Comet Holmes now appears almost twice the diameter of the full Moon in the night sky.
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Because the comet is so large in the sky, it is spread out, making it appear much fainter in the night sky. But it is still visible to the unaided eye when well away from light pollution.
The best way to observe the comet now is with a pair of binoculars that are large (to collect a lot of light) but with low magnification (because the comet is so large in the sky).
The apparent size and brightness of Comet Holmes is regularly estimated by amateur astronomers world wide. A list of estimates is available at the IAC/ICQ/MPC website. Using averages of these estimates, I have plotted the apparent size of Comet Holmes against time (below).
The size of Comet Holmes
In this graph, you can see the number of days along the bottom since 24 October, 2007 - the date when Comet Holmes suddenly increased in brightness.
Up the left hand side of the graph, I show the angular size of the comet - that is how big the comet appears to us in the night-time sky. The apparent size of the Full Moon, which is half a degree across (or 32 arc-minutes) is labelled for comparison.
Up the right hand side of the graph, I show the actual size of Comet Holmes in millions of km (assuming that the comet is at a fixed distance of 1.7 AU away - although the comet is moving away from us, it has not moved too much over the last 2 months).
Note how within days of the outburst in October, the comet was bigger than the separation of the Earth and the Moon, and within weeks it was physically bigger than the Sun!
Currently, it appears about 1 degree (60 arc-mins) across in the night sky - that's twice the diameter of the full Moon. In physical size, the nucleus of the comet is now surrounded by a cloud of gaseous water that is over 2.5 times larger than the Sun.
What an amazing comet!