Page:The story of the comets.djvu/35

II. with any modern observations of large comets in respect of which such precise language is used.

The transformations which comets undergo are so varied and numerous that it is not easy to reduce them to writing in any very orderly fashion. The following is an excellent instance of these transformations. On August 8, 1769, Messier, while exploring the Heavens with a 2-foot telescope, perceived a round nebulous body which turned out to be a comet. On August 13 a tail about 6° long was visible to the naked eye; on Aug. 28 it measured 15°; on September 2 it measured 36°; on the 6th 49°; and on the 10th 60°. The comet then plunged into the Sun's rays and ceased to be visible. On October 8 the perihelion passage took place; on Oct. 24 the comet again became visible but with a tail only 2° long; on November 1 the tail measured 6°, on the 8th it was only 2$1⁄2$° long, on the 30th only 1$1⁄2$°. After that the comet ceased to be visible. Changes of this character may not unfrequently be noticed.

Transits of comets across the Sun no doubt occasionally happen, but there is no clearly authenticated instance known. The German sun-spot observer Pastorff noticed on June 26, 1819 a round dark nebulous spot on the Sun. It had a bright point in its centre. Subsequently when the orbit of the Comet of 1819 (ii.) came to be investigated, Olbers pointed out that the comet must have been projected on the Sun's disc between 5h and 9h a.m. Bremen M. T. Pastorff asserted that his "round nebulous spot" was the comet. Olbers, and with him Schumacher, disputed the claim, and the matter seems not free from doubt. The Comet of 1826 (v.) was calculated to cross the Sun on Nov. 18 of that year, but owing to bad weather in Europe only 2 observers, Gambart and Flaugergues, saw the Sun on that day, and neither of them obtained any trace of the comet in transit. The Comet of 1823 is said also to have crossed the Sun but without having been seen.