Comets are among the most spectacular objects that can be seen in the sky. In ancient times they inspired fear and awe. Often they were considered heralds or omens of some momentous event, such as the death of a king. As recently as the 19th century, the tail of a comet was thought to be composed of poisonous gases, and the approach of one was feared, because it was believed that, if the Earth were to pass through the tail, everyone on it would be gassed to death. Today, we know that comets are just objects consisting of dirt and watery ice and other volatile materials, and that the tail is caused by the heating of the Sun as the comet approaches it out of the cold vast reaches of space. We know that a comet, no matter how spectacular, is incapable of causing any damage to our planet, unless it were to hit us directly, as comet Shoemaker-Levy did Jupiter in 1994, and as the comet in the film Deep Impact did. We know all these things are true, yet when we see the approach of a cosmic leviathan in the sky, we may not feel fear any longer, as ancient man once did, but we still feel awe.
Because each comet is unique , it is difficult to predict in advance how spectacular and bright it will become. Comet ISON, which is now rapidly approaching us, and is due to come closest to the sun on Nov 28, 2013, only a little more than a month from now, was at one time predicted to become so bright that it might rank as 'Comet of the Century'. While the rate of increase in its' brightness has slowed in recent months, it is still expected to be an impressive object, and should be easily visible to the naked eye at its' time of maximum brightness, which is likely to occur somewhat after it's closest approach to the sun. Part of the reason it is so difficult to predict how bright any comet, including Comet ISON, will become is that each comet is unique, with a different composition and characteristics. In the case of Comet ISON, there is also the question of, since it will approach so close to the sun (about 1.7 solar radii), whether or not it will break apart due to the gravitational and heat stresses of being so close to our nearest star. Also, this is the first time Comet ISON has approached the sun. unlike Comet Halley, which as we know approaches us once every eighty-six years, and is therefore somewhat more predictable. Thus, part of the excitement of observing Comet ISON as it brightens in the night sky is our not knowing exactly what will happen, unlike most astronomical phenomena, where we know exactly what will happen. For it is only by seeing what we do not expect to see that we can hope to learn anything new.