What Is a Total Lunar Eclipse?

Cause

• When the moon is full, it is directly opposite the sun in the sky. When it is in this position it passes through the Earth's shadow. This blocks part or all of the sun's light, therefore causing an eclipse. The shadow of the Earth is divided into two portions: the penumbra and the umbra. The penumbra is the name for both of the two outer zones; these do not block all of the sunlight from reaching the moon. The umbra, or inner section, blocks all the light. It is when the Moon is lined up with the umbra that a total lunar eclipse occurs.

Appearance

• The total lunar eclipse is very dramatic because of the intense red color the moon takes on during the phase of totality (the period when the moon is completely in the Earth's shadow). The reason the Moon appears red is because of the indirect sunlight which illuminates it even when it is in the shadow of the Earth. This light passes through the Earth's atmosphere, where most of the blue light is separated out. What is left is red or orange in color, and nowhere near as bright as sunlight, and this is what gives the moon its red glow.

Danjon Scale

• The French astronomer Andre-Louis Danjon came up with a five point scale for rating the appearance and brightness of the moon during a total lunar eclipse. Each eclipse is given an L rating between one and five which is defined as follows:

  1. A very dark eclipse where the moon is almost invisible.

  2. A gray or brown eclipse with only a few features of the moon visible.

  3. A dark red or rust colored eclipse with light edges.

  4. A brick red eclipse with a yellow color at the edges of the umbra.

  5. An orange eclipse where the edge of the umbra is very brightly colored, even appearing bluish.

Other Eclipses

• The other two types of lunar eclipse are penumbra and partial, but neither of these is quite as dramatic as the total eclipse. The penumbra eclipse is where the moon passes through the penumbra shadow of the Earth; these are difficult to see but interesting scientifically. The partial lunar eclipse is where part of the moon moves through the umbra shadow of the Earth. This type of eclipse is very easy to observe and can be seen quite easily with the naked eye.