Comet Hunting

By Bill Bradfield

Some new comets have been discovered accidentally by amateur astronomers whilst making variable star or deep sky observations. Surprisingly a few bright comets displaying tails have been discovered with the naked eye by observant members of the general public (including for example farmers and in-flight airline pilots). However, the vast majority of visually discovered comets are found by comet hunters carrying out systematic telescope searches of selected areas of the night sky.

As the motion of all comets is directed around the Sun and because these bodies brighten as they move towards the Sun, there is a greater potential for the visual discovery of a new comet in the skies towards the Sun rather than away from the Sun's direction. This means that searches should be concentrated on the western sky after evening twilight and on the eastern sky before morning twilight.

From the astronomical viewpoint there is no particular time during the year which favours the discovery of new comets. A search can be undertaken at any time providing there is no bright Moon above the horizon.

Searches of the prime sky areas should be made twice a month, the first being early in the Moon free period and the second towards the end of that period. The first search is often rewarding because it could reveal a comet which could have been brightening substantially in a sky covered by bright moonlight. Furthermore, the first search could show a comet which has moved out from the Sun's direction. Some comets may normally never brighten sufficiently to be detected in small telescopes even when they are inside the Earth's orbit. However, if they undergo an outburst of brightness they could become easily seen. Thus, the twice-a-month search provides an increased chance to discover these outburst comets.

Studies undertaken by various investigators on the discovery circumstances of previously discovered comets show that more comets have been discovered in the morning sky than in the evening sky. Thus the prospective comet hunter should ensure that morning activity receives as much as if not more attention than evening effort.

In recent times, the application of advanced CCD technology to discover very faint near Earth orbit objects will in the future severely limit the opportunities for new comets to be found visually with small telescopes. A prime example of the new technique is the very successful LINEAR program operating in New Mexico, described in the magazine "Astronomy" April 1999, pages 60 to 61. However, until a similar system is based in the southern hemisphere there will continue to be opportunities for the visual discovery of new comets which come from the south.

William Bradfield was one of the most prolific comet hunters of the 20th Century, having been credited with the discovery of 18 comets visually between 1972 and 2004. William passed away on 9th June 2014.