Page:The story of the comets.djvu/88

54 conjectured to have been impressed by a comet, was observable, and a few days later was easily picked up with a telescope. Though the comet had no particular interest in itself, its orbit soon attracted notice because it was seen to resemble very closely the orbit of Wolf's Comet, and Schulhof pointed out the extreme probability of the 2 bodies being two portions of one original—a case of Biela over again. It may be added that a good photograph generally discloses much more detail in a comet than can be recognised by the eye with a telescope.

Barnard's Comet of 1892 (v.) does not stand alone as an instance of astronomers having been assisted in their labours by the photographic art. After their discovery in the usual way it was found that several recent comets had impressed themselves on photographic plates long before their discovery visually. Thus Brooks's Comet of 1904 (i.) was photographed in May 1903, giving an interval thus covered by observation of 753 days. Again, Kopff's Comet of 1905 (iv.) was found on a plate exposed on Jan. 10, 1904, 783 days before its visual discovery, so that its period of visibility thus regarded might be said to have been more than 3$1⁄2$ years. Such records are of great value in tracing the movements of a comet because, thanks to the perfection which has been attained in celestial photography, photographic plates can be read and brought into line with the stellar co-ordinates of Right Ascension and Declination as effectively as if the services of a meridian instrument had been available for fixing the place of the comet at a given time on a given day.

So many of our observations of ancient comets (and those observations by far the most valuable) depend on Chinese records and descriptions of them that it may interest the reader to see how an ancient Chinese astronomer sought to convey his information to posterity. The following is Matuan-liu's account of Halley's Comet in 837:—

"In the 2nd year of the Epoch Kae Ching, the 2nd Moon, day Ping Woo, there was comet in S. D. Wei. It was about 7 cubits in length. It pointed towards Nan Tow. On the day Woo Shin it was to the south-west of S. D. Wei. It was bright, and moved rapidly. On the day Kwei Chow its place was in S. D. Heu. On the day Sin Yew its length was about 10 cubits. It went to the west, gradually pointing to the South. On the day Jin Seuh