Causes of Lunar Eclipses

Luna-tastic: Do You Know What Actually Causes a Lunar Eclipse?

Lunar eclipses―an astronomical incident and a splendid sight! Know what causes them.
UniverSavvy Staff
Last Updated: Dec 10, 2017
Moon, Earth's only natural satellite, is a cold, rocky mass, about 3476 kilometers in diameter. It orbits the Earth every 29½ days, with its orbit tilted at 5 degrees from the plane of the Earth. During its revolution, it passes above or below the Earth's shadow. But it's only two times in a year that the Sun, Moon, and the Earth stand lined up. Due to their relative positions, a part of the Earth's shadow falls on the Moon and a lunar eclipse is caused.
The Moon, as we know, has no light of its own, and illuminates itself by stealing sunlight. The period of 29.5 days between two consecutive new Moons is known as lunation. On the night of a new Moon, the illuminated side of the Moon points away from the Earth and we see 'no Moon' in the sky. When the Moon is full, it is directly opposite to the Sun. The night of a lunar eclipse is a Full Moon night.

What are the causes of lunar eclipses? On the night of a lunar eclipse, the Earth, the Sun, and the Moon are positioned in one straight line, with the Earth in between the Sun and the Moon. With this placement of these three celestial bodies, the Earth casts a shadow on the surface of the Moon. To put it simply, the Moon travels through some portion of the Earth's shadow, causing a lunar eclipse.

The Earth casts a shadow that can be divided into two parts. The outer and fainter portion is known as the 'penumbra' while the inner, darker portion is called 'umbra'.
Lunar eclipse
  • Penumbral Eclipse: During this, the Moon passes through the Earth's penumbra causing no significant observable change in the Moon's appearance. When the Moon lies within the penumbra of the Earth, a total penumbral eclipse is said to occur. These eclipses seldom occur. When they do, that part of the Moon which is nearest to the umbra, appears darker.
  • Partial Lunar Eclipse: When the Moon partially enters the umbra; a partial lunar eclipse is caused. It can easily be seen with bare eyes.
  • Total Lunar Eclipse: It is caused when the Moon travels totally into the Earth's umbra. The time that elapses between the first and last contact between the Moon and the Earth's shadow can be about 3.8 hours. When in the Earth's umbra, the Moon illuminates by the colors refracted and filtered by the Earth's atmosphere, thus appearing red, orange, or yellow. This lunar illumination makes the event of a lunar eclipse worth witnessing.
In case a lunar eclipse occurs just after sunrise or just before sunset, it is likely that the eclipsed Moon and the Sun are seen at the same time. This phenomenon is termed as a 'selenelion'. The Moon and the Sun appear on opposite points in the sky. This is known as a 'horizontal eclipse'. This phenomenon is evident during every lunar eclipse on those parts of the Earth where it is sunrise or sunset at the time of the eclipse.

Generally, two partial lunar eclipses occur every year. Total lunar eclipses occur less frequently. In that part of the world where it is night, a lunar eclipse can be seen. Around 35% of the lunar eclipses are penumbral and are not easily detectable. Without the Earth's atmosphere, it would have been impossible to watch the red coloration of the Moon during a lunar eclipse. If not for the Earth's atmosphere, the Moon would have appeared entirely dark during a lunar eclipse.
This astronomical phenomenon of the Earth coming between the Sun and the Moon and casting a shadow on the lunar surface, is definitely a spectacle.