Barnard’s comet, 1889 I., although it never became visible to the naked eye, was visible with a telescope from September 2, 1888, to August 18, 1890, or 715 days—the longest period of visibility of any comet on record. When last seen it was 6¼ times the earth’s distance from the sun, or about 580 millions of miles,[224] or beyond the orbit of Jupiter!
Messier, who was called “the comet ferret,” discovered “all his comets with a small 2-foot telescope of 2¼ inches aperture, magnifying 5 times, and with a field of 4°.”[225]
It is a very curious fact that Sir William Herschel, “during all his star-gaugings and sweeps for nebulæ, never discovered a comet;”[226] that is an object which was afterwards proved to be a comet. Possibly, however, some of his nebulæ which are now missing, may have been really comets.
Sir William Herschel found the diameter of the head of the great comet of 1811 to be 127,000 miles. The surrounding envelope he estimated to be at least 643,000 miles, or about three-fourths of the sun’s diameter.
On a drawing of the tails of the great comet of 1744 given in a little book printed in Berlin in that year, no less than 12 tails are shown! These vary in length and brightness. A copy of this drawing is given in Copernicus.[227] The observations were made by “einen geschichten Frauenzimmer,” who Dr. Dreyer identifies with Christian Kirch, or one of her two sisters, daughters of the famous Gottfried and Maria Margaretta Kirch (Idem, p. 107). Dr. Dreyer thinks that the drawing “seems to have been carefully made, and not to be a mere rough sketch as I had at first supposed” (Idem, p. 185).
The tails of some comets were of immense length. That of the comet of 1769 had an absolute length of 38 millions of miles. That of 1680, 96 million of miles, or more than the sun’s distance from the earth. According to Sir William Herschel, the tail of the great comet of 1811 was over 100 millions of miles in length. That of the great comet of 1843—one of the finest in history—is supposed to have reached a length of 150 millions of miles![228]
In width the tails of comets were in some cases enormous. According to Sir William Herschel, the tail of the comet of 1811 had a diameter of 15 millions of miles! Its volume was, therefore, far greater than that of the sun![228]
According to Hevelius the comet of 1652 was of such a magnitude that it “resembled the moon when half full; only it shone with a pale and dismal light.”[229]
Halley’s comet at its next appearance will be examined with the spectroscope for the first time in its history. At its last return in 1835, the spectroscope had not been invented.
For the great comet of 1811, Arago computed a period of 3065 years; and Encke found a period of 8800 years for the great comet of 1680.[230]