Night sights worth look
ONE of the great delights of running planetarium shows is to project a realistic night sky onto the planetarium dome, and explain to the audiences what they can see in the sky that night.
With the sun setting early, the colder months are a great period to be out in the evenings, identifying the famous star patterns called the constellations. Join me tonight while we take a look at a couple of the most famous ones.
Before we do, it is important to know that there is quite a distinction between a constellation and a star cluster. On many occasions I have heard of genuine clusters of stars, such as the famous Pleiades Cluster (the “Seven Sisters”), being referred to as a “constellation” of stars. However, that is the wrong terminology.
For mapping purposes, we think of the sky as being a sphere surrounding us, and a constellation is a region of that sphere. It’s rather like imagining that you are standing at the centre of a transparent globe of the Earth, with no oceans and the Earth’s surface being divided up into countries everywhere.
The big difference is that the sphere is not real, and that stars within a constellation are all at different distances from us, so the apparent star pattern is just that — apparent. For example, the main stars in the constellation of Crux Australis — the Southern Cross — look like a cross only from our viewpoint. If we were to travel even a fraction of the distance across our galaxy, the stars would all appear in significantly different directions and the pattern would be disrupted.
However, it is generally the pattern that the stars seem to make that has defined the constellations. Official borders between these regions were approved by the International Astronomical Union nearly 90 years ago, and of course these regions contain many stars that don’t actually form part of the main pattern.
In many cases, the pattern is so obscure that it’s hard or impossible to imagine such a shape. With these, it is important not to try too hard; some, for example, were named after navigational or scientific instruments, without regard for the pattern itself.
To find the Southern Cross tonight, go out once the sky is dark and face in the direction the sun went down earlier, and make an almost complete halfturn to your left, so that you are facing approximately south-southeast. Looking high in the sky, you will see the cross tipped over slightly in an anticlockwise direction, with two bright stars below it and to its left. They are called the “pointers” for an obvious reason.
The Southern Cross is the smallest of all the constellations, but there is another, much larger, one that is easy to spot: it is the constellation of Scorpius, the scorpion. Like the Southern Cross, it is one of the few that actually does resemble the object or creature after which it is named.
To find this arachnid, turn a little farther to the left so that you are facing to the southeast and look fairly low in the sky. You can see shape of the scorpion running roughly parallel to the horizon, with his heart marked by the bright reddish-orange star Antares, whose name means “rival to Mars”.
Cruz Australis and Scorpius are but two of 88 constellations whose names and borders are officially recognised, even though they have no scientific value. However, in many cultures the star patterns have been seen differently. It is wonderful to think about these, especially those such as some Australian Aboriginal people seeing our Southern Cross as a stingray, and the Mayans seeing part of the constellation of Orion, The Hunter, forming a turtle.