Albedo

The fraction of impinging sunlight reflected by a celestial body.

Aphelion

The greatest distance in its orbit of the Earth (or any other celestial body) from the Sun.

Apogee

The greatest distance in its orbit of the Moon from the Earth; may also be applied to other objects, such as artificial satellites.

Appulse

When two celestial bodies come into conjunction but do not quite eclipse, occult, or transit each other (e.g., an asteroid passing very close by a star in the sky).

Arcsecond

A measure of angle equivalent to one-sixtieth of an arcminute, which in turn is one-sixtieth of a degree, there being 360 degrees in a complete circle.

Astronomical Unit (AU)

The average distance between the Earth and the Sun (1 AU is approximately 93 million miles, or 150 million kilometers).

Azimuth

The angular distance measured along the horizon from a fixed point (usually clockwise from due north).

Baily’s beads

The visual phenomena seen just before or during totality in a solar eclipse, the light from the photosphere reaching the eye through valleys around the periphery of the Moon appearing to form moving beads.

Barycenter

The combined center of mass of two or more orbiting bodies.

Celestial equator

The equatorial plane of the Earth extrapolated out into the sky.

Celestial latitude

The angle north or south of the ecliptic plane.

Celestial longitude

The angle around the ecliptic from the spring equinox position to some specified point; this is measured counterclockwise (i.e., in the direction of the orbital motion of the Earth, the Moon, and the other planets).

Chord

The path apparently taken by an eclipsing body across an eclipsed body (e.g., Venus or Mercury in transit across the Sun).

Chromosphere

A layer a few thousand miles thick between the photosphere and the corona that may be seen fleetingly during an eclipse as a circle of red spikes around the outside of the Sun.

Coma

The vast cloud of gas and dust around the solid nucleus of a comet formed when sunlight causes some of its icy content to evaporate.

Conjunction

The alignment of two celestial bodies when they are at the same celestial longitude. Lunar superior conjunction (i.e., when the longitudes are 180 degrees different making the Moon full, such that a lunar eclipse may occur) is usually termed opposition. Inferior conjunction (when the Moon may eclipse the Sun) is often simply termed conjunction; many accounts incorrectly refer to this as being the time of new moon.

Contact points

The points (and instants of time) when the silhouettes and shadows of eclipsing bodies come into contact; these define the periods of the total and partial phases of an eclipse. The contact points may be either internal or external (e.g., when Venus touches the solar disk on the outside, and

when it wholly enters that disk a short time later) leading to the concepts of ingress and egress, or immersion and emersion.

Corona

The outermost layer of the solar atmosphere, spreading millions of miles out into space, which may be seen as a white halo around the Moon during a total solar eclipse.

Diamond-ring effect

The visual phenomenon usually seen just before totality is reached in a solar eclipse, the corona and chromosphere providing the apparent ring while the final visible part of the photosphere sparkles like a diamond set on that ring.

Eclipse season

A period of time during which the Sun (or the Moon) is between the ecliptic limits.

Eclipse year

The time between successive passages of the Sun through the lunar nodes; duration 346.6 days. Eclipses of the Sun and the Moon can only occur during the eclipse seasons that straddle the middle and the beginning/end of each eclipse year.

Ecliptic

The plane of the Earth’s orbit, and also the apparent path of the Sun across the sky.

Ecliptic limits

The range of possible celestial longitudes within which an eclipse can occur.

Flare

A massive ejection of solar material into space, seen as a bright globule. Larger flares appear as prominences during an eclipse.

Ground track

The shadow of an eclipsing body drawn across the surface of the Earth.

Inclination

The tilt of the Moon’s orbit relative to the ecliptic. (May also be applied to other objects such as planets, comets, and asteroids.)

Ion

An atom or molecule with either fewer or more electrons than usual, giving it a net positive or negative electrical charge.

Light-curve

A graph of the way in which the brightness of a body changes as it is eclipsed, or otherwise changes in intensity (e.g., variable stars display intrinsic cyclic changes in brightness).

Metonic cycle

A period of 19 solar years, which is very close to 235 synodic months in duration. Apart from providing for eclipse sequences, the Metonic cycle is used in various calendar schemes (e.g., the calculation of dates of Easter).

Node

Either of two diametrically opposite points where the orbit of the Moon, a satellite, a planet, or some other celestial object crosses the ecliptic. If passing from south of the ecliptic to the north that point is termed the ascending node; the other is the descending node.

Occultation

The eclipse of a star or some other distant body such as a galaxy or quasar by the Moon or another solar system body (a planet, asteroid, or comet).

Opposition

The time at which a celestial object is opposite the Sun in the sky such that their celestial longitudes differ by 180 degrees; see also Conjunction.

Penumbra

The area of partial shadow surrounding that of complete shadow (the umbra) in an eclipse.

Perigee

The closest approach in its orbit of the Moon to the Earth; may also be applied to other objects such as artificial satellites.

Perihelion

The closest approach in its orbit of the Earth (or any other celestial body) to the Sun. See also Aphelion.

Photosphere

The visible surface of the Sun. During a total eclipse the photosphere is obscured by the Moon, allowing the chromosphere, corona, and prominences to be seen.

Precession

The gradual shift in some celestial angular measure

due to gravitational perturbations. For example, the Earth’s spin axis precesses in a clockwise direction under the influence mainly of the Moon and the Sun, taking 25,800 years to complete a 360 degree turn, and this results in the precession of the equinoxes. The perihelion point of the Earth’s orbit around the Sun precesses in the counterclockwise direction under gravitational tugs by the other planets, taking 110,000 years to make a full revolution.

Prominence

A cloud of gas protruding outwards from the chromosphere into the corona. Prominences are spectacular reddish structures often seen during total solar eclipses; they may be over 100,000 miles high.

Redshift

The displacement of spectral lines toward the red end of the spectrum, due to an object’s recessional speed; caused by the Doppler effect.

Refraction

The bending of light rays as they pass from one medium (e.g., air) into another of different density (e.g., glass).

Saros

The cycle of 18 years plus 10 or 11 days over which eclipses repeat. The saros is due to the near synchronicity of integer multiples of the synodic, anomalistic, and nodical months, as explained in the Appendix.

Seeing

A measure of the twinkling of stars caused by atmospheric turbulence. At any particular observatory the seeing may vary from hour to hour and night to night, and is measured in arcseconds.

Shadow bands

Short-lived alternating bright and dark bands of light that may be seen on the ground and other suitable structures in the last few seconds before a solar eclipse becomes total. Their origin is similar to the seeing of stars, being caused by the propagation through the turbulent atmosphere of light

from a small source, in this case the very slender crescent of the solar disk shortly before it is obscured.

Spicules

The numerous short, spiky prominences seen during an eclipse to project up from the chromosphere into the lower corona.

Sunspot

A dark patch seen on the photosphere, slightly cooler than its surrounds.

Synodic month

The time from one full moon to the next; also known as a lunation. There are other types of lunar month (sidereal month, anomalistic month, draconic or nodical month), as discussed in the Appendix.

Syzygy

Either of the two points in the lunar orbit that are aligned with the Earth and Sun in terms of celestial longitude (i.e., inferior and superior conjunction, or opposition), where an eclipse is possible.

Transit

The passage of Mercury, Venus, or some other celestial body (e.g., an asteroid) across the face of the Sun. May also be applied to other cosmic situations (e.g., transit of a moon across the face of Jupiter, or transit by one star in a binary pair across the other).

Umbra

The region of complete shadow in an eclipse.

Universal Time (UT)

The reference system used to coordinate most time-keeping worldwide, especially in astronomical observations. It may be thought of as being the mean solar time for the Greenwich meridian (i.e., GMT) although the definition is slightly different.

Zenith

The point in the sky directly above the observer.