What is a Lunar Eclipse?

Generally, an eclipse happens when an object passes through another object’s shadow. A shadow consists of a region of total shadow called umbra and an outer region of partial shadow called penumbra.


When the moon passes through the Earth’s umbra, an observer on the Earth looking at the moon will experience a lunar eclipse. This can occur when the sun, the earth and the moon are aligned exactly.

What is Lunar Eclipse

Geometry of a Lunar Eclipse

Therefore lunar eclipse occurs, when the moon passes behind the Earth’s shadow (it’s umbra) and when the Earth, the Moon and the sun are in a straight line. Significantly, a lunar eclipse occur only at the night of a full moon and lasts for a few hours, unlike a solar eclipse that lasts for a few minutes.


Conditions for Lunar Eclipse:
  • The Moon must be in the full phase.
  • The Sun, the Earth and the Moon must be in a straight line.
  • The Earth should be in the middle of the sun and the moon.


Stages in Lunar Eclipse:

There are particularly, seven stages in the lunar eclipse.

1. The first contact of the Moon and the Earth’s penumbra takes place.

2. The Moon enters the Earth’s umbra and hence this is the start of the Partial lunar eclipse.

3. When the moon touches the umbra internally, the total lunar eclipse starts.

4. Middle of the eclipse occurs, when the centre of the Moon is close to the centre of the Earth’s umbra.

5. Total eclipse ends when the moon touches the umbra again, internally.

6. Moon touches the umbra externally and hence the end of the partial lunar eclipse occur.

7. Moon leaves the penumbra.

Stages of Lunar Eclipse

Depicting the stages of the lunar eclipse


Types of Lunar Eclipse:

According to the observations of the astronomers, the basic types of lunar eclipses are:

1. Penumbral lunar eclipse: It is occurred when the moon passes through the earth’s penumbral shadow and such an eclipse is of little interest to astronomers. They are also hard to observe.

2. Partial lunar eclipse: This occurs, when the portion of the moon, passes through Earth’ umbral shadow. These eclipses can be seen clearly, even with the naked eye.

3. Total lunar eclipse: The entire Moon passes through the Earth’s umbral shadow. These eclipses can be observed very clearly due to the significant red color of the moon.


During a penumbral eclipse, an astronaut on the Moon’s surface can witness a partial solar eclipse, with the Earth blocking a portion of the sun. While during the partial and total lunar eclipses, he would witness the total solar eclipses, with the Earth masking the sun entirely.


Lunar Eclipse

Lunar Eclipse (Image credit: Oliver Stein)

Why does the Moon appear red during a Total Lunar Eclipse:

The total phase of a lunar eclipse is interesting and beautiful, because of the refracting and filtering effects of the Earth’s atmosphere. The absence of atmosphere on the earth would have resulted in a total lunar eclipse, in complete black. The color of the moon can vary from dark brown and red to yellow and bright orange, depending on the number of clouds present and the amount of the dust accumulated on the Earth’s atmosphere.


The reason of the color is that the sunlight refracts or bends as it passes through the Earth’s atmosphere. The dust particles remove the bluer colors in the sunlight and hence making red color appear predominantly. This can be viewed as a similar case of sunset appearing red on the Earth.


Observing Lunar Eclipse:

Lunar eclipses are easy and safe to watch without the need of any protective material, unlike the solar eclipses. Naked eye is sufficient to witness the lunar eclipses, while a pair of binoculars can help in magnifying the red color of the moon.


The overall darkness of the lunar eclipses can be measured from the Danjon scale (devised by Andre Danjon).

  • L=0, Moon is almost invisible.
  • L=1, Grey or Brown in color.
  • L=2, Very dark central shadow formation, with deep red or rust colored eclipse.
  • L=3, Brick red eclipse, with yellow rim.
  • L=4, Very bright copper red or orange eclipse


Frequency of Lunar Eclipse:

The moon’s orbital plane is tilted at an angle of about 5°10 to the Earth’s orbital plane, making eclipses occur only when the three bodies of the Earth, the Sun and the Moon are aligned and when the Moon is crossing the Earth’s orbital plane.


Since the orbits of the Earth and moon are known accurately, the eclipses can be predicted in advance. From the astronomers’ observations, the number of lunar eclipses in an year is expected to range from 0 to 3.


Lunar Eclipse in Mythology:

According to science, the eclipses are just the phenomena, occurring due to the coincidence of the cycles, while myths and beliefs of lunar eclipse vary.


In Chinese mythology, lunar eclipses occur when the Dragon is attempting to eat the moon. It was traditional in ancient China, to make loud noises to frighten the dragon away to counteract the effect.


According to the Serrano Indians of California, an eclipse is the beliefs of the dead, trying to eat the sun or the moon and hence they sing, dance and shout to scare the spirits away.


Some tribes in Brazil believe that eclipses are a result of the fight between the sun and Moon. They say the eye of the sun or Moon is pierced by a small boy, who shoots with an arrow. The wound bleeds, which is symbolized by the reddish color of the Moon.