annular eclipse: A solar eclipse in which a ring of the Sun's photosphere remains visible when the Sun, Moon and Earth are aligned. Since the orbits of the Earth around the Sun and of the Moon around the Earth are elliptical, the angular diameters of the Sun and Moon vary slightly as their distances from the Earth change. A solar eclipse that would otherwise have been total is seen as annular if the Moon's angular diameter at the time is less than the Sun's.

aphelion: The point furthest from the Sun in the orbit of a body, such as a planet or comet, that is travelling around the Sun.

apogee: The point furthest from the Earth in the orbit of the Moon or of an artificial Earth satellite.

ascending node: The point where an orbiting object crosses the plane of reference for its orbit, travelling from south to north. The opposite point in the orbit, where the object crosses the plane from north to south, is the descending node. For a planet or comet, the reference plane is the ecliptic.

conjunction: An alignment of two bodies in the solar system with the Earth, so that they appear to be at the same place (or nearly so) in the sky as seen from the Earth. A planet is said to be at conjunction when it is at the same ecliptic longitude as the Sun, and so approximately in line with it. The planets Mercury and Venus can form such a line by being between the Earth and the Sun, when they are said to be at inferior conjunction, or behind the Sun as seen from the Earth, an alignment called superior conjunction. The planets further from the Sun than the Earth can only come to superior conjunction. Conjunctions can also occur between planets or between the Moon and one or more planets but, in that context, the word is often used more loosely to describe an approach within a few degrees.

descending node: The point where an orbiting object crosses the plane of reference for its orbit, travelling from north to south. The opposite point in the orbit, where the object crosses the plane from south to north, is the ascending node. For a planet or comet, the reference plane is the ecliptic.

ecliptic: 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 traveling 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 and additional constellation, Ophiuchus.

elongation: The angular distance between the Sun and a planet or the Moon as viewed from the Earth, i.e. the angle Sun-Earth-Moon/planet. For the inferior planets, Mercury and Venus, elongation is restricted to a limited range. The maximum values, east and west, reached during each orbit are called greatest elongation. The greatest elongation for Mercury lies between 18 and 28 degrees according to circumstances; the equivalent range for Venus is 45-47 degrees. Any elongation is possible for the planets further from the Sun than the Earth. An elongation of 90 degrees is called quadrature, of 0 degrees conjunction, and of 180 degrees opposition.

Mercury at greatest eastern elongation (19 degrees, January 11, 2002)

equinox: Either of the two points at which the celestial equator intersects the ecliptic, and also the times when the Sun passes through either of these points. The Sun passes from south to north at the northern vernal (spring) equinox and from north to south at the northern autumnal equinox. The approximate dates are March 21 and September 23. The position of the northern vernal equinox is also traditionally known as "the first point of Aries". However, the effects of precession have gradually moved the point so that it now actually lies in the adjacent constellation of Pisces.

heliocentric: Pertaining to a Sun centered solar system.

inferior conjunction: The position of either of the planets Mercury or Venus when it lies directly between the Earth and the Sun. Because of the relative tilts of the planetary orbits, only rarely does Mercury or Venus actually transit the face of the Sun. Normally, at inferior conjunctions, the planets pass in the sky to the north or south of the Sun.

Mercury in inferior conjunction (January 27, 2002)

inferior planet: Either of the two planets Mercury or Venus, whose orbits around the Sun lie inside that of the Earth.

libration: Any of several effects that alter precisely which hemisphere of the Moon's surface is visible from the Earth. Despite the fact that the Moon's rotation and orbital periods are equal, so that the Moon very nearly keeps the same face towards the Earth all the time, a total of 59 percent of the Moon's surface can be viewed from the Earth at some time or other as a result of libration. Physical libration is a real irregularity in the Moon's rotation; the larger effect is geometrical libration, in both latitude and longitude. Libration in latitude results from the Moon's orbit being inclined to the ecliptic by an angle of 5 degrees, 9 minutes of arc. The elliptical shape of the Moon's orbit means that its orbital velocity is not constant, and this produces libration in longitude of 7 degrees, 45 mintues of arc. Additionally, diurnal libration is a small effect that results from observing the Moon at different times of day.

meridian: (1) The great circle on the celestial sphere passing through the poles and the zenith. (2) A line of longitude on the Earth, or on another astronomical body. On the Earth, the meridian through Greenwich, England marks the zero of longitude and is sometimes referred to as the prime meridian.

occultation: The passage of one astronomical object directly in front of another so as to obscure it from view as seen by a particular observer.

opposition: The position of one of the superior planets when it is opposite the Sun in the sky, that is, when its elongation is 180 degrees. At opposition planets are at full phase and reach their highest point in the sky at midnight. At the same time, they achieve their closest approaches to the Earth. As the orbits of the planets are elliptical rather than perfectly circular, some oppositions bring the planets closer to Earth than others. This effect is particularly marked with Mars.

