A list of astronomical terms used on this website.

This page was last amended on 1st January, 2010

Asterism. A group of stars, similar to a constellation, given a collective name. The important distinction, however, is that constellations are officially recognised  by the International Astronomical Union , and asterisms are not. (By international treaty, only the International Astronomical Union can name and define constellations.) Perhaps the best known asterism in the UK is The Plough, part of the much larger constellation Ursa Major.

Bayer designation. A system introduced by the German astronomer Johannes Bayer in 1603 in which he assigned a lower-case Greek letter to each star in a constellation, so that alpha (α) was the brightest, beta (β) the second-brightest and so on in the Greek alphabetic order. He attached the Greek letter to the Latin name of the star’s parent constellation in genitive (or possessive) form to indicate that the star belonged exclusively to that constellation. Unfortunately, the order of brightness is not always followed exactly, and a further complication is that stars with a common name are also assigned a Bayer designation, (e.g. Polaris in Ursa Minor has been designated the Greek letter "alpha" to become alpha Ursae Minoris), so such stars have two names!

Bolide. A fast, exceptionally bright meteor, which often explodes or disintegrates in flight.

Celestial equator. An imaginary line  directly above the Earth’s equator.

Circumpolar. Refers to a celestial object that never sets. It circles the celestial pole, is always above the horizon, and can be seen in the sky any night of the year. Whether or not a particular object is circumpolar depends on position of the observer on the Earth's surface. From Kidderminster UK those objects with a declination (see below) greater than about +37.5° will be circumpolar.

Conjunction. See "Superior conjunction".

Culmination. The time when a star or other celestial body reaches the observer's meridian and is thus at its highest point in the sky. From Kidderminster, this means when the body is in the south.

Declination. The angular distance of a celestial body north or south of the celestial equator. It is analogous to latitude on the surface of the Earth and is normally preceded by "+" to represent northern values and "-" to represent southern values.

Earthshine. Sunlight reflected from the day side of the Earth onto the Moon's surface. Also known as the Moon's "ashen glow" or "the old Moon in the New Moon's arms". An earthshine Moon is a brightly glowing slender crescent Moon joined with the pale glow of the nearly full "dark" side of the Moon. Earthshine is best seen during a few days either side of a New Moon and during the months of April and May. Particularly bright earthshine is sometimes considered to be an indication of bad weather to come in the UK, as rain-bearing clouds travelling from W to E over the Atlantic reflect much more light than if the skies were clear.

Ecliptic. The plane of the solar system and the path of the Sun in the sky with respect to the stars.

Elongation. The apparent angular distance of a planet from the Sun. 

"First quarter". The time at which first quarter of the Moon's period of revolution around the Earth ends; sometimes known as "half Moon".

Gibbous. The phase of Moon between first quarter and full Moon and between full Moon and last quarter.

Inferior conjunction. Occurs when a planet moves between the Earth and the Sun so that the three bodies are in almost a straight line, with the planet in the middle. The bright glare of the Sun makes it impossible to see the planet when this occurs. Being the only planets whose orbits lie nearer the Sun than the Earth's orbit does, Mercury and Venus  are the only ones that can exhibit an inferior conjunction.

Magnitude. Normally refers to the apparent or visual magnitude. It is a measure of the apparent brightness of a celestial object as seen with the naked eye. The greater the brightness, the lower is the value of the magnitude. For example, a star of magnitude +1 is brighter than a star of magnitude +3. This seemingly illogical convention is analogous to winning a prize in a raffle: the first prize is normally bigger, brighter or of a higher value than the third prize. Very bright objects have a magnitude of negative value. For example, Venus has typically a magnitude of -4, while Sirius, the brightest star in the sky, has a magnitude of -1.46. The Sun has a magnitude of -26.

Meridian. The great circle on the celestial sphere which passes through the zenith and both celestial poles, and therefore cuts the observer's horizon at exactly north and south.

Meteor. A so-called "shooting star". They are not stars at all, but specks of dust or other space debris, often no bigger than grains of sand, which enter the Earth’s atmosphere at high speed. Friction with the atmosphere causes the meteor to heat up and glow, white hot, as it burns up. This can be seen as a short-lived streak of bright light speeding across the night sky. Typically, about10 meteors an hour can be seen from any given location on the Earth's surface, quite at random. But several times a year the Earth passes through clouds of this debris left behind by visiting comets, when many more meteors per hour ("meteor showers") can be seen.

Occult (verb). To shut off from view; to cover. The Moon can occult a star or a planet, a planet can occult a star, and a planet can (very rarely) occult another planet

Occultation. The covering up of one celestial body by another.

Opposition. The position of a planet when it is exactly opposite the Sun in the sky. It can be seen due south at local midnight and can be considered to be at its closest to the Earth until it reaches opposition again, so the planet appears particularly bright. Venus and Mercury, whose orbits lie between the earth and the Sun, can never be in opposition.

Orbital eccentricity. For an ellipse, defined as (1 - 2//=((ra/rp) + 1)) where:

Perihelion. The point in the orbit of member of the Solar System that is nearest to the Sun.

Precession. The  slow, periodic conical motion of the rotation axis of a spinning body. In the case of Earth's precession it is due to the fact that Earth's axis of rotation is not perpendicular to the ecliptic but is inclined about 23°.5 and is thus affected by gravitational perturbations from other bodies in the solar system.

Radiant. The imaginary point in the sky from which all the meteors of a particular shower appear to originate. It is a manifestation of perspective, as all the meteors actually move  parallel to each other. Meteor radiants are not stationary, as the Earth's motion around the Sun causes them to move by about one degree of ecliptic longitude per day.

Retrograde motion. Normally planets in orbit around the Sun appear to move from west to east against the background stars. Occasionally, the planet will appear to change direction and move from east to west; this is known as "retrograde motion". It is purely a line of sight effect caused by the relative positions of the planet and the Earth in their respective orbits.

"Seeing". The clarity with which stars and other celestial objects can be observed.

Superior conjunction. Occurs when a planet is on the opposite side of the Sun from the Earth, so that the three bodies are in almost a straight line, with the Sun in the middle. The bright glare of the Sun makes it impossible to see the planet when this occurs.

Sporadic. Meteors that can not be associated with any meteor shower. They can be seen any night (weather permitting), in any part of the sky, in any direction. Up to about 10 sporadic meteors per hour can be seen most of the time, but you might be a few more during July or August.

Terminator. The dividing line between the illuminated and non-illuminated parts of the Moon or planets disk. At dawn and dusk (close to the terminator), surface features cast their longest shadows, indicating the size and shape of the objects casting the shadows. 

"Train". A trail of faintly glowing ionized gas which can still be seen "superimposed" on the path of a meteor after the meteor itself has burnt out. Trains normally exist for only a second or two, but very occasionally a train of 10 seconds duration, or even longer, may occur.