Double tails of comets; comets of 1823, 1850, and 1851–Tails multiple, fan-shaped, rectilinear, curved–Variable number of tails belonging to the same comet; comets of Donati, of 1861 and of Chéseaux.

Generally a comet has but one tail, which varies considerably in form or size, or, at all events, appears to do so. Sometimes these changes take place very rapidly, but still, as a rule, the tail consists of one luminous train. Nevertheless, examples may be adduced of double and even multiple tails. The comets of 1807 and 1843 were furnished with double tails, or, what comes to the same thing, single tails formed of two branches of very unequal length. It was the same with the comet of 1823, about which Arago gives the following details:–

‘On the 23rd of January, 1824, the comet, in addition to its ordinary tail opposite to the sun, had another which was directed towards the sun, so that it resembled somewhat the great nebula of Andromeda. The first tail appeared to include a space of about 5°, but the length of the second was scarcely 4°. Their axes formed between them a very obtuse angle of nearly 180° (fig. 25). In the close vicinity of the comet the new tail was hardly to be seen. Its maximum brightness occurred at a distance of 2° from the nucleus. During the first few days in February the tail opposite to the sun was alone visible ; the other had disappeared, or had become so faint that the best telescopes in the clearest weather failed to show any trace of it.’