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

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

The distance between the Sun and an object in orbit around it when they are at their closest approach. See also: orbital elements.

The time after which a cyclical phenomenon repeats itself. See also: orbital elements.

The relationship between the absolute luminosity and the period of variability of Cepheid variable or W Virginis stars.

A comet in a closed, elliptical orbit within the solar system. Periodic comets are observed at their regular returns to the Earth’s vicinity, as long as their orbit carries them near enough to be recoverable. The term is usually applied to comets with periods of less than 200 years, more strictly called short-period comets.

A major annual meteor shower, the radiant of which lies near the star Eta Persei η Per. The peak of the shower occurs on 12 August and the normal limits are 23 July-20 August. The meteor stream is associated with Comet 109P/Swift-Tuttle. This is one of the best, most reliable annual showers, with peak rates typically between 50 and 100 per hour. Records of it date back around 2,000 years.

A large and interesting constellation of the northern hemisphere lying in a rich part of the Milky Way. It is one of the 48 listed by Ptolemy c. AD 140 and contains ten stars brighter than fourth magnitude, including the noted variable star Algol. Perseus also includes a magnificent pair of open clusters visible to the naked eye, known as the Double Cluster in Perseus.

One of the spiral arms of the Milky Way Galaxy. It winds around from the far side of the galactic centre to the region of the Galaxy beyond the Sun.

A systematic error in observations made by a particular individual.

A temporary or localized gravitational disturbance in the otherwise uniform motion of a body under a stable gravitational force. For example, the motion of a comet, which is largely determined by the gravitational force of the Sun, suffers perturbations if the comet passes close enough to a planet for its pull to be significant in comparison with that of the Sun. An encounter with the most massive planet, Jupiter, can alter very significantly the orbit of a comet, to the extent that a comet moving in a parabolic orbit may be captured into a short-period elliptical orbit.

A large lunar crater, 176 km 110 miles in diameter, near the south-east limb of the Moon. A prominent rille runs across the crater floor between the multiple central peak and the terraced walls.

Asteroid 3200, diameter 6 km, discovered in 1983 by the Infrared Astronomical Satellite IRAS. It is in a highly eccentric, Earth-crossing orbit and appears to be the parent body of the Geminid meteor shower. It may be the dead nucleus of a former comet.

The ratio of the illuminated area of the apparent disc of a celestial body to the area of the entire apparent disc, taken as a circle. The phases of the Moon are the recurring cycle of apparent forms of the Moon. New Moon, first quarter, full Moon and last quarter are formally defined as the times at which the excess of the apparent celestial longitude of the Moon over that of the Sun is 0°, 90°, 180° and 270° respectively. The Moon and planets show phases because bodies of this kind emit no light of their own, shining only by reflected sunlight. The hemisphere of a moon or planet facing the Sun is bright, its other hemisphere dark. The phase as seen from the Earth depends on the relative positions of the Earth and Sun with respect to the body, since this determines what proportion of its illuminated half is visible.

The angle between the two lines formed by joining the centre of a planetary body to the Sun and to the Earth.