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When most people think of an eclipse, they think of totality, the apex of a total solar eclipse, where the Sun, Moon and Earth are in perfect alignment and the Moon completely covers the Sun. Even here, the Sun's light doesn't completely disappear. With the central brightness gone, it's possible to see the beautiful arcing curves of the Sun's corona, while Earth is plunged into a false twilight.
Totality can only be seen along a narrow corridor, known as the path of totality. Observers situated away from this track will see a partial eclipse of varying magnitude, depending on their distance from it. Some parts of the Earth are so far from the track that they won't see an eclipse at all.
That total solar eclipses can happen at all is the result of a fantastic cosmic coincidence — the Moon is both 400 times smaller than the Sun and 400 times closer to us, meaning they appear to be the same size in the sky — most of the time.
The Moon's orbit around the Earth is not a perfect circle, which causes the Moon's apparent size to change over the course of each month by 14 per cent. When the Moon appears smallest it no longer fills the Sun's disc. When eclipses happen during this time, they are annular instead of total: a thin ring of solar disc remains visible around the edge of the Moon's silhouette, and this can be just as beautiful as totality. There are also extremely rare hybrid eclipses, which transition from total to annular mid event.
We know solar eclipses occur when the Sun, Moon and Earth line up in the sky. Why then don't we see eclipses every month at new Moon? It's because the Moon's orbit is inclined by 5.3° to the ecliptic, the plane in which Earth orbits the Sun. That means that even if the Earth, Moon and Sun are aligned in a straight line as seen from above (known as a 'syzygy'), the Moon may be too high above or too low below the orbital plane to block the Sun's light.
While every eclipse is partial somewhere on the planet, there are some during which the darkest part of the Moon's shadow misses the Earth, meaning there is no totality anywhere on the planet. This happened on 23 October 2014, when there was a partial eclipse that could be seen from North America — but in order to see totality you would have had to have been several hundred kilometers above the North Pole.
Lunar eclipses, where the Moon passes into Earth's shadow, are much more relaxed affairs than their solar counterparts, typically lasting for over an hour rather than a matter of minutes.
Because the Sun is much bigger than Earth, it splits our planet's shadow into two parts: the darkest, called the umbra, and a lighter outer ring, called the penumbra. The intensity of a lunar eclipse depends on how much of the Moon passes into the Earth's shadow, and which part of the shadow it passes through.
In a total lunar eclipse, the entire Moon passes through the penumbra and into the umbra, gradually darkening until it is completely covered, a point known as totality. During totality no sunlight shines directly on the Moon, but some is refracted onto it via Earth's atmosphere. As our atmosphere filters out blue light, the Moon often gains a strange orange-brown color.
As the Moon goes into eclipse and dims, the sky gets darker too. You may not have realized how bright a full Moon can be. It lights up the sky around it with a blue haze, out of which only the brighter stars are visible. During a total lunar eclipse, the darker Moon means that the fainter stars can come out and we end up with the eerie sight of a deep-red Moon surrounded by twinkling stars.
The darkness of the Moon gets during a total lunar eclipse is described by the Danjon Scale, which runs from L0 through to L4. As the Moon is only lit by light that has passed through Earth's atmosphere, its precise color and darkness will depend on how much dust, volcanic ash and water vapor is in the atmosphere to affect the sunlight's path. The eclipse in 1884, after the huge volcanic eruption of Krakatoa, was so dark that the Moon could only just be made out, such was the amount of dust in the atmosphere.
There are two other types of lunar eclipse: partial, where only a portion of the Moon passes through Earth's dark umbral shadow, and penumbral, where part of the Moon only passes through the lighter, outer shadow. Partial eclipses can be quite noticeable, but penumbral eclipses often only cause a slight dimming.
Lunar eclipses can be observed without optical aids. For solar eclipses, you always need to use equipment with certified filters, or project the event onto a piece of card. The one exception is during the brief window of totality during a total solar eclipse. This is the only time it is safe to look directly at the Sun. The simple rule is: if you're not absolutely sure about safety, don't do it.
The point at which the Moon first touches the solar disc, marking the beginning of the eclipse.
The moment the Moon is fully within the solar disc, marking the start of annularity or totality. Partial eclipses do not have second or third contacts.
The point of totality or annularity.
The instant the lunar disc touches the other side of the solar disc, ending totality/annularity and marking the start of egress.
The point when the edge of the Moon's trailing edge breaks contact with the solar disc, ending the eclipse.
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