Constellations

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If asked most people would be able to name the twelve constellations of the zodiac and possibly some more of the well known constellations such as Orion. A constellation being a group of stars that appear to make a pattern in the night sky, these patterns typically represent animals, mythological gods or creatures, or even inanimate objects.

The origins of the earliest constellations go back to the clay writing tablets from Mesopotamia dating to around 3000BC. Ancient China also has a long history of observing the heavens, star names have been found on oracle bones dating back to the Shang dynasty. The ancient civilisations used constellations to relate stories of their beliefs, experiences, or mythology.

There are 88 recognised constellations, the 48 traditional Western constellations are Greek and all are in the Northern Hemisphere as these were the only ones visible to them. The constellations in the Southern Hemisphere were named by astronomer Johan Bayer as he made his notes about the south regions of the sky, following the tradition of the Greek names mainly connected to the sea and its creatures. The astronomer La Caille added the last 13 constellations, but broke with the traditional name giving and used scientific equipment or instruments.

It wasn’t until 1922 that the International Astronomical Union (IAU) formally accepted the modern list of 88 constellations, and followed this in 1928 by adopting the official constellation boundaries that cover the entire celestial sphere.

Of the current constellations, there are 12 through which the Sun and the planets travel over the course of the year. These are known as the "signs of the zodiac" and are thought by many to have special significance. The signs of the zodiac we know today reach back to the days of the Babylonians, 2000 to 3000 years ago. At that time the sun passed through twelve constellations we still assign to the zodiac, but since then there have been several changes to the assignment of the stars. The sun now passes through thirteen constellations, as after the sun leaves Scorpius, the Scorpion, in the last week of November it moves through Ophiuchus, the Serpent Holder, then in December moves into Sagittarius, the Archer.

There are other star patterns and groups called asterisms. These are not constellations under the formal definition, but are used by observers to navigate the night sky. Asterisms may be several stars within a constellation, or they may share stars with more than one constellation. The most famous example of an asterisms is The Plough (or the Big Dipper) within the constellation of Ursa Major (The Great Bear). Other examples include the Pleiades and Hyades in the constellation Taurus and Venus' Mirror in the constellation of Orion.