The year's best meteor display – the Geminids – is set to wow skygazers on Thursday and Friday.

The meteors, which got their name because they appear to originate from the constellation Gemini, are unusual because they come from an object that looks like an asteroid.

Most other meteor showers are caused by the Earth ramming into a cloud of rocky debris left behind by a comet decades or centuries earlier. The debris, most of which is smaller than a pebble, hits the atmosphere at blistering speed, burning up and leaving behind bright trails seen as "shooting stars".

In the case of the Geminids, however, the particles follow the path of a 5-kilometre-wide object called 3200 Phaethon that was discovered in 1983 by NASA's Infrared Astronomical Satellite.

The object, which lacks a tail, may be an asteroid – it spends part of its orbit in the main asteroid belt between Mars and Jupiter. If that is so, it may have collided with another space rock there to create debris that then spread along its orbit – ultimately producing the Geminid meteors.

Alternatively, 3200 Phaethon could be a comet that has lost all of its ice after too many close passes by the Sun. In that case, the meteors would be produced by material shed from the comet long ago.
Dark skies

Whatever their source, the Geminids produce fine displays each December, generally producing more meteors than the next best show, the August Perseids. This year should be especially good for the Geminids, since the Moon will appear as a thin crescent and therefore will not interfere much with the shower.

The Earth is expected to pass through the thickest part of the cloud of debris at 1745 GMT on Friday 14 December. Observers in Asia should watch on the night of 14-15 December.

For observers in North and South America, the peak occurs during daylight hours. For them, the display will be best before dawn on Friday morning, when a few dozen meteors per hour should be visible from a dark site at mid-northern latitudes, according to Sky & Telescope magazine.

Southern hemisphere observers will see meteors at a lower rate. Observers in North and South America can also try watching on the evening of Thursday 13 December, when some meteors will already be arriving, although in smaller numbers.

For observers in Europe, the display before dawn on Thursday morning may be about equal to that on Friday evening.

Although the meteors will point back towards Gemini, they can appear anywhere in the sky. To see the most meteors, do not focus on Gemini, but watch as large a patch of the sky as possible. The best displays will be visible from darker sites away from city lights, which wash out the fainter meteors.

Interestingly, 3200 Phaethon happens to be passing near Earth during this year's meteor display, though it cannot be seen without a telescope. Its closest approach was on 10 December, when it was about 18 million kilometres away.