Composed mainly of frozen debris and gases left over from the formation of the solar system, as comets approach the earth these icy pieces are heated by the warmth of the Sun, releasing the gases within. These freed gases and dust particles will catch the Sun’s light, this causing them to glow. As these gases get warmer and begin to expand, solar winds blow and disperse this material, producing the comet’s beautiful tail.
With a telescope, it is probable that at least one comet will be observed on any night but those comets bright enough to be seen by the naked eye are rare. Only once or twice every ten years are comets easy to see by the naked eye.
There are many factors that influence the visibility of a comet, the main ones being its distance from the Sun (as it is the Sun’s warmth helping to release the gasses and particles within the ice pieces that makes the tail visible) and its distance from the Earth after the comet has been warmed by the Sun.
Some comets can be viewed better, with a decent pair of binoculars. Also influencing the visibility of a comet is the location from where it is observed. In June, 2010, for instance, a bright green comet was visible for the first and only time, making its way across the UK skies. Although not particularly bright, this green comet could be observed from the UK for a very short time before it returned for perpetuity to the solar system.
Astronomy websites will often suggest the best chances of seeing specific comets and from where on earth visibility will be highest as well as giving their likely positions in the night sky. Towards the end of October 2010 for instance, Comet Hartley 2 will be making its closest approach to Earth. The Comet will be visible in the northern hemisphere late October while in November those living in the southern hemisphere might be able to catch a glimpse of the comet as it moves away from the earth.
Attempting to observe comets from a city environment plagued by light pollution isn’t likely to be as successful as from a location giving a sweeping access to the dark and starry sky. It is also more difficult to see comets when they are close to bright stars or the brightness of a full moon.
Comets spend most of their time moving around the solar system some distance away from the Sun. So it is only as their path brings them closer to the earth and the Sun that their icy pieces begin to warm. The dust particles are released from the ice and the solar wind will push on the gases, sweeping them into the long tail that always points away from the Sun.
The brighter the comet, the easier it will be to see it with the naked eye but often it is recommended that binoculars will make a difference.
The closer comets come to the earth, the more likely it is that they will be observed by those beneath the night-time skies. Other factors influencing the visibility of a comet include on which side of the sun it makes its closest approach to the earth and how high above the horizon it is when we see it.
Sources
http://www.bbc.co.uk/solarsystem/other_solar_system_bodies/comet
http://www.dailymail.co.uk/sciencetech/article-1287121/Bright-green-comet-visible-UK-skies-rest-week.html
http://nineplanets.org/comets.html
