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A comet is an icy body that (normally) orbits the Sun. Wikipedia:

A comet is an icy, small Solar System body that, when close enough to the Sun, displays a visible coma (a thin, fuzzy, temporary atmosphere) and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet. Comet nuclei are themselves loose collections of ice, dust, and small rocky particles, ranging from a few hundred meters to tens of kilometers across. Comets have been observed since ancient times and have historically been considered bad omens.

A Great Comet is a particularly notable and impressive comet. There might be only one Great Comet every ten years.

Comets are the cause of some meteor showers. Wikipedia:

As a result of outgassing, comets leave a trail of solid debris. If the comet's path crosses Earth's path, then at that point there are likely to be meteor showers as Earth passes through the trail of debris. The Perseid meteor shower occurs every year between August 9 and August 13, when Earth passes through the orbit of Comet Swift–Tuttle. Halley's comet is the source of the Orionid shower in October.

Comet McNaught

On 7 August 2006 British-Australian astronomer Robert McNaught discovered a comet that in January 2007 was easily visible in the Southern Hemisphere and was the brightest comet for more than 40 years. Formally known as C/2006 P1 and also called the Great Comet of 2007.

It was very dim when first discovered; at magnitude +17 to faint even for Auckland observatory's largest telescope. It brightened to magnitude +9 (still far too dim to see with the naked eye) then for most of December 2006 was lost in the glare of the sun.

However, when it reappeared it was brightening rapidly, and achieved an estimated magnitude -5.5 around 13 January 2007.

Good weather on Thursday 18 January 2007 allowed a trip to the top of Mt Roskill to take photographs of Comet McNaught.