Constellations (3)

Some constellations are not at all easy to recognize and the only way to learn their positions is to become familiar with the night sky. For this, you need no telescope, although a planisphere (a circular star chart rotating under a mask that shows the night sky) is helpful.

For northern skies, begin with the Plough, or Dipper. These seven stars are always above the horizon unless you are close to the Equator, and the constellation is easy to detect. An imaginary line drawn through its two right-hand stars -- the Pointers -- leads to Polaris (the Pole Stars), which is at the end of the tail of Ursa Minor (the Little Bear). The stars of the Plough are part of the constellation of Ursa Major (the Great Bear), but the remaining stars are dimmer and not so easy to identify.

On the opposite side of Polaris is Cepheus, and at the same distance but over to the right is the W of stars known as Cassiopeia. Keep in mind, though, that the Earth rotates on its axis, so the constellations appear to circle around Polaris. As a result, you will see them in all positions and orientations - on their sides or even upside down - so the W of Cassiopeia may appear like an M, for example.

Some other constellations are also easy to recognize and can be used as guides to the more difficult ones. Such are the Square of Pegasus, Gemini and Taurus (which you can recognize by its bright red star, Aldebaran, and the tiny cluster of stars known as the Pleiades). One of the easiest constellations to identify is Orion (the Hunter), which with its two bright stars, Betelgeuse and Rigel, its diagonal belt of stars in the middle and the sword hanging downward from the centre of the belt. Using the line of Orion's belt as a guide, look downward and you come across Sirius in Canis Major (the Great Dog); sight upward along the belt and you find Aldebaran in Taurus. Unfortunately, Orion is not visible in northern skies during summer.

For observers in the Southern Hemisphere, the circumpolar constellation, Crux (the Southern Cross) is always visible. The vertical bar of the cross points towards the south celestial pole. Other notable constellations of the Southern Hemisphere are Carina (the Keel), Scorpius (the Scorpion), Centaurus (the Centaur), Canis Major (the Great Dog), Puppis (the Poop) and Vela (the Sails).

That certain collection of stars forms a constellation is purely an optical effect; they are not connected, as astrologers believed. Taking Orion as an example, its stars are all at different distances from Earth. From Earth to Betelgeuse is almost as far as from Betelgeuse to Rigel, and the left-hand star in the belt is nearly 9 times nearer the Earth Betelgeuse, while the centre one is 6 times nearer, and the right-hand one is more than twice the distance of Rigel. That we see them as a group is a line-of-sight effect.

 (above) A detail from a British Astronomical Association star chart drawn by the Dutch amateur astronomer Wil Tirion shows some Southern Hemisphere stars. The coordinates of right ascension and declination have been corrected for precession - a movement of the Earth's axis - so the stars are drawn, as they will appear in the year 2000. The Southern Cross is present, as is the larger Magellanic Cloud, which is a companion galaxy to our own.