Eclipse

An eclipse is an astronomical phenomenon in which one celestial body becomes obscured by another, either through the casting of a shadow or the interposition of an intervening object along the line of sight. This alignment of three celestial objects, known as a syzygy, can occur in many combinations throughout the Solar System and beyond. Eclipses may involve the Earth, Moon and Sun, or they may occur among planets, moons and stars in more distant systems. Their nature depends on relative sizes, distances and orbital geometry, creating a diversity of eclipse types observable across different contexts.

Nature and Types of Eclipses

Eclipses fall into two principal categories depending on the visibility of the body being obscured. Occultation takes place when one object is completely hidden by another, while a transit occurs when a smaller body passes only partially across the face of a larger one. A deep eclipse or deep occultation refers to cases where a smaller object is positioned entirely behind a larger body, resulting in full concealment for a period. These terms apply to terrestrial observations as well as to complex eclipsing arrangements in binary star systems and planetary–satellite environments.
Within the Earth–Moon–Sun system, these phenomena take their most familiar forms. A solar eclipse occurs when the Moon’s shadow sweeps across the Earth, and a lunar eclipse occurs when the Moon passes into the Earth’s shadow. More broadly, similar events occur when a moon enters the shadow of its host planet, when a planet crosses the shadow of a moon, or when stars of a binary system periodically obscure each other. For eclipses to take place between Earth and the Moon, the bodies must align closely near the lunar orbital nodes during an eclipse season—two periods each year when the plane of the Moon’s orbit intersects the ecliptic.
If the Earth’s and Moon’s orbits lay in the same plane, eclipses would be monthly occurrences. However, the Moon’s orbit is inclined by about five degrees, making precise alignments uncommon. Even when alignment occurs, the type of eclipse depends on the apparent sizes of the Sun and Moon, which vary with their distances from Earth at the time.

Terminology and Etymology

The word eclipse derives from the ancient Greek term for abandonment or darkening, linked to verbs signifying disappearance or cessation. This linguistic origin reflects the perceived “loss” or momentary fading of a celestial body during such events.
Important terms describing eclipse geometry include the umbra, penumbra and antumbra, which define shadow regions created when one object blocks light from a luminous source.

• The umbra is the darkest region in which the light source is completely obscured.
• The penumbra is the region of partial shadow, where the light source is only partly blocked.
• The antumbra extends beyond the tip of the umbra, where the blocking object appears entirely within but smaller than the light source.

Observers within the umbra witness a total eclipse, those within the penumbra observe a partial eclipse, and those in the antumbra may see an annular eclipse. In lunar eclipses, only the Earth’s umbra and penumbra are relevant because its antumbra extends far beyond the Moon’s orbit.
Key phases of an eclipse are described in contacts: first contact marks the moment the eclipsing object begins to obscure the light source; second contact occurs when it is fully within the light disc; third contact signals its departure; and fourth contact marks its exit from the disc entirely.

Shadow Geometry and Orbital Considerations

In an eclipse involving spherical bodies, the length of the umbral shadow depends on the radii of the objects and their separation. For the Earth–Moon system, the Earth’s umbra extends roughly 1.38 million kilometres into space, easily long enough to envelop the Moon during a lunar eclipse. The Moon’s shadow, cast during a solar eclipse, moves across Earth’s surface at speeds near 1 km per second.
The geometry of eclipses is influenced heavily by the inclination of the Moon’s orbit. Because this orbit is tilted relative to the ecliptic, the nodes where the two orbital planes intersect appear to travel around the Earth due to gravitational interactions, producing an eclipse season approximately every six months. Eclipses during these seasons follow predictable cycles driven by the orbital periods of the bodies involved.
A prominent example is the saros cycle, lasting about 18 years and 11 days. This interval corresponds to near-integer relationships between several lunar orbital periods, allowing eclipses of similar geometry to recur. Each saros series contains solar and lunar eclipses separated by these intervals, although they occur in different parts of the world owing to the fractional day remainder in the cycle.

Solar Eclipses

A solar eclipse arises when the Moon passes between the Earth and the Sun. The type of eclipse observed depends on the Moon’s distance from Earth and the alignment of the three bodies.

• A total solar eclipse occurs when observers enter the Moon’s umbra, resulting in complete occultation of the Sun’s photosphere.
• An annular eclipse takes place when the Moon is too distant to fully cover the Sun, leaving a bright ring, or annulus.
• A partial eclipse occurs when observers fall within the penumbra, with only a portion of the Sun concealed.

The magnitude of a solar eclipse describes the fraction of the Sun’s apparent diameter obscured by the Moon. Values greater than or equal to one indicate total coverage, while smaller values denote partial or annular events.
Solar eclipses are relatively brief. Under the most favourable geometric conditions, totality can last up to 7 minutes 31 seconds and follow a track up to about 250 kilometres wide. Outside this narrow corridor, vast regions may observe a partial eclipse. The Moon’s shadow typically travels eastward at speeds approaching 1,700 km per hour as it intersects Earth’s surface. The near-equality of the apparent diameters of the Sun and Moon enables the dramatic total occultations seen from Earth, a coincidence not shared by many planetary systems.

Lunar Eclipses

A lunar eclipse occurs when the Earth’s shadow falls upon the Moon. These events are visible to all observers on the night side of Earth and last longer than solar eclipses because the Earth’s umbra is much larger than the Moon. Only total and partial lunar eclipses are possible, as the Earth’s antumbra extends far beyond the Moon’s orbit. Lunar eclipses take place during the full moon phase of an eclipse season when alignment places the Moon near one of its orbital nodes.

Eclipse Cycles and Frequency

Within a single calendar year, between four and seven eclipses may occur, involving combinations of solar and lunar events. Maximum sets of seven are relatively rare but have occurred in years such as 1591, 1787, 1918 and 1982. Over centuries, these patterns repeat according to saros and other cycles involving nodical, synodic, anomalistic and sidereal orbital periods.
Because the shadow paths of eclipses shift with orbital evolution, each event varies in its visibility from different locations on Earth. Regions experience long intervals without a total solar eclipse, while lunar eclipses are more commonly observed from wide geographical areas.

Eclipses Beyond Earth

The principles governing eclipses apply throughout the Solar System. Moons of planets may regularly enter each other’s shadows or produce mutual occultations. Jupiter’s large moons frequently cast shadows across the planet’s cloud tops, creating eclipses observable through telescopes. Similar phenomena occur in other planetary systems and in binary star pairs, where eclipsing binaries are valuable for measuring stellar masses, radii and orbital dynamics.