Great comets are rare. Comets are occasional visitors to the inner parts of the Solar System. Since 1960, 10 comets have been visible with the naked eye. Few of these qualified as Great comets, sufficiently bright to draw worldwide attention. Comet McNaught, currently visible around sunset, is the brightest such comet since 1965 (see Figure 1).

Where can I see it?
The comet is still very close to the Sun at the moment. The best time to see it is immediately after sunset. Find a location with a clear horizon towards the southwest where the Sun sets. Figure 2 shows where the comet will be visible.

Immediately after sunset, the comet will be visible to the left of where the sun disappeared. At the same time, Venus is visible as a bright evening star some what to the right of the setting Sun. According to reports from 15 Jan, the comet is is now comparable in brightness to Venus. The short tail of the comet should be noticable. Predicting the brightness of comets is notoriously difficult. It is expected to fade over the next few days, although it will also become easier to see as it will be higher in the evening sky. It will remain visible for at least the next week. At peak brightness there were reports that the comet was visible in bright daylight. The comet has faded a bit now so that this is no longer the case. Be aware that looking directly at the Sun will do permanent damage to your eyes.

What is a comet?
Comets have been likened to dirty snowballs. They consist largely of frozen, fluffy water (hence snow), with dust particles embedded in them. They reside in the outer solar system, beyond the orbit of Pluto (recently downgraded to 'dwarf planet'). Small disturbances to their orbits, induced by planets, occasionally cause one to fall in towards the Sun. This is what is likely to have happened to comet McNaught.

What is the tail?
When close to the Sun, the heat begins to evaporate the snow. The water vapour (which includes other chemicals as well) drifts away from the comet and is pushed out by solar wind - a stream of particles emitted by the Sun. This vapour forms the tail. Not surprisingly, it always points away from the Sun. The tail can reach enormous lengths, many million of kilometres. But the amount of gas in it is in fact very small. It would fit inside a soccer ball!

What will happen next?
Comets would normally be expected to return to the outer solar system, albeit on an orbit which will eventually return to the Sun. We do not yet know the precise orbit of comet McNaught, but current estimates indicate it may be travelling on an orbit which will allow it to escape the Sun altogether. This can happen if a comet picked up some extra speed while falling towards the Sun, from coming too close to a planet. (Spacecraft travelling to the outer planets use the same trick, normally using the gravity of Jupiter.)

If this is correct, comet McNaught will never come back to us. Instead it will leave the Solar System, and may eventually (after millions of years) fall towards another star. We can expect other comets (a few each decade, typically, become visible) but it may be decades before one becomes as bright as McNaught. Or we may be lucky and get another one this year-but don't bet on it.

Is there any danger to Earth?
No.

Really?
Yes. The orbit of McNaught stays far away from the Earth. But even if it did come close, the danger of a collision would still be astronomically small. Besides, comets are relatively small and fluffy: they tend to explode while still high up in the atmosphere.

What is a Great Comet?
This is a phrase used for a particularly impressive comet. The brightness of a comet depends on how big it is, how close it gets to the Sun, and/or how close it gets to the Earth. Comet Halley is a large comet, but in 1987 it did not come very close to the Sun, and it remained very far from Earth. So it did not qualify as a Great Comet. On a previous appearance, in 1910, it did come close to Earth and became very impressive-and so it was a Great Comet at that time. (1910 was a good year for comets: a second comet became visible even during day light.)
Great comets of recent decades have been Hale-Bopp (1995), Hyakutake (1994), Comet West (1975) and Ikeya-Seki (1965). Of these, only Ikeya-Seki was brighter than the current comet McNaught.
The brightest comets of the past 200 years have been the Sun grazers: comets which get incredibly close to the Sun. It appears that all the brightest comets (include the one from 1965) are remnants of a much larger comet which broke in pieces during its last visit, in AD 1106. The fragments have gone off on slightly different orbits and now are coming back to the Sun one by one. Comet McNaught is not part of this family, however. It travelled well inside the orbit of Mercury, 6 times closer to the Sun than we are. Sun grazers get far closer to the Sun, however.

Albert Zijlstra, Jan 16, 2007