Jupiter, in opposition, will rule the night sky for a few weeks.

Jupiter, in opposition, will rule the night sky for a few weeks. Photo: Maurice Valimberti

THE word ''opposition'' means a very different thing in political circles than it does in astronomy. No matter how much of a masochist a politician may be, he or she would like to spend as little time as possible in opposition. The exact opposite is true of astronomers.

Astronomically, the word means an object - usually a planet or other solar system body - that is directly opposite the sun. When the sun is setting in the west in late afternoon, this object rises in the east. At present, Jupiter is in this favourable position. The king of the planets has come to opposition this week and is visible from sunset until sunrise.

It sometimes surprises people that the planets are not visible all the time. I often get disappointed looks from the public when I say that Saturn, for example, can't be seen through my telescope that particular evening. Because of the geometry of the solar system and the different periods planets take to orbit the sun, they're at their best usually only at opposition. Mars is particularly unimpressive, except when it's at, or near, opposition. For the planets with outer orbits, opposition means that's when they're at their closest, and brightest and biggest in the eyepiece.

Jupiter spends a number of months out of our evening sky. Then suddenly, or so it appears to the uninitiated, it pops out and is visible in the early evening, low in the eastern sky. In fact, to some it's so startling to notice a bright new ''star'' in the east that they ask me if a new supernova has been discovered. In the next few weeks it will rise at sunset and be in the north-east sky about 10pm.

To its left will be the little group known as the Pleiades, or colloquially as ''the Seven Sisters''. Above it will be the slightly orange star Aldebaran in a group of stars resembling an upside-down V, and part of the constellation Taurus. Further to the right is the ''saucepan'', or Orion, with the bright-red supergiant Betelgeuse under the saucepan heating it up, and the blue supergiant Rigel above it as the steam. I love this time of year.