What is an eclipse of the Moon?

A lunar eclipse is an eclipse in the true sense of the word because it happens when our Moon passes into shadow – the shadow of the Earth in space. This is a quite different event to what is termed an eclipse of the Sun because the Sun is physically blotted out by the Moon rather than put in its shadow. (You can find out about eclipses of the Sun here.)

A total lunar eclipse by Paul Sutherland
A total lunar eclipse by Paul Sutherland

Lunar eclipses can only occur at Full Moon because the Sun, Earth and Moon have to be in line. The reason they do not occur at every Full Moon is that the plane of the Moon’s orbit is tilted by about five degrees to that of the Earth around the Sun so that it normally passes above or below the Earth’s shadow. They occur at the points where the two orbits cross, called the nodes.

Whereas a total eclipse of the Sun can only be seen at points on the Earth hit by the Moon’s shadow, a total eclipse of the Moon will be visible from any part of the world from which the Moon itself can be seen. This is because it is the Moon’s own physical appearance that changes. It means we get to see many more lunar eclipses than solar in our lives.

The Moon usually shines from reflected sunlight. The Sun also causes the Earth to cast a shadow which is normally invisible in the sky. When the Moon drifts into the shadow, its light begins to fade as light from the Sun is cut off.

At first the effect is only slight because the eclipse is in its penumbral phase – from the Moon the Earth would appear only partly to be obscuring the Sun. Most people casually glancing at the Moon at this time would probably not notice anything amiss. During some eclipses the Moon only enters this twilight zone of the shadow in which case the event is termed a penumbral eclipse.

However, when the Moon passes through the central shadow, or umbra, the Sun’s light is completely blocked from falling on the Moon. Which begs the question, why can we still see it? During most total eclipses, the Moon remains visible, glowing in colours ranging from orange to a deep red. This is sunlight that has shone through the Earth’s atmosphere and been scattered onto the lunar surface. As it is the longer reddish wavelengths of light that get scattered the most, the Moon takes on a reddish hue.

At some lunar eclipses, the Moon only partly enters the central umbral shadow. Events like these are termed partial eclipses of the Moon. All total eclipses have a partial phase as the Moon enters and later leaves the umbra.

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Astronomers find it interesting to check how dark or bright a total lunar eclipse will be because this can be affected by the state of our atmosphere. Especially dark eclipses can occur in months after a major volcanic eruption that has sent dust to high altitudes.

Sometimes the Moon will take on a three-dimensional appearance due to the different levels of brightness within the shadow.

The total phase of a lunar eclipse can last up to an hour and three quarters, depending on how far into the centre of the Earth’s shadow the Moon has travelled. Passage through the entire shadow, including the penumbra, can take several hours.

 

Eclipse of the Moon diagram
Eclipse of the Moon diagram

Key moments during an eclipse are called contacts and are as follows:

  • P1 (First contact): Start of the penumbral eclipse. The outer limb of the Moon makes contact with the Earth’s penumbra.
  • U1 (Second contact): Start of the partial eclipse. The outer limb of the Moon makes contact with the Earth’s umbra.
  • U2 (Third contact): Start of the total eclipse. The Moon lies completely within the Earth’s umbra.
  • Greatest eclipse: A total eclipse reaches its peak with the Moon at its closest to the centre of the umbra.
  • U3 (Fourth contact): End of the total eclipse. The Moon’s outer limb leaves Earth’s umbra.
  • U4 (Fifth contact): End of the partial eclipse. The whole of the Moon exits Earth’s umbra.
  • P2 (Sixth contact): End of the penumbral eclipse. The Moon completely leaves the Earth’s outer penumbral shadow.
Note that the circular shadow bands shown in the diagram are not themselves visible in the night sky but only on the Moon as it passes through them. The edges of each band are also not sharp but diffuse.
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By Paul Sutherland

Paul Sutherland has been a professional journalist for nearly 40 years. He writes regularly for science magazines including BBC Sky at Night magazine, BBC Focus, Astronomy Now and Popular Astronomy, plus he has authored three books on astronomy and contributed to others.

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