Asterism

A group of stars in a recognizable shape. Stars may all be within a single constellation, or may come from several. Examples: the Big Dipper / Plough, Orion’s Belt, the Summer Triangle, the Winter Hexagon.

Bayer notation

A system of naming stars within their constellation, using Greek letters according to visual brightness. Universally used, you will come across this method of referring to stars, but its given name is equally acceptable, if it has one.

Circumpolar

We use this phrase a little wrongly. All stars are circumpolar, around either the north or south celestial pole. Usually, however, we use this word to indicate stars that do not set at your latitude. For the whole of Europe, for example, the Plough and Cassiopeia are circumpolar. From the Midlands northwards, Deneb and Capella just about stay above the horizon.

Constellation

An area of the sky. Constellations are not regularly shaped, they have been drawn around selections of stars that have traditionally been viewed as a group. Thus we can speak of a comet, for example, being “in” a constellation – simply in which patch of sky you’ll find it.

Ecliptic

The path the sun appears to take across the sky. The planets and moon are also to be found very close to or on this path. So are some stars!

Equinox

The point at which the sun enters the celestial northern or celestial southern hemisphere, and the halfway point between the solstices. The daylight sky will change colour over the few days around the equinox, from greeny-blue in the winter half to purply-blue in the summer months.

Magnitude

The apparent brightness of an object. The lower the number, the brighter the object, and both positive and negative numbers are used. Vega (in Lyra) has a magnitude of 0. Betelgeuse is 0.45, Sirius is -1.45. As a rule of thumb, anything with a magnitude of 4 or less should be visible to the naked eye.

Solstice

The highest or lowest point the sun reaches in the year. The height of the ecliptic is lower or higher each day as we move from one solstice to the other. Often called “Longest Day” or “Shortest Day” in popular calendars. See equinox.

Zodiac

Normally a term reserved for astrology, but occasionally pops up in astronomy in phrases such as “Sagittarius is a zodiacal constellation.” The Zodiac is the circle in the sky around the Earth, drawn by the Sun’s rising points each day for a year. The Zodiac “starts” on the day of the spring equinox and “zodiacal constellations” are those through which the circle passes. The “signs of the zodiac” are strictly an astrological concept and are not the same as the zodiacal constellations.