Constellations

Look at the stars for long enough and you start to see shapes: a hunter, a plough, a cross. People everywhere have done this, and different peoples saw different pictures in the same stars. These join-the-dots pictures are constellations.

The stars in a picture are not really together. Take Orion, the hunter. His shoulder, the red star Betelgeuse, is about 500 light years away. His foot, Rigel, is about 860. They only look like parts of one figure because we see them in nearly the same direction.

Today astronomers use 88 official constellations. They work like countries on a map of the sky: every star belongs to one, which makes it easy to say where something is.

Which ones you can see depends on where you live and the time of year. As the Earth goes round the Sun, the night side faces a different part of space, so the winter sky and the summer sky are not the same.

In the 3D view the figures are drawn as lines between the stars. Leave the Solar System and you can watch them pull out of shape.

The word constellation is used in two ways. To most people it is a figure made by joining bright stars. To astronomers it is one of 88 areas that together cover the whole sky, so that every star, galaxy and passing comet is in exactly one of them. The figures are old and came first. The areas are the modern, official version.

A pattern has depth. These are the seven bright stars of Orion, with distances from the star catalogue this site draws.

Star	Part of the figure	Distance in light years
Bellatrix	Shoulder	250
Betelgeuse	Shoulder	500
Saiph	Knee	650
Mintaka	Belt	690
Alnitak	Belt	740
Rigel	Foot	860
Alnilam	Belt, the middle star	Given as nearly 2,000

The nearest is about 250 light years away and the furthest several times that. Catalogue distances beyond a thousand light years are uncertain, so Alnilam’s may be well out, but the point stands: seen from the side, Orion is a long, thin scatter of stars, not a figure.

Why the sky changes with the seasons. The stars are there all the time, but you can only see them at night, and night is the side of the Earth facing away from the Sun. As the Earth goes round the Sun, that side looks out on a different part of space. Each night the stars are a little further west than at the same time the night before, and after a year they are back where they started. A constellation that lies in the direction of the Sun is up only in the daytime and cannot be seen at all.

Why it depends on where you are. The northern and southern halves of the Earth face different ways. Someone in Australia can see some constellations that someone in Britain cannot, and the other way round.

The zodiac. The Sun, Moon and planets all keep close to one line across the sky, called the ecliptic. The constellations that line runs through are the zodiac. Astrology uses them to make claims about people’s lives and futures. Those claims are not based on scientific evidence. Astronomy uses the same constellations only as places.

What they are used for now. The stars hold nearly the same positions for many lifetimes, so constellations make good landmarks. Many stars and nebulae are named after the constellation they are in, and a meteor shower is named after the constellation its meteors seem to come from. The Orionids appear to come from Orion.

What the 3D view shows. The view draws each figure as lines joining its stars, and each end of a line sits at that star’s real place in space, not on a painted dome. Close to the Sun, out as far as the Oort cloud, the figures look just as they do from the Earth, with their names beside them. A few light years further out they begin to change. A star that is close to us shifts a long way against the rest, and a distant one hardly moves, so every figure is pulled out of shape in its own way. Further off still the lines are faded out, because by then they are only a tangle pointing back at the Sun.

A few lines are missing. Nine stars the figures use have no usable distance in the catalogue, so they have no place in space to draw to, and the lines that end on them are left out. Camelopardalis, Cepheus and Sagittarius are the figures that lose most.

The lines themselves are a convention, taken from one modern set of figures that is credited on the sources page. The official constellations are areas of sky, not line drawings.