annular eclipse

A solar eclipse that occurs when the apparent size of the Moon is not large enough to completely cover the Sun. A thin ring of very bright sunlight remains around the black disk of the Moon.

anomalistic month

The time taken for the Moon to orbit the Earth from apogee to perigee and back to apogee.

antumbra

That part of the Moon's shadow that extends beyond the umbra. An annular eclipse is seen by an observer in the antumbra.

aphelion

The point in an object’s orbit when it is farthest from the Sun. Currently the Earth reaches aphelion in July.

apogee

The point in an object’s orbit when it is farthest from the Earth.

astronomical unit

The average distance between the Earth and the Sun (149 597 870 km).

Baily's beads

The effect seen just before and just after totality when only a few points of sunlight are visible through valleys around the edge of the Moon.

central eclipse

A solar eclipse in which the central axis of the Moon's shadow traverses the Earth. Central solar eclipses can be total, annular or hybrid

chromosphere

The lower atmosphere of the Sun just above the photosphere that appears as a thin crimson ring around the edge of the Sun during a total solar eclipse.

contact

One of the instances when the apparent position of the edges of the Sun and the Moon (for eclipses) and the Sun and a planet (for transits) cross one another during an eclipse or transit. They are designated as first, second, third and fourth contact.

corona

The upper atmosphere of the Sun that appears as a halo around the Sun during a total solar eclipse.

crescent

The phase of the Moon when it appears to be less than half illuminated, or something of similar shape such as the Sun during a partial solar eclipse.

diamond ring

The effect seen in the few seconds just before and just after totality of a total solar eclipse when there is a single point of sunlight brilliantly shining through a valley on the limb of the Moon.

eclipse

The alignment of celestial bodies so that one is obscured, either partially or totally, by the other.

eclipse season

The period of time when the Sun is near alignment with a lunar node, during which eclipses may take place. For solar eclipses, this time window of 31-37 days occurs every 173.3 days.

eclipse year

The length of time it takes for the apparent motion of the Sun to take it from one node of the Moon to the other and back to the original node (about 346.6 days).

ecliptic

The plane of the Earth's orbit around the Sun. As seen from the Earth, the Sun appears to move along the ecliptic during the course of a year.

gibbous

The phase of the Moon when it appears more than half illuminated.

H-alpha line

The strongest spectral line of hydrogen in the visible part of the spectrum.

hybrid eclipse

A solar eclipse which appears annular and total along different sections of its path.

lunar eclipse

The passage of the Moon into the shadow of the Earth, which can only occur at a full moon.

lunar month

The time from one full moon to the next (29.53 days).  More properly the synodic month.

magnitude

(of a solar eclipse) The fraction of the apparent diameter of the Sun covered by the Moon. By convention it is usually quoted at maximum phase.

mare

A large flat area on the Moon formed by volcanic material flowing from beneath the lunar surface. (plural is maria)

node

The two points where a tilted orbit intersects a geometrical plane e.g. the Moon's orbit intersecting the ecliptic.

obscuration

(of a solar eclipse) The fraction of the Sun's area covered by the Moon.

partial lunar eclipse

A lunar eclipse where a portion (but not all) of the Moon enters the Earth's umbra.

partial solar eclipse

A solar eclipse seen from within the Moon's penumbra. The Moon appears to block part (but not all) of the Sun's photosphere.

path of totality

The path (up to about 270 km wide) that the Moon's shadow traces on the Earth during a total solar eclipse.

penumbra

The part of a shadow (as of the Moon or the Earth) within which the source of light (the Sun) is only partially blocked. Also, the lighter outer area of a sunspot.

penumbral eclipse

An eclipse of the Moon when the Moon enters the penumbra of the Earth’s shadow.

perigee

The point in an object’s orbit when it is closest to the Earth.

perihelion

The point in an object’s orbit when it is closest to the Sun. Currently the Earth reaches perihelion in early January.

photosphere

The bright, visible surface of the Sun. 

prominence

A large-scale gaseous formation above the surface of the Sun shaped by the Sun’s magnetic field.

Rayleigh scattering

The scattering of light by particles smaller than the wavelength of the light, resulting in separation of colours. This causes the sky to be blue and sunsets to be reddish.

regression

The movement of points in an orbit in the direction opposite from the motion of the orbiting body. For example, the Moon travels from west to east but its nodes are regressing from east to west.

saros

The eclipse cycle with a period of 223 synodic months or 6 585.32 days (18 years and about 11.3 days).

shadow bands

Faint ripples of light sometimes seen on flat, light-coloured surfaces just before and just after totality.

sidereal month

The time taken for the Moon to make one orbit of the Earth with reference to the fixed stars (27.32 days).

solar eclipse

The passage of the new moon directly between the Sun and the Earth when the Moon's shadow is cast upon the Earth. The Sun appears in the sky either partially or totally covered by the Moon.

solar flare

An explosive eruption on the Sun.

sunspot

A magnetic disturbance on the Sun that appears as a dark blotch on its surface.

synodic month

The time from one full moon to the next (29.53 days). Also called a lunar month.

terminator

The edge between night and day on the Moon or planet.

total lunar eclipse

A lunar eclipse where the Moon enters completely the Earth's umbra.

total solar eclipse

A solar eclipse seen from within the Moon's umbra. The Moon appears to completely block the Sun's photosphere.

totality

The period during a solar eclipse when the Sun's photosphere is completely covered by the Moon and the period for a lunar eclipse when the Moon is in the complete shadow of the Earth.

umbra

A complete shadow (as of the Moon or the Earth) within which the source of light (the Sun) is totally hidden from view. Also, the dark inner area of a sunspot.

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