Eclipse

An eclipse is an astronomical event which happens when one astronomical object moves into the shadow of another, or when another object passes between it and the viewer. This special alignment is called a syzygy. Eclipses can happen in different ways, such as one object being completely hidden (occultation) or only partly hidden (transit). Sometimes a small object can disappear behind a bigger one, which is called a "deep eclipse."

Totality during the solar eclipse of August 11, 1999, showing the solar corona and prominences

We most often hear about two kinds of eclipses: solar eclipses and lunar eclipses. In a solar eclipse, the Moon's shadow falls on the Earth. In a lunar eclipse, the Moon moves into the Earth's shadow. But eclipses can happen in other places too, like when a planet goes into the shadow of one of its moons, or when a moon passes into the shadow of its planet.

Solar and lunar eclipses only happen during special times called "eclipse seasons," which happen twice each year. This is when the Earth's orbit around the Sun crosses the Moon's orbit around the Earth. Because the Moon's orbit is tilted, eclipses do not happen every month. If the orbits were perfectly lined up, we would see eclipses much more often. Total solar eclipses are rare and can be many decades apart for any one place on Earth.

Etymology

The word "eclipse" comes from an ancient Greek word, ἔκλειψις (ékleipsis), which means 'the darkening of a heavenly body'. This word is derived from a verb that means 'to darken' or 'to cease to exist'. It shows how people in ancient times described the mysterious event when the Sun or Moon seems to disappear for a while.

Umbra, penumbra and antumbra

Main article: Umbra, penumbra and antumbra

An eclipse happens when one space object moves into the shadow of another object. This creates a shadow area that can be seen from certain places on Earth or in space.

During an eclipse, the shadow has three parts: the umbra, where the object completely blocks the light; the antumbra, where the object is in front of the light but too small to block it fully; and the penumbra, where the object only partly blocks the light. If you are in the umbra, you see a total eclipse. If you are in the antumbra, you see an annular eclipse, and if you are in the penumbra, you see a partial eclipse.

Eclipse cycles

An eclipse cycle is a pattern that happens when eclipses repeat after a certain amount of time. One well-known example is the saros, which causes similar solar or lunar eclipses to occur about every 6,585 days, or just over 18 years. Because this time period isn’t an exact number of days, each eclipse in the series can be seen from different places around the world. The movements of the Earth and Moon create these repeating patterns, making eclipses appear in a predictable way over many years.

Earth–Moon system

An eclipse involving the Sun, Earth, and Moon happens when these three are nearly in a straight line, allowing one to hide behind another from our view. Because the Moon's path around the Earth is tilted compared to Earth's path around the Sun, eclipses can only happen when the Moon is near the points where these paths cross. This happens twice a year during a time called eclipse season, and during this period, there can be between four to seven eclipses in a year.

A solar eclipse occurs when the Moon moves in front of the Sun as seen from Earth. This can result in a total solar eclipse, where the Moon completely covers the Sun, or an annular eclipse, where the Moon only partially covers the Sun leaving a ring of sunlight visible. These eclipses are brief and can only be seen in totality along a narrow path on Earth.

A lunar eclipse happens when the Moon moves into Earth's shadow. This can only occur during a full moon and can be seen from a large area of Earth. Lunar eclipses last longer than solar eclipses and can be total, partial, or penumbral, where the Moon only skims Earth's shadow. Even during a total lunar eclipse, the Moon often appears reddish, which is why it is sometimes called a "Blood Moon."

Other planets and dwarf planets

Gas giants

A picture of Jupiter and its moon Io taken by Hubble. The black spot is Io's shadow.

The large planets in our solar system, called gas giants, often have eclipses because they have many moons. Jupiter especially shows beautiful eclipses, where its big moons move in front of it and cast shadows. These shadows can be seen moving across Jupiter's clouds.

Scientists used these eclipses to discover how fast light travels. By watching when eclipses happened a little late, they figured out the time it takes for light to reach us from far away.

Mars

Saturn occults the Sun as seen from the Cassini–Huygens space probe

Main article: Transit of Phobos from Mars

On Mars, the moons are too small to cover the Sun completely, so we only see parts of the Sun being covered. However, Mars's moons often pass in front of each other, creating their own eclipses. These have been captured in pictures from Mars itself.

Pluto

Transit of Phobos from Mars, as seen by the Mars Opportunity rover (10 March 2004).

Main article: Solar eclipses on Pluto

Pluto and its large moon Charon often eclipse each other. This happened regularly between 1985 and 1990, helping scientists learn more about both of them.

Mercury and Venus

Eclipses are not possible on Mercury and Venus because they have no moons. But we can see them pass in front of the Sun from Earth. Transits of Venus are rare, happening in pairs every eight years, but these pairs occur less than once every hundred years. The next pair will be in the year 2117. Transits of Mercury happen more often, about thirteen times every hundred years.

Eclipsing binaries

A binary star system has two stars that orbit each other around a common point called the centre of mass. When the path of these stars lines up with where we are, we can see one star pass in front of the other. This creates a special kind of variable star known as an eclipsing binary.

The brightest light we see from an eclipsing binary is when both stars are fully visible. When one star moves in front of the other, the total light we see gets dimmer before returning to normal. The first eclipsing binary discovered was Algol, located in the constellation Perseus. Its brightness normally stays at a level called 2.1, but every 2.867 days, it dims to 3.4 for over nine hours. This happens when the dimmer star passes in front of the brighter one. The idea that this dimming was caused by an eclipse was suggested by John Goodricke in 1783.

Types

An eclipse happens when one space object moves into the shadow of another, blocking it from view. There are different kinds of eclipses depending on which objects are involved. For example, when the Moon moves between the Sun and Earth, we see a solar eclipse. This can be total, annular, hybrid, or partial. When Earth comes between the Sun and the Moon, we see a lunar eclipse, which can also be penumbral, partial, or central.

Eclipses can also happen on other planets. For instance, the moons Phobos and Deimos can pass in front of the Sun as seen from Mars, causing transits or eclipses. Similar eclipses can also be observed on planets like Jupiter, Saturn, Uranus, Neptune, and Pluto.

Sun – Moon – Earth: Solar eclipse | annular eclipse | hybrid eclipse | partial eclipse

Sun – Earth – Moon: Lunar eclipse | penumbral eclipse | partial lunar eclipse | central lunar eclipse

Sun – Phobos – Mars: Transit of Phobos from Mars | Solar eclipses on Mars

Sun – Deimos – Mars: Transit of Deimos from Mars | Solar eclipses on Mars

Other types: Solar eclipses on Jupiter | Solar eclipses on Saturn | Solar eclipses on Uranus | Solar eclipses on Neptune | Solar eclipses on Pluto