Comet Holmes has dramatically increased in size over the last few weeks - it is now even bigger than the Moon in the night sky! However, it has also faded significantly - it is now a challenge to see from light-polluted London, but it is still easily visible from anywhere remotely dark. The photograph to the right was taken by planetarium presenter Tony Sizer on the night of 15 November 2007.
A spectacular sequence of images, taken by John Pane, shows exactly how Comet Holmes has changed over the last month.
What we believe happened is that the surface of the comet somehow ruptured, exposing fresh ice to the heat of the Sun. This ice then vapourises into a halo around the comet, similar to clouds in the sky on Earth. As we all know, clouds in the sky reflect sunlight, which is exactly what the cloud around this comet is doing. As the cloud increases in size, it appears brighter.
In Tony's picture, you can actually see stars through the cloud of water surrounding the nucleus of the comet.
More details about the comet can be found in the Wikipedia entry.
If you you want to observe Comet Holmes yourself, look for a fuzzy object very close to the bright star Mirphak in Perseus. This map (click on it for a larger version) will help guide your eye towards Mirphak, high in the east in the evening - just look down from the easy-to-spot "W" of stars that form the constellation Cassiopeia.