Comets have both fascinated and terrified man since ancient times. A comet's visible presence in the night sky is something that most people find magical and it will generate enough interest to get people gazing skyward in their backyards and to appear over and again in newspapers and media in general.
Although comets visible to the naked eye are a rarity they are not unusual in terms of their presence in the solar system. There are literally thousands of comets in orbit around the sun and are a common sight through telescopes, often with more than one comet at a time in the sky.
So, if there are so many comets why do we see them so rarely with the naked eye? In order to understand the reasons it is necessary to understand what a comet is.
- Explaining comets -
Comets are the icebergs of space, born along with the solar system thousands of millions of years ago and all in orbits of varying length around the sun. Some orbits are relatively short such as that of Halley's comet, last seen in 1986, which completes one complete orbit every 74 to 79 years and is what is termed as a short period comet. Other comets take thousands of years to complete an orbit.
Comets are made of dust, rocky particles and ice. When the comet's proximity to the sun lessens, the surface of the comet is heated and begins to evaporate seperating gas, ice and dust particles away from the comet's nucleus. This process is known to astonomers as sublimation. The pressure from solar winds pulls on this material and helps to form the comet's tail. Tails can extend into space as much as 100 million kilometres.
Particles absorb solar radiation and light and reflect it causing the comet to gain its brightness. How bright a comet becomes will depend on how much of a comet's icy crust is melted. Once a comet has started to move away from the sun again it will be less visible as the sun's effects lessen.
It isn't only the sun's proximity to a comet however which will affect whether or not it is visible. Some comets, especially those approaching the sun for the first time, are able to resist the sun's effects; the ice won't be broken down enough to release the debris which makes a comet more easily observed.
- Other factors influencing comet visibility -
Besides the proximity to the sun and the comet's resistance to the sun's effects there are some other factors that may come into play.
Comets are known as fickle and unpredicatable solar system bodies. So much is still unknown about comet's including their composition and astronomers are often unable to explain why some comets become visible to the naked eye while others don't. Neither are they able to accurately predict a comet's brilliance.
When a comet appears low on the horizon they are often less visible due to atmospheric conditions.
Certain phases of the moon will also make for poor comet viewing, most notably the full moon and the first quarter when it is waxing and coming towards fullness.
Light pollution will also have some affect on comet visibility but this can often be easily remedied by comparatively short trips outside built up urban areas.
Scientists continue to study comets and regularly new comets are being charted. Astronomy sites on the Internet are a good source for keeping up to date with comet activity and, as stated earlier, several viewings are possible annually through a telescope.