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What is a lunar eclipse?

Kulvinder Singh, News reporter
Jun 13, 2015, 20:36 UTC

Sen—There is nothing more spectacular than a total solar eclipse, when the Moon passes directly in front of the Sun. From a particular vantage point on Earth an area can be plunged into total darkness. But such an event is only visible over a narrow strip, where the Moon blocks out 100 per cent of the Sun's light. A total lunar eclipse on the other hand, is potentially visible over half of the entire planet when it occurs.

Eclipses occur when bodies such as planets and moons obscure one another—either partially or totally—in relation to light from a sun. In other words, one casts a shadow on the other.

A moon or planet's shadow has two parts to it. The umbra is the darkest region. It tapers in the shape of a cone—in a direction away from the source of light. Anything within this region will be totally eclipsed. By contrast the surrounding penumbra fans outwards and is less dark. Anything in this region will only be partially eclipsed. Anything outside the shadow is not eclipsed at all. These definitions are a bit different for lunar eclipses.

The Earth's umbra is around 9,000 km in diameter at a distance of 384,403 km—the mean distance to the Moon. The Moon itself has a mean diameter of 3,475 km, indicating that Earth's umbra—nearly 2.6 times larger—can totally obscure it with ease.

When the Moon completely passes into Earth's umbra it is totally eclipsed. Sometimes it will pass only through the penumbra. When this happens it is called a penumbral eclipse. These can be very difficult to notice. Sometimes the Moon will straddle the umbra and penumbra. This is a partial lunar eclipse and is easily visible.


Diagram showing a total lunar eclipse, with the Moon within Earth's umbral shadow. Image credit: Kulvinder Singh.

The orbit of the Moon around the Earth, as well as Earth's orbit around the Sun, are not exactly aligned. The plane of the Moon's orbit differs from the plane of Earth's orbit by five degrees. It does not sound like very much but this is why solar and lunar eclipses do not occur every month—at every new moon and full moon, respectively. The Sun and Moon usually miss one another as the pass along the sky, and the Moon usually misses Earth's shadow.

When a lunar eclipse of any kind does occur, the Moon does not disappear. This may surprise anyone who has never witnessed one before. Instead the Moon is bathed in orange or rust-coloured light, often referred to as a 'blood moon'. The reason this happens is because sunlight is scattered by Earth's atmosphere and diffuses into the shadow. The amount of scattering depends on dust or clouds in the atmosphere. This strongly influences the amount of reddening of an eclipsed Moon and can be measured on the 'Danjon Scale'.

Lunar eclipses can last anywhere between two to nearly four hours, with total eclipses usually lasting over an hour. Sometimes a total lunar eclipse can last for 100 minutes or more, and sometimes just five.

There can be up to three total lunar eclipses in any one year, or up to seven partial eclipses in the same period.