The night sky: Comets show up - but only just

By GRANT CHRISTIE

The two comets predicted to be visible to the naked eye in May have met that expectation - but only just.

Comets are notoriously fickle objects and there is no reliable way to predict their brightness. The density and size of a comet's light-reflecting coma depends on the condition of the comet's solid surface, and this varies from one comet to the next.

You can see the two comets with the unaided eye from dark rural sites when there is no bright moonlight. From urban centres you will need binoculars.

Even if they are not a visual spectacle, these comets are of scientific interest. At the Stardome we have been following Comet NEAT because it is too far south to be seen from the northern hemisphere.

We have imaged the comet using the Nustrini Automatic Telescope to get precise positional measurements. These measurements track small changes in the comet's orbit.

Comets have tiny masses and their orbital paths are mostly determined by the gravity of the sun and planets. However, as they warm up closer to the sun, gases shooting out from the frozen surface act as small jets, slowly changing their orbit. The comas are pushed around by the solar wind and guided by the sun's magnetic field, forming several tails.

Scientists at Jet Propulsion Laboratory in California are using our images to study the sun's magnetic field in a region of space seldom studied before.

Over the next few nights, Comet NEAT is passing above the bright star, Sirius, and is easy to find using binoculars.

Venus, Mars and Saturn are together low in the northwest just after sunset while Jupiter is the brilliant object due north at 8pm in the constellation Leo.

Orion is setting at about 9pm to be replaced by the winter constellations of Scorpius and Sagittarius rising in the east. In the south, the Southern Cross reaches its highest point at around 10pm. With Centaurus to its east and Carina to its west, this is a wonderful region to explore with binoculars or a small telescope.

* The Stardome is open for public viewing from Wednesday to Saturday. Phone (09) 624 1246. Dr Grant Christie is an astronomical researcher and chairman of the Auckland Observatory and Planetarium Trust Board.