Different colours of the heads and tails of comets–Examples of colour taken from the observations of the ancients : red, blood-red, and yellow comets–Difference of colour between the nucleus and the nebulosity–Blue comets–The diversity of colour exhibited by comets is doubtless connected with cometary physics, and with the temperature and chemical nature of cometary matter.

The light of many comets has been sensibly coloured. The comet of B.C. 146 exhibited a reddish tinge, according to Seneca: ‘A comet as large as the sun appeared. Its disc was at first red and like fire.’ A little further on Seneca again observes: ‘ Comets are in great number, and of more than one kind; their dimensions are unequal, their colours are different; some are red, without lustre ; others are white and shine with a pure liquid light.… Some are blood-red, sinister presage of that which will soon be shed.’ The ancients had, therefore, observed the difference of colour in the light of comets. And we shall mention a number of similar examples taken from the chronicles of the Middle Ages and from modern observers.

The comets of 662 and 1526 are cited by Arago as having been ‘of a beautiful red;’ and we have seen that Pliny in his classification speaks of comets whose ‘ mane is the colour of blood.’ Such was the comet which appeared in November 1457; according to an ancient chronicle ‘its coma or tail resembled the colour of flame.’