Opposition of Mars (June 13, 2001)

penumbral: (1) A region of partial shadow. During a solar eclipse, when the Moon's shadow sweeps across the surface of the Earth, observers in the penumbral zone see only a partial eclipse. (2) The lighter periphery of a sunspot, surrounding the darker umbra. In the penumbra the magnetic field is horizontal and spreads radially.

perihelion: In orbital motion in the solar system, the point of closest approach to the Sun.

perigee: In orbital motion, the point of closest approach to the Earth of the Moon or an artificial Earth satellite.

precession: The uniform motion of the rotation axis of a freely rotating body when it is subject to turning forces (torque) due to external gravitational influences. Precession causes the Earth's rotation axis to sweep out, over a period of 25,800 years, a cone of angular radius about 23 degrees, 26 minutes around the perpendicular to the plane of the Earth's orbit (ecliptic). The main source of the torque is the combined effect of the gravitational pulls of the Sun and Moon on the Earth's equatorial bulge. (Precession would not occur if the Earth were a perfect sphere. Rotation, however, causes the equatorial radius to exceed the polar radius by about 0.3%). The combined effect of the Sun and Moon is called lunisolar precession. The Moon's contribution is about twice as large as the Sun's, because of its smaller distance. The gravitational force exerted on the Earth by the other planets causes small changes in the Earth's orbital elements, giving rise to planetary precession. The sum of planetary and lunisolar precession is called general precession. A consequence of the precession of the Earth's rotation axis is that the celetial poles trace out circles in the sky over 25,800 years. So, for example, about 13,000 years from the present, the nearest bright star to the north celestial pole will be Vega rather than Polaris.

quadrature: The position of the Moon or of a planet when its angular distance from the Sun, as viewed from the Earth, is 90 degrees.

retrograde: Motion of an object on the celestial sphere in the east-west direction, or, for orbital motion or axial rotation in the solar system, motion that is clockwise as observed from north of the ecliptic.

selenographic: Pertaining to a Moon-centered coordinate system.

solstice: The two points on the ecliptic where the Sun reaches its maximum and minimum declination, or the times at which the Sun is at these points. They lie approximately midway between the vernal and autumnal equinoxes. The solstices occur on about June 21 and December 21. At the summer solstice, the Sun reaches its highest altitude in the sky and the duration of daylight is maximum. At the winter solstice the Sun's altitude at noon is the lowest as seen from a particular latitude, and the duration of daylight is a minimum.

superior conjunction: The point in the orbit of either Mercury or Venus when the planet lies on the far side of the Sun as viewed from the Earth.

Venus in superior conjunction (January 14, 2002)

superior planet: Any of the major planets whose orbits lie outside that of the Earth - Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.

transit: (1) The passage of a star or other celestial body across the observer's meridian in the course of the daily apparent motion of the celestial sphere. (2) The passage of either of the planets Mercury or Venus across the visible disc of the Sun. (3) The passage of a natural satellite across the disc of its parent planet.

umbra: (1) An area of total shadow, such as the zone on the surface of the Earth from which totality is observed during a solar eclipse. (2) The dark central region of a sunspot, where the magnetic field is vertical and typically has a strength a few thousand times that at the surface of the Earth. The temperature is about 3500 degrees K, compared with 6000 degrees K for the surrounding photosphere.

zodiac: A belt of twelve constellations through which the Sun's path in the sky, the ecliptic, passes. They are Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricornus, Aquarius, and Pisces. Though the ecliptic formerly went through these twelve constellations, the effects of precession and the precise definitions of constellation boundaries mean that it now also goes through a thriteenth, Ophiuchus. Since the orbits of all the planets, apart from Pluto, lie very nearly in a plane, the apparent paths of the planets remain in or close to the zodiacal constellations.