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HISTORY OF PHYSICAL ASTRONOMY.
endued with a rotation on an axis. Hooke is one of the first astronomerswho arrived at this conclusion by reasoning of a strictly legitimate cha-racter, founded upon his own observations of the planet. On the 28th ofMarch, 1666, he communicated a paper to the Royal Society , containingan account of observations of certain spots on the disk of the planