A phenomenon in which the light from a celestial body is temporarily cut off by the presence of another. This may be: 1 the passage of a planetary satellite, such as the Moon, into shadow so that the direct illumination from the Sun normally causing it to shine is cut off; 2 the obscuration of all or part of the Sun by the passage of the Moon directly across it solar eclipse; 3 the passage of a star belonging to a binary system behind its companion so that the total light received from the system is reduced. There is some confusion of usage between the terms eclipse and occultation. If eclipse is reserved for the cutting off of sunlight by shadow, solar eclipses and the phenomena observed in eclipsing binary stars are, strictly speaking, occultations. However, the use of eclipse in these contexts is firmly established and normal. In the description of the motion of the moons of other planets, such as Jupiter, it is usual to distinguish between eclipses and true occultations. The Moon’s orbit around the Earth is inclined at only 5° to the plane of the Earth’s orbit around the Sun. From time to time, the three bodies become aligned and an eclipse of the Sun or Moon occurs. A solar eclipse can occur only at or very close to new Moon. Though the Moon is much nearer than the Sun, their apparent diameters are nearly equal at about half a degree. This coincidence makes total solar eclipses possible, with a maximum duration of 7,5 minutes. However, there are small variations in the apparent sizes of the Sun and Moon because the orbits of the Moon and Earth are elliptical rather than circular. The ratio of the diameters of the Moon and Sun is described as the magnitude of a solar eclipse. If a solar eclipse that would otherwise have been total occurs when the Moon’s diameter appears less than the Sun’s, an annulus ring of the Sun’s disc remains visible when the centres of the two bodies are aligned. Such a solar eclipse is described as annular. The Moon’s shadow on the Earth is only a few hundred kilometres wide. It traces out a curved path as the motion of the three bodies makes the eclipse visible at successive locations. Over a wider region either side of the path of totality, a partial eclipse is seen. Partial eclipses may occur when no part of the Earth witnesses a total eclipse. During the brief moments of a total solar eclipse, darkness falls, and the outer parts of the Sun, the chromosphere and the corona, whose light is normally swamped by the bright photosphere, become visible. Lunar eclipses occur when the Moon passes into the shadow of the Earth. They can take place only close to full Moon and can be seen from any location where the Moon has risen. The Moon does not normally disappear completely: its disc is illuminated by light scattered by the Earth’s atmosphere. It often takes on a deep reddish hue. The full shadow umbra cast by the Earth is surrounded by a region of partial shadow, called the penumbra. In the early and late stages of the progress of a lunar eclipse, the Moon enters the penumbra. It is possible for lunar eclipses to occur which are only penumbral. The length of the Moon’s path through the umbra, divided by the Moon’s apparent diameter, defines theÔmagnitude of a lunar eclipse. The relative motions of the Sun, Earth and Moon are such that at least two eclipses of the Sun must occur in any year though most will be partial. The maximum number of eclipses in any one year is seven, two or three of which must be lunar. It is theoretically possible for solar eclipses to occur at successive new Moons, and for there to be a lunar eclipse in between. However, lunar eclipses at two successive full Moons are not possible. See also: eclipsing binary.

The time between successive passages of the Sun through the same node of the Moon’s orbit, which is equal to 346.620 03 days. This period is less than a sidereal year because the changing orientation of the Moon’s orbit causes the node to change position in the sky. See also: saros.

A binary or multiple star system whose total brightness varies on a regular cycle because the orbital motion of the member stars causes them to pass one in front of another as viewed from Earth. If the two stars in an eclipsing binary are of unequal luminosity, the light curve displays a primary minimum at A when the dimmer star passes in front of the brighter one, and a secondary minimum at C when the opposite occurs. The best-known example of an eclipsing binary is Algol.

eclipsing binary.

The mean plane of the Earth’s orbit around the Sun. The name arises because eclipses of the Sun or Moon can occur only when the Moon passes through this plane. From the point of view of an observer on Earth, the relative orbital motion of the Earth and Sun makes it look as if the Sun were travelling around the Earth once a year. The Sun’s path on the celestial sphere traces out the ecliptic plane, and is often marked as the ecliptic on celestial charts. Since the orbits of the other planets are inclined to the ecliptic plane by only very small angles, their observed positions in the sky are always close to the ecliptic. The band of constellations through which the ecliptic passes defines the traditional zodiac, though the effects of precession and the precise definition of constellation boundaries mean that the ecliptic now passes through an additional constellation, Ophiuchus.

A celestial coordinate system, based on the plane of the ecliptic. This system has applications in the study of planetary and solar system dynamics. In the ecliptic system, the position of an object in the sky is defined by the coordinates ecliptic latitude β, beta and ecliptic longitude λ, lambda. Latitude is measured in degrees north and south of the ecliptic; northerly latitudes are positive and southerly ones negative. Longitude is measured in degrees along the ecliptic. The zero point is the northern vernal equinox, the point at which the ecliptic and the celestial equator intersect. Precession is making this zero point change slowly with time. See also: obliquity of the ecliptic.

The maximum angular distances the Sun or Moon can be at full or new Moon from the points where the Moon’s orbit crosses the ecliptic the nodes in order for an eclipse to be possible. If the Moon’s orbit coincided with the ecliptic, a solar and lunar eclipse would be observed at each new and full Moon, respectively. Since the Moon’s orbit is inclined at 5° to the ecliptic and the apparent diameters of the Sun and Moon are only half a degree, eclipses can occur only when the Earth, Sun and Moon are lined up in space, or very nearly so. From the point of view of an observer on Earth, this means that both the Sun and Moon must be within a certain angular distance of one of the nodes of the Moon’s orbit. For a lunar eclipse to occur, the Moon must be within 24° of its node; the Sun must be within 37° of the node for a solar eclipse to take place.

An upper limit on the ratio of the luminosity to mass of a stable star generating energy through the conversion of hydrogen to helium. Arthur S. Eddington 1881-1944 showed that the limit is 40,000 when units of the Sun’s mass and luminosity are used. If the limit is exceeded, the outer layers of the star are blown away by radiation pressure and a planetary nebula is formed. The Eddington limit also sets an important constraint on the rate at which a black hole can grow by accreting matter.

Kuiper Belt

A parameter that expresses how effective a radio telescope is at absorbing radiation of a particular frequency and from a particular direction, and at making the power available for measurement.

A measure of the energy output of an object, such as a star, defined as the temperature of a black body having the same total luminosity as the observed object. Effective temperature is often quoted as one of the physical characteristics of a star. Since the spectrum of a normal star is similar to that of a black body, effective temperature is a good indication of the actual temperature of its photosphere.

The location 40 kilometres 25 miles south-west of Bonn, Germany, of the observatory of the Max-Planck-Institut für Radioastronomie. The radio telescope located there is a 100-metre 330-foot steerable dish, the largest of its type in the world.

Asteroid 13, diameter 244 km, discovered by A. de Gasparis in 1850.

Abbreviation for evaporating gaseous globule. An EGG is a condensed ball of gas surrounding a star in the process of formation; it is exposed when the less dense gas surrounding it is dispersed by the action of ultraviolet radiation.

Popular name for a planetary nebula also known by the catalogue number CRL2688. It is a very young planetary nebula and the central star is hidden behind a ring of dust. Numerous shells of gas ejected by the star, which was a red giant until a few hundred years ago, are illuminated by light from the central star.