Prof. E. C. Pickering estimates that the total number of stars visible on photographs down to the 16th magnitude (about the faintest visible in the great Lick telescope) will be about 50 millions.[272] In the present writer’s enumeration, above referred to, many stars fainter than the 16th magnitude were included.
Admiral Smyth says, with reference to Sir William Herschel—perhaps the greatest observer that ever lived—“As to Sir William himself, he could unhesitatingly call every star down to the 6th magnitude, by its name, letter, or number.”[273] This shows great powers of observation, and a wonderful memory.
On a photographic plate of the Pleiades taken with the Bruce telescope and an exposure of 6 hours, Prof. Bailey of Harvard has counted “3972 stars within an area 2° square, having Alcyone at its centre.”[274] This would give a total of about 41 millions for the whole sky, if of the same richness.
With an exposure of 16 hours, Prof. H. C. Wilson finds on an area of less that 110′ square a total of 4621 stars. He thinks, “That all of these stars belong to the Pleiades group is not at all probable. The great majority of them probably lie at immense distances beyond the group, and simply appear in it by projection.”[274] He adds, “It has been found, however, by very careful measurements made during the last 75 years at the Königsbergh and Yale Observatories, that of the sixty-nine brighter stars, including those down to the 9th magnitude, only eight show any certain movement with reference to Alcyone. Since Alcyone has a proper motion or drift of 6″ per century, this means that all the brightest stars except the eight mentioned are drifting with Alcyone and so form a true cluster, at approximately the same distance from the earth. Six of the eight stars which show relative drift are moving in the opposite direction to the movement of Alcyone, and at nearly the same rate, so that their motion is only apparent. They are really stationary, while Alcyone and the rest of the cluster are moving past them.”[275] This tends to show that the faint stars are really behind the cluster, and are unconnected with it.
It is a popular idea with some people that the Pole Star is the nearest of all the stars to the celestial pole. But photographs show that there are many faint stars nearer to the pole than the Pole Star. The Pole Star is at present at a distance of 1° 13′ from the real pole of the heavens, but it is slowly approaching it. The minimum distance will be reached in the year 2104. From photographs taken by M. Flammarion at the Juvisy Observatory, he finds that there are at least 128 stars nearer to the pole than the Pole Star! The nearest star to the pole was, in the year 1902, a small star of about 12½ magnitude, which was distant about 4 minutes of arc from the pole.[276] The estimated magnitude shows that the Pole Star is nearly 10,000 times brighter than this faint star!
It has been found that Sirius is bright enough to cast a shadow under favourable conditions. On March 22, 1903, the distinguished French astronomer Touchet succeeded in photographing the shadow of a brooch cast by this brilliant star. The exposure was 1h 5m.
Martinus Hortensius seems to have been the first to see stars in daylight, perhaps early in the seventeenth century. He mentions the fact in a letter to Gassendi dated October 12, 1636, but does not give the date of his observation. Schickard saw Arcturus in broad daylight early in 1632. Morin saw the same bright star half an hour after sunset in March, 1635.
Some interesting observations were made by Professors Payne and H. C. Wilson, in the summer of 1904, at Midvale, Montana (U.S.A.), at a height of 4790 feet above sea-level. At this height they found the air very clear and transparent. “Many more stars were visible at a glance, and the familiar stars appeared more brilliant.... In the great bright cloud of the Milky Way, between β and γ Cygni, one could count easily sixteen or seventeen stars, besides the bright ones η and χ,[277] while at Northfield it is difficult to distinctly see eight or nine with the naked eye.” Some nebulæ and star fields were photographed with good results by the aid of a 2½-inch Darlot lens and 3 hours’ exposure.[278]
Prof. Barnard has taken some good stellar photographs with a lens of only 1½ inches in diameter, and 4 or 5 inches focus belonging to an ordinary “magic lantern”! He says that these “photographs with the small lens show us in the most striking manner how the most valuable and important information may be obtained with the simplest means.”[279]
With reference to the rising and setting of the stars due to the earth’s rotation on its axis, the late Sir George B. Airy, Astronomer Royal of England, once said to a schoolmaster, “I should like to know how far your pupils go into the first practical points for which reading is scarcely necessary. Do they know that the stars rise and set? Very few people in England know it. I once had a correspondence with a literary man of the highest rank on a point of Greek astronomy, and found that he did not know it!”[280]