How do solar and lunar eclipses happen?

Two shadows, two eclipses

When the light source is broad, a shadow has two regions: the dark core where the light is blocked completely (the umbra) and the penumbra where only part of it is blocked. In a solar eclipse the Moon's shadow falls on the Earth: along the narrow band the umbra crosses, the Sun is covered entirely, while across the much wider penumbral region a partial eclipse is seen. In a lunar eclipse the roles are reversed; this time the Earth's shadow falls on the Moon, and it can be watched from the whole of the hemisphere then in darkness.

Why isn't there an eclipse every month?

A solar eclipse is possible only at new moon and a lunar eclipse only at full moon; but an eclipse does not occur in every phase cycle. The plane of the Moon's orbit is tilted about 5 degrees to the plane of the Earth's orbit around the Sun. So at most new moons the Moon passes a little above or below the Sun. The two points where the planes intersect are called the nodes. An eclipse occurs only if the new or full moon happens near these nodes, and those conditions are met roughly twice a year, in periods called eclipse seasons. In a given year there are at least two and at most about five solar eclipses; in most of them the Sun is not completely covered from any point, that is, only a partial eclipse occurs. There is a distinction here that is often confused: what is narrow is the central track along which the total or annular phase is seen, and this band is generally a few hundred kilometres wide. The partial eclipse, by contrast, can be watched from a far wider geography extending on either side of that track, typically on the scale of thousands of kilometres.

Total, annular and partial eclipses

A pleasing coincidence of the sky is that the Sun is about 400 times larger than the Moon but also about 400 times further away, so the two appear almost the same size in the sky. Because the Moon's orbit is not a perfect circle, this balance is sometimes upset: in an alignment that happens while the Moon is relatively far away, it cannot cover the Sun completely and a bright ring is left around its edge; this is called an annular eclipse. If the Moon is close enough, a total eclipse occurs and daylight turns to twilight for a few minutes. Where the alignment is imperfect, a partial eclipse is seen.

Where does the red colour of a lunar eclipse come from?

During a total lunar eclipse the Moon usually does not disappear; it turns a coppery red. The reason is the Earth's atmosphere: part of the Sun's light is refracted in the atmosphere and bent into the shadow. Because blue light is scattered and filtered out along the way, the light reaching the shadow is predominantly red, just like the process that reddens a sunset. In a sense the Moon is lit at that moment by the light of all the sunrises and sunsets around the Earth.

What have eclipses given science?

Eclipses are not only watched; they have been producing knowledge for centuries. Babylonian astronomers worked out from their records that eclipses recur on a cycle of about 18 years (the saros cycle); this is among the first examples of predictive power in antiquity. In 1868, during a total solar eclipse, the spectrum of the light from the Sun's outer layers was examined and the trace of an element unknown until then was seen: helium takes its name from the Greek word for the Sun, and it was discovered on the Sun before it was found on Earth. The corona, the Sun's outer atmosphere, could likewise be studied for a long time only during eclipses; the road to today's space observatories was opened by data gathered in those few brief minutes. The most famous scientific scene of an eclipse, though, is the observation of 1919, which we come to shortly.

Watching an eclipse safely

A lunar eclipse is entirely safe to watch with the naked eye, and binoculars make the experience better still. A solar eclipse is a serious exception: looking at the Sun unprotected during the partial phases can cause permanent eye damage. The Sun must be viewed only through undamaged eclipse glasses made for the purpose; methods such as sunglasses, smoked glass or X-ray film are not safe. Eclipses have also been science's proving ground through history; the observation of a total solar eclipse in 1919 made it possible to test the bending of light predicted by general relativity. We explain the logic of such tests in our guide to falsifiability.

Frequently asked questions

This guide draws on NASA's eclipse calculation and education pages and on the content of standard astronomy sources.