Introduction

Until the great advances of very recent years in almost every branch of theoretical astronomy, comets had come to be regarded as perhaps the most puzzling and mysterious of all the many types of heavenly bodies. This was not due to any great difficulty in the way of observing comets, as could be urged with a problem such as the structure of the solar corona, for they are extremely numerous, and some of them at times so extraordinarily bright as to outshine Venus herself and be easily visible in broad daylight with the naked eye. Their various properties are almost the direct antithesis of those of planets, and the two types of object are often referred to as 'the two solar families'. The times and intervals of occurrence of comets in the sky seem quite irregular compared with stars and planets, some remaining visible for months and others only for a few days. Brooks’ comet of 1904 was observable for more than twelve months, Comets 1889 and 19361 were observed for over twenty-four months, Comet 1927 IV for four years, while Grigg's comet of 1901 was observable for only twelve days, and a few very faint comets have been observed perhaps only on a single occasion.

The word ‘comet’, according to the Oxford Dictionary, comes from the Greek word which means a ‘long-haired star’—presumably a comet—and is doubtless also connected with coma, the Latin word for ‘hair’. The name thus fancifully refers to the most obvious feature of many comets, which resemble a star embedded in a mildly luminous fog from which there appears to be carried away a long streaming trail itself faintly luminous. It is this tail that no doubt suggested to the ancients long tresses of feminine hair streaming in the wind. Naturally the brightest comets (as seen from the Earth) and those with the most extensive tails are the ones that have made the strongest impressions, and so appear more typical of comets than they may really be; in fact, most comets are very faint objects detectable only with powerful telescopes.