Most people have heard of a solar eclipse even if they have not seen one. In this the moon passes between the Earth and the sun. This will block all or part of the sun’s light from reaching the Earth depending if it is a full or partial eclipse.
But what about a lunar eclipse? The moon does not produce its own light rather it reflects the sun’s light. For a lunar eclipse to be produced the Earth must pass between the moon and the sun so blocking this light from reaching the moon.
The moon orbits the earth every 29.5 days. During the full moon phase the moon is directly opposite the sun. It is only during this phase of the moon that the Earth can interfere with the light reaching its surface. There is not a lunar eclipse every month because the plane of the moons orbit is tipped about five degrees away from the Earth’s solar orbit. The moon, therefore, spends most of its time away from the Earth’s shadow.
There are three types of lunar eclipse: penumbral, partial and total. Unlike solar eclipses lunar eclipse are safe to observe with the naked eye.
Penumbral eclipses are not seen by the naked eye. The moon passes into the penumbral shadow of the Earth. This shadow can be described as being formed by two lines from the outer edge of the sun that cross over before touching the outer edge of the earth and continuing to diverge into space. The moon in the penumbral shadow still receives light from non-intersecting rays so is still fully visible. Penumbral eclipses can only be observed with optical instruments.
Partial eclipses can be seen with the naked eye. The moon passes part way into the umbral shadow of the Earth. The umbral shadow is formed by two lines taken from the outer edge of the sun which touch the outer edge of the earth and intersect in space beyond the moon’s orbit. As the moon passes into the umbral shadow it appears to have had a bite taken out of it.
Total lunar eclipses can also be seen with the naked eye. In these the moon passes completely into the earth’s umbral shadow. At the start of a total eclipse the moon has a similar appearance to a partial eclipse but the bite gets larger until totality when the moon takes on an amazing red color. This color comes from light that is transmitted through the Earth’s atmosphere. The atmosphere absorbs light at the blue end of the spectrum but allows the light at the red end to be transmitted. When this red light reaches the moon it is reflected back giving the moon a red appearance.
A diagram of the penumbral and umbral shadow as well as pictures of a lunar eclipse can be found at: http://www.mreclipse.com/Special/LEprimer.html
