Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/876

1873, while showing excellent agreement with the chart as respects the southern parts of Mars, present some features in the northern hemisphere (more favourably seen in 1871 and 1873 than for many previous years) which are want ing in the above chart. This will be manifest from the views here given (figs. 41, 42, 43, and 44), obtained by Mr Knobel, F.R.A.S., with an 8-inch reflector of the

Browning- With construction.

The shape of the planet has been repeatedly measured, but the results are not accordant. A set of measures made at Greenwich with a double-image micrometer in 1845 gave g^g- for the ellipticity, and another set in 1852 gave ^. Arago contends for an ellipticity of -^j-, the result of the Paris measures. But some observers can detect no differ ence between the polar and equatorial diameters ; some even find the polar diameter longer than the equatorial. Adhuc subjudice lisest.

The planet s rotation has been determined more accurately than was possible in Herschel s time. Beer and Madler, from their observations in 1830-1837, deduced the rotation- period 24 h. 37 m. 23 8 s., and showed how Herschel had omitted to count one complete rotation when he deduced the period above mentioned. Kaiser of Leyden, by com paring his own observations in 1862 with those of Beer and Madler, Sir W. Herschel, and finally (carrying back the research) with an observation made by Huyghens in 1672, deduced the period 24 h. 37 m. 22 "62 s.

Proctor, by a similar process, extending from observa tions made in 1869 backwards to an observation by Hooke in 1666 (N.S.), deduced the period 24 h. 37m. 22 - 7 35 s. Kaiser contended for the accuracy of his result, asserting that Hooke s pictures were less trustworthy than Huyghens s. But recently, in 1873, Proctor has detected three errors in Kaiser s computation, who has reckoned the years 1700 and 1800 as leap years instead of common years, and made a further error of a day in correcting Hooke s date from old style into new. When account is taken of these corrections, Hooke s observations and Huyghens s are easily reconciled. Due weight being given to each, the period 24 h. 37m. 22 72 s. results, a value probably lying within O Ol s. of the true sidereal rotation- period of the planet.

It remains only to be added, that the spectroscopic observations of Mr Huggins show that the vapour of water is present in the atmosphere of Mars, a discovery of great interest and importance. A volume, however, instead of the limited space which is here available, would be required to discuss fully all that has been discovered respecting the planet Mars.

Chapter XIII.—Asteroids, or Minor Planets.

On the first day of vue present century a new planet was discovered, which, although in one sense seeming to fill up a gap in the solar system, was yet unlike any hitherto known member of the planetary family. Kepler and Titius, followed in later times by Bode, had adopted the view that a planet was wanting to complete the symmetry of the solar system, as indicated by a certain law of progres sion in the planetary distances. This law is presented in the following table, which includes Uranus (known to Bode, but not to his predecessors in the recognition of this law):—

Mercury. Venus. Earth. Mars. Missing Planet. Jupiter. Saturn. Uranus. 4 4 4 4 4 4 4 4 3 6 12 24 48 96 192 4 7 10 16 28 52 100 196 3-9 7-2 10 15 52 95 192

Here the series 7, 10, 16, &c., is obtained by adding tc 4 successively the numbers 3, 6, 12, 24, &c., forming a geometrical series ; and the distances of Venus, the earth, Mars, Jupiter, &c., which are indicated in the lowest row, are found to correspond with the members thus obtained very closely indeed. Mercury is included in the above table, though the proper number to add to the constant 4 would be 1^, not zero, and thus the distance of Mercury is really less than T 8 T ths of the proper distance according to the law of progression. And in passing it may be noticed that the distance of Neptune on the outside of the system differs as markedly from that indicated by the above law as the distance of the innermost known planet; for the law gives the distance of the next planet beyond Uranus as 388 on the scale above used, whereas the real distance of Neptune is 300 on that scale. However, towards the close of the last century, when Uranus had but lately been discovered and found to obey the law of distances, it was natural that the attention of astronomers should be directed to the strange gap in the series between Mars and Jupiter. So great was the confidence with which many now regarded the law, that the theory was advanced that a planet invisible to the naked eye was travelling within the seemingly vacant space ; and through the exertions of Baron de Zach, an association of 24 astro nomers was formed to search the zodiac for the unseen planet. Yet the discovery was not made by any member of this society. Wollaston had laid down a star in his catalogue where no star could be &een. Piazzi carefully examined the neighbourhood, to determine whether some star really existing there, had been misplaced by Wollaston. On January 1, 1801, he noted a small star in Taurus, which on January 3 had changed its place. He wrote to Oriani and Bode; but the planet, which was at its station ary point, following opposition, on January 1 2, had become invisible owing to its approach to the sun, before the letter reached them, and Piazzi himself fell ill soon after. But Gauss the eminent mathematician undertook to compute the new planet s orbit and motions from the observations made by Piazzi, and at length, after long searching, De Zach rediscovered the planet on December 3, 1801, Olbers independently discovering it on the following evening. The planet was found to correspond well with the theory which had suggested the search for it, since its distance is 2 767 where the earth s distance is represented by unity, or 2 7 6 7 on the scale used in the above table. Piazzi gave to the new planet the name Ceres. Besides differing from the other known planets in being so small as apparently to belong to a different order, Ceres was found to have an inclination (of more than 10-^) exceeding largely even that of Mercury. But these anomalous relations were soon found to be but the first indications of a discovery alto gether more remarkable.

During his long and arduous search for Ceres, Dr Olbers

of Bremen had had occasion to examine with peculiar care the stars near her path. On March 28, 1802, he observed a star near Bode s stars 20 and 191 Virginis, where he felt sure that no star had existed during his former observations. After two hours it had moved very much as Ceres had done when he had first seen her almost in the same spot. Subsequent observations showed that this was a small planet (to which the name Pallas was given) travelling in an orbit having a mean distance of about 2770 (earth s as 1), slightly exceeding, therefore, that of Ceres, but with an eccentricity of 248, considerably exceeding that of any known planet, and an inclination of 34 39 , exceeding the sum of the inclinations of all the other planets together. The effect of this great inclination on the geocentric position of the

planet is even more remarkable than the heliocentric range