FOOTNOTES
[1] Galileo Galilei is very generally called by his christian name, but I depart from this practice here.
[2] ‘Observatory,’ vol. ii. p. 364.
[3] Reproduced, by permission, from Cassell’s ‘New Popular Educator.’
[4] Reproduced, by permission, from Cassell’s ‘New Popular Educator.’
[5] Such objects show considerable glare in a very large instrument. The advent of Jupiter into the field of the 6-foot has been compared to the brightness of a coach-lamp. The outer satellite of Mars was seen twice with this instrument in 1877, “but the glare of the planet was found too strong to allow of good measures being taken.”
[6] My 10-inch reflector by With-Browning was persistently used for four years without being resilvered or once getting out of adjustment.
[7] In this and future references to reflectors the Newtonian form is alluded to. The direct-vision reflectors of Gregory and Cassegrain have gone out of use, and the present popularity of Newtonians may be regarded as a case of the “survival of the fittest.”
[8] Chambers’s ‘Descriptive Astronomy,’ 4th ed. vol. ii., also contains some useful references and diagrams.
[9] The Rev. F. Howlett measured this spot on the following day, June 20, and found it 63″ in its largest diameter. He used a small refractor, and projected the Sun’s image on to a screen sufficiently distant for it to have a diameter of 3 feet.
[10] On May 13, 1890, at 3h, I tested the three methods alluded to on a scattered train of small spots, and derived the following measurements of length:—
By glass micrometer 76,570 miles.
“ cross wires 76,610 ”
“ cardboard disk 75,770 “
In this comparison I used an excellent 4-inch Cooke refractor, belonging to a friend.
[11] The maximum duration of totality, under every favouring circumstance, appears to be about 8 minutes. The great eclipse which occurred on August 18, 1868, maintained the total phase for nearly 6 minutes 50 seconds in the Gulf of Siam. In reference to this eclipse, Dr. Weiss says:—“In the records of ancient eclipses there are to be found only two which may be compared in size with that of August 18, 1868, but none in which the totality lasted so long. The first of these is the eclipse of Thales (28 May, 585 B.C.), which is said to have been the first predicted, and to have terminated a bloody war between the Lydians and the Medes. The second was visible on June 17, 1433, in Scotland, and the time of its occurrence was long remembered by the people of that country as ‘the black hour.’”
[12] Carrington found that spots near the equator gave a shorter rotation-period than those far removed from it. This offers an analogy to the spots on Jupiter, which move with greater celerity near the equator, though the rule is not absolute.
[13] In 1852 Dawes observed and measured a rotatory motion affecting a spot at the rate of about 17° per day.
[14] Lalande, in 1778, asserted that “there are spots of very considerable magnitude, which, reappear in the same physical points of the solar disk.”
[15] A spot was visible on June 30, 1889, in 40° South latitude. Its recorded duration was 2 days. This object was observed at the Stonyhurst Observatory and at a station in North America.
[16] In September 1889 Prof. Thury, of Geneva, reported a change in the centre of the crater Plinius. With a 6-inch refractor he saw, instead of the usual two hills in the interior, a circular chalk-like disk “with a dark spot in its centre like the orifice of a mud-volcano.”
An extensive walled plain, 110 miles in length.
[18] The objects for observation when the Moon’s age is from 2 to 4 days may be suitably re-examined a few days after the full.
[19] A large walled plain containing a small crater, Cleomedes A.
[20] A curious double crater, with comet-like rays crossing the Mare Fœcunditatis.
[21] A circular ring-plain, 42 miles in diameter.
[22] The interior of this crater exhibits some interesting features as the Sun rises higher above it.
[23] A fine ring-plain, 25½ miles in diameter.
[24] Mädler says “the full Moon knows no Maginus,” meaning that this object is invisible under a vertical Sun.
[25] Mösting, Lalande, and Herschel form a fine triangle when the Sun has attained a great altitude. Mösting is a ray-centre.
[26] A ring-plain 37½ miles in diameter, with very irregular terraced walls.
[27] A range of mountains, with intervening valleys.
[28] Mädler describes this as a square enclosure with rampart-like boundaries, which “throw the observer into the highest astonishment.”
[29] A great walled plain, 91 miles in diameter.
[30] A walled plain, 55 miles in diameter, in which Schröter suspected changes.
[31] An extensive mountain-range on the E. by S. limb.
[32] A walled plain, 95 miles in diameter, and probably the deepest in the N.E. quadrant, for the S.E. side of its wall rises to nearly 17,000 feet.
After the full the same objects should be re-examined under the reversed illumination.
[33] Chambers, in his ‘Descriptive Astronomy,’ 4th edition, 1889, devotes a chapter to the discussion of facts having reference to Vulcan; and the reader desiring full information will find it here.
[34] This period was probably derived erroneously by Bianchini. It includes 25 periods of 23h 22m, which corresponds with the times of rotation by Cassini and others given in the table.
[35] Schröter’s final result. In 1788 he had derived a period of 23h 28m from observations of faint dark spots, and in 1789-91 irregularities in the S. horn of Venus gave him a period of 23h 20m 59s.
[36] This was believed by Sir J. Herschel to be due to “an ochrey tinge in the general soil, like what the Red-Sandstone districts on the Earth may possibly offer to the inhabitants of Mars, only more decided.”
[37] Herschel’s earlier observations were made in 1777-79, and his period, like that of his predecessors, is about 2 min. in excess of the correct value; but Mädler pointed out that, by giving Mars an additional rotation on his axis, Herschel’s value will agree within 2 sec. of his own. Herschel appears to have adopted 768 rotations instead of 769, and may have been led to this by the excessive periods of Cassini and Maraldi and by the want of intermediate data between his own observations in April 1777 and May-June 1779. His second determination, made in 1784, is more correct.
[38] Deduced from observations extending over 15 years only, at Bristol.
[39] The question of periodicity is an extremely interesting one as affecting the disposition, form, and colours of the markings on Jupiter. Certain features visible in 1869-70 were unmistakably reproduced in 1880, and it has been suspected that the cycle of these changes accords with the length of the Jovian year. Future observations must be compared with old drawings and records for the identification of similar features if they are recurrent.
[40] On the morning of Dec. 5, 1887, I made a drawing of Saturn, the image of the planet being remarkably well defined, though the Moon was only 1° distant.
[41] Amongst the first observers of these dark transits were Cassini (Sept. 2, 1665), Romer (1677), and Maraldi (1707).
[42] Huygens appears to have used a refractor of 2-1/3-inch aperture and 23-feet focal length, with a power of 100, in effecting this discovery.
[43] Schröter, Harding, Schwabe, and others have observed luminous points on the rings, but they have remained stationary, so that the period of rotation announced by Herschel has never been confirmed, but rather disproved by counter-evidence. Herschel wrote, in November 1789:—“I formerly supposed the surface of the ring to be rough, owing to luminous points like mountains seen on the ring, till one of these supposed luminous points was kind enough to venture off the edge of the ring and appear as a satellite. I have always found these appearances to be due to satellites.”
[44] Galle, at Berlin, had, twelve years previously, made an observation which, if it had been interpreted correctly, would have given him priority. In June 1838 he remarked, on several nights, that the inner boundary of the inner ring was very indistinct and “gradually lost itself towards the body of the planet.” The space between the ring and Saturn was half filled with a dim veil, extending inwards from the ring. These observations failed to attract the notice their importance deserved, and Galle himself did not appreciate their full significance until the announcements of Bond and Dawes in 1850.
[45] Struve wrote, in 1883:—“That changes do take place in the ring-system is sufficiently proved.” Trouvelot, Schiaparelli, and others have also remarked variations of a sufficiently decided character to be placed on record.
[46] Herschel remarks that he saw this satellite in his 20-foot speculum two years before, viz. on Aug. 19, 1787, but he was then much engaged in observations of the satellites of Uranus.
[47] Donati’s Comet of 1858 and Coggia’s Comet of 1874 may be mentioned as good examples of the gradual approach and development of these visitors witnessed by means of the telescope.
[48] It ought, perhaps, in the present state of our knowledge, to be termed “the Neptune of comets;” for it has the longest period of any comet whose path has been definitely ascertained by multiple returns to perihelion.
[49] Encke’s Comet has the shortest period of all the known comets.
[50] Newton conjectured that comets formed “the aliment by which suns are sustained,” his opinion being that the former bodies finally coalesced with the suns round which they revolved. He remarked:—“I cannot say when the Comet of 1680 will fall into the Sun,—possibly after five or six revolutions; but whenever that time shall arrive, the heat of the Sun will be raised by it to such a point that our globe will be burnt and all the animals upon it will perish.”
[51] During the seven months from May to November 1890 I noted ninety-five telescopic meteors while engaged in comet-seeking.
[52] A list of these was published in the ‘Monthly Notices,’ vol. 1. p. 466. See also ‘Monthly Notices,’ vol. xlv. pp. 93 et seq.
[53] There are several forms of this instrument: for particulars of construction and use the reader is referred to Thornthwaite’s ‘Hints on Telescopes,’ and Chambers’s ‘Astronomy,’ 4th ed. vol. ii.
[54] Mr. George Knott, of Cuckfield, mentions that the radius of the first bright diffraction-ring of a stellar image, for a 7-1/3-inch aperture, is 1″·01, and for one of 2 inches 3″·70 (‘Observatory,’ vol. vi. p. 19; see also vol. i. pp. 107 and 145). Mr. Dawes is quoted as giving 1″·25 for a 7-inch, 1″·61 for a 5½-inch, and 3″·57 for a 2·4-inch. These figures exceed the theoretical values, if the latter are adopted from Sir G. B. Airy’s ‘Undulatory Theory of Optics,’ where, for mean rays, we have:—
Radius of object-glass in inches × radius of bright ring in seconds = 3·70.
[55] The number visible to different persons varies according to eyesight. Some observers see thirteen or fourteen stars in the Pleiades, while others cannot discern more than six or seven.
[56] About 2 seconds. Sir W. Herschel found the diameter of α Lyræ with a power of 6450 to be 0″·3553. Tycho Brahe, before the invention of telescopes, estimated the diameter of Sirius as 120″. J. D. Cassini, with a telescope 35 feet long, found the diameter of the same star 5″.
[57] Dr. Doberck gives some valuable information with reference to the computation of binary star-orbits in ‘The Observatory,’ vol. ii. pp. 110 and 140.
[58] The star α Canis Minoris (Procyon) was also inferred to be a binary and to have a similar period. Several close companions appear to have been discovered (Ast. Nach. no. 2080). But Prof. Hall, using the 25·8-inch refractor at Washington, says:—“I have never been able to see any of these companions that would stand the test of sliding and changing the eyepiece, turning the micrometer, &c., and am therefore doubtful of their existence. This is an interesting star for the powerful telescopes of the future.” It has been surmised that the companion is a non-luminous one, and therefore invisible.
[59] It is remarkable that nearly all the temporary stars have appeared in the region of the Milky Way.
[60] This expert comet-finder would appear to have more acute, sensitive vision on faint stars than Burnham (see ‘Monthly Notices’, vol. xlix. p. 354).
[61] Sir W. Herschel at first entertained this view, finding that with every increase of telescopic power more nebulæ were resolved. But in 1791 he said, “perhaps it has been too hastily surmised that all milky nebulosity is owing to starlight only.” Lacaille had remarked in 1755 that “it is not certain the whiteness of parts of the Milky Way is caused by clusters of stars more closely packed together than in other parts of the heavens.”
[62] This is exclusive of 47 new nebulæ discovered by Prof. Safford, which form the appendix to the catalogue.
[63] Chambers says only four examples are known, but this is erroneous, as Lord Rosse’s telescope has added five ring-nebulæ to the four previously catalogued.
[64] Some of the nebulæ in Messier’s list were discovered by Mechain at Paris, who, like Messier, earned celebrity by his cometary discoveries. He was born at Laon in 1744, and died at Valencia in 1805.
[65] O. Struve had expressed views identical with these in 1857 (see ‘Monthly Notices,’ vol. xvii. p. 230).
[66] Humboldt says this “name is evidently derived from the voyage of Magellan, although he was not the first who observed them.”
[67] I have selected the various objects in these lists from the New General Catalogue.
[68] These forms are more numerous than the annular nebulæ. They often exhibit a blue colour, and the spectroscope shows them to consist of gas.