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Travelling to the southern part of the world, where the Plough may only be visible for half an hour in mid-April, or indeed may have totally vanished below the horizon, we need something else that can help us on our stargazing travels. Indeed, as for seeing the Plough (even for the briefest of periods), places near 23°S, like Alice Springs, Australia, São Paolo, Brazil, or Gaborone, Botswana, are really your most southerly locations.

A comparison in size between the Plough, a part of Ursa Major, and Crux, the Southern Cross.

What we’re looking for in the southern skies is a small constellation, the smallest in fact, known as Crux, the Southern Cross.

Of course, as with everything else, the years have performed transformations and rearrangements of the part of the sky where the Cross that we know today appears. For example, in Ptolemy’s day – the second century AD – the stars of Crux were part of the next-door constellation Centaurus, the Centaur. It was only in the late 16th century that the Cross began to take on its own personality as modern astronomers placed it in their star atlases.

Another change of names involves something you can see – or not see! – within the borders of Crux: a cloud of blackening dust and gas which obscures the Milky Way stars behind (we’ll be hearing a lot more about the Milky Way later in the book). Known today as the Coal Sack, in history this cloud has also been the Soot Bag and the Black Magellanic Cloud – which is a mysterious name I really like. It was once darkly described as ‘the inky spot – an opening into the awful solitude of unoccupied space’.

Crux, and some friends that we shall meet very shortly, are the southern equivalent of the Plough and Pole Star combined, because they too can be used to find your way about in the dark. By following various imaginary lines you can fairly easily discover the South Pole of the sky – the point about which the stars seem to revolve.

Unfortunately when you get to this point, darkness prevails, for there is no star equivalent of Polaris awaiting your arrival – there is no South Star, or Polaris Australis, as you could have called it. Astronomers with big telescopes who do not wash much will harp on about the star σ Octantis, which almost marks the southernmost point. However, it is extremely faint, difficult to find and therefore almost useless. So the Southern Cross and a couple of dazzlers next door do the admirable job of locating this starry (or celestial) pole of the south.

As the Earth spins and travels around the Sun, you’ll find Crux in different parts of the sky depending on the time and date. Its highest appearances occur during the autumn and winter evenings; in spring and summer evenings it’s nearer the horizon.

Round and round the Southern Cross goes. Using the Crux ‘Pointers’ and some useful jiggery-pokery with Rigel Kentaurus and Hadar, you easily can find the South Pole of the sky!

You can just glimpse a few stars of Crux from the Canary Islands off the north coast of Africa, but you’ll need to go below about 23°N – to Aswan, Egypt, Hong Kong or Dacca, Bangladesh, for example – to see it in all its glory. If your latitude is further south than about 34°S, like Sydney, Australia, Montevideo, Uruguay, or Cape Town, South Africa, then technically the Southern Cross never sets – although it still just skims the horizon until you go even further south, which you’ll have to do by boat as you run out of land!

Anyway, that’s the ‘where’ bit. Next, what about how big things are?

The appearance of the Plough we know and love really depends on where we live. In the northern hemisphere it may be visible whenever it’s dark. However, the further south you go, the less you see of it. Around 23°S it appears low over the northern horizon only during evenings in April – and even then it’s upside down!