universe Today. Here’s an article about moon eclipse, and here’s an article about the lunar phases.
If you’d like more info on the Lunar Eclipse, check out NASA Lunar Eclipse Page, and here’s a link to NASA Science Discussion on Lunar Eclipse.
We’ve also recorded an entire episode of Astronomy Cast all about the Eclipse. Listen here, Episode 160: Eclipses.
Sources: Hyperphysics, NASA
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