The altitude of a star or other object in the sky is the angular distance of that object above the horizon. For example, if a star is at the zenith, or overhead point, its altitude is 90° and if it is on the horizon, its altitude is 0° (see also Angular Measurement).

Apogee is the point in the orbit of the Moon or other satellite around our planet at which it is furthest from the Earth.

A grouping or collection of stars, located within one or more constellations, that form an apparent and distinctive pattern as seen from Earth. Examples include the False Cross (formed from stars in Carina and Vela), the Square of Pegasus in Pegasus, the Summer Triangle (formed from stars in Lyra, Cygnus and Aquila) and what is perhaps the best known asterism, the Plough in Ursa Major.

See Double Star

The celestial sphere is the imaginary sphere which surrounds the Earth and upon which the stars and constellations appear to lie.

A circumpolar star is one which never sets from the observer’s latitude (see Circumpolar Stars).

A constellation is an arbitrary grouping of stars forming a pattern or imaginary picture on the celestial sphere. There are 88 official constellations, many of which have traditional names and are associated with folklore and mythology. The constellations together cover the entire sky, each one referring to, defining and identifying that particular region of the celestial sphere. As a result, every object that we see in the sky is described as being within one particular constellation or another.

Double stars are two stars which appear to be close together in space. Most of these are comprised of stars that are gravitationally linked and orbit each other, forming a genuine double-star system (also known as a binary star). Some double stars, however, (known as optical doubles) are made up of two stars that only happen to lie in the same direction as seen from Earth and are not linked to each other. Alcor and Mizar, in the Plough, are not actually associated with each other and only form an optical double. Mizar itself, however, is a binary star, the two stars being gravitationally linked.

An eclipse occurs when one celestial object is either totally or partially obscured by another, such as the Sun by the Moon during a solar eclipse or when the Earth’s shadow falls onto the lunar surface during a lunar eclipse.

The ecliptic is the apparent path of the Sun through the sky, seen superimposed against the background of stars and passing through a band of constellations called the Zodiac.

A galaxy is vast collection of stars, gas and dust bound together by gravity. Measuring many light years across, galaxies occur in a wide variety of shapes and sizes including spiral, elliptical and irregular and most are so far away from us that their light has taken many millions of years to reach our planet. Our solar system is located in the Milky Way Galaxy, a spiral galaxy containing several billion stars. A member of the Local Group of Galaxies (see below), the Milky Way Galaxy is often referred to simply as the Galaxy.

To express distances to the stars and other galaxies in miles or kilometres would involve numbers that are so huge that they would be unwieldy. Astronomers therefore use the term ‘light year’ as a unit of distance. A light year is the distance a beam of light, travelling at around 186,000 miles (300,000 km) per second, would travel in a year, and is equivalent to 5.88 trillion miles (9.46 trillion kilometres).

The Local Group is a gravitationally-bound collection of galaxies containing over 50 individual members, one of which is our own Milky Way Galaxy. Galaxies are usually found in groups or clusters and, apart from our own Local Group, many other groups of galaxies are known, typically containing anywhere up to 50 individual members. Even larger than the groups are clusters of galaxies which can contain hundreds or even thousands of individual galaxies. Groups and clusters of galaxies are found throughout the universe.

A node is the point at which the orbital path of a planet or comet intersects, or crosses, the ecliptic. An orbiting body moving north produces an ascending node, and if moving south, a descending node. The overall effect is that the orbital plane (the imaginary circle effectively traced out as an object orbits the Sun) is ‘tilted’ to the ecliptic. The plane of reference for the orbit of the Moon around the Earth is taken to be the ecliptic, not the equatorial plane.

Perigee is the point in the orbit of the Moon or other satellite around our planet at which it is closest to the Earth.

A star is a self-luminous object that shines through the release of energy produced by nuclear reactions at its core. Stars come in a whole range of different colours, sizes and temperatures, our Sun being a fairly average, medium-sized yellow star.

Stars are seen to have many different colours, a prominent example being the bright red Betelgeuse in the constellation of Orion, which contrasts sharply with the nearby brilliant blue-white Rigel which marks Orion’s foot. Our own Sun is yellow and Procyon, the brightest star in Canis Minor, also has a yellowish tint. Other stars with conspicuous colours include orange-red Aldebaran in Taurus and the red super giant Antares in Scorpius.

The colour of a star is a good guide to its temperature, the hottest stars being blue and blue-white with surface temperatures of 20,000 degrees or more. Classed as a yellow dwarf, the Sun is a fairly average star with a temperature of around 6,000 degrees. Red stars are cooler still, with surface temperatures of only a few thousand degrees. Betelgeuse in Orion and Antares in Scorpius are both red giant stars that fall into this category.

Variable stars are stars whose brightness varies over a period of time. There are many different types of variable star, although the variations in brightness are basically due either to changes taking place within the star itself, such as with Mira in Cetus, or the periodic obscuration, or eclipsing, of one member of a binary star by its companion, as is the case with Algol in Perseus.

The zodiac is the band of 12 constellations which straddles the ecliptic through which the Sun and planets appear to travel throughout the year. The constellations forming the Zodiac are Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricornus, Aquarius and Pisces.