Eclipse

An eclipse occurs when one astronomical body moves into the shadow of another, or when one body passes in front of another as seen from a given viewpoint. The word usually refers to two familiar events: a solar eclipse, when the Moon blocks the Sun, and a lunar eclipse, when Earth's shadow falls on the Moon.

Eclipses are striking because they depend on a precise alignment of three bodies, and because the sizes and distances involved line up in a way that makes the effect possible at all.

Solar eclipses

A solar eclipse happens when the Moon passes between the Sun and Earth, casting a shadow on part of the planet. In a total eclipse the Moon completely covers the Sun's bright disc, and day briefly turns to twilight as the solar corona becomes visible. In an annular eclipse the Moon is too far away to cover the Sun fully, leaving a bright ring. In a partial eclipse only part of the Sun is hidden. Totality is visible only along a narrow track; elsewhere the eclipse is partial.

Lunar eclipses

A lunar eclipse occurs when Earth lies between the Sun and the Moon, and the Moon enters Earth's shadow. In a total lunar eclipse the whole Moon darkens and often glows a coppery red, an effect caused by sunlight bent through Earth's atmosphere. A partial eclipse shades part of the disc, and a penumbral eclipse only dims it subtly. Unlike solar eclipses, lunar eclipses are safe to watch with the naked eye and are visible from the entire night side of Earth.

Why they do not happen every month

The Moon orbits Earth roughly once a month, so it seems that alignments should be common. They are not, because the Moon's orbit is tilted relative to Earth's orbit around the Sun. Most months the Moon passes just above or below the line between Earth and Sun. Only when the geometry lines up near the points where the two orbits cross can an eclipse occur, which is why they come in limited seasons rather than monthly.

Predicting eclipses

Eclipses follow repeating cycles that ancient astronomers noticed long before they understood the cause. The most famous is the saros, a period of roughly eighteen years after which a similar eclipse recurs. Because the motions of Earth and Moon are now known with great precision, modern astronomers can predict the timing, duration and path of eclipses centuries ahead, to the second.

History and culture

For most of history an eclipse was sudden, unexplained and frightening, and many cultures recorded them as omens or signs from the gods. Eclipse records in old chronicles are valuable to historians, because they help date events in regions where calendars were inconsistent. The ability to predict eclipses was also an early triumph of science, showing that the heavens follow regular laws rather than the whim of deities.

Watching safely

Solar eclipses must never be viewed with the naked eye or through ordinary sunglasses, cameras or telescopes, because the Sun's light can damage the retina even when partly covered. Safe methods include certified eclipse glasses and indirect projection. The only safe moment to look without a filter is during the brief total phase of a total solar eclipse. Lunar eclipses carry no such danger.

Eclipses as a scientific tool

Eclipses have long been more than spectacle. A total solar eclipse briefly reveals the Sun's outer atmosphere, the corona, which is normally drowned out by the Sun's glare. Measurements taken during such events helped confirm ideas about the Sun and, famously, offered some of the earliest tests of gravity theories by showing how starlight bends near the Sun.

Lunar eclipses provide a different kind of information. Because Earth's shadow falls on the Moon, the way sunlight filters through the atmosphere reveals the composition of the air and the amount of dust and volcanic haze high above. Historical records of eclipses, including their colour and darkness, help scientists reconstruct past volcanic eruptions and climate. In these ways, events once treated purely as omens have become precise instruments for studying the Sun, the Earth and their long history.

Transits and other linked events

Eclipses belong to a wider family of astronomical events where one body passes in front of another. A transit occurs when a smaller body crosses the face of a larger one, such as a planet moving across the Sun or a moon crossing its planet. These events are less dramatic than eclipses to the eye but invaluable to science, since they reveal sizes, orbits and, for distant stars, the presence of planets.

Occultations, in which a nearer object hides a more distant one, let astronomers study faint sources briefly revealed or blocked. All these events share the same underlying geometry as eclipses, and all depend on careful prediction. They show that alignment in the sky is not only a spectacle but a measurement tool, turning the clockwork of the solar system into a way of studying objects far beyond it.

Key points

• An eclipse is one body blocking another's light, or casting a shadow on it.
• Solar eclipses can be total, annular or partial, depending on the alignment.
• Lunar eclipses can be total, partial or penumbral, and are safe to view directly.
• Eclipses are not monthly because the Moon's orbit is tilted.
• They repeat in cycles such as the saros, and can now be predicted precisely.
• Solar eclipses require eye protection; only totality is safe to view bare-eyed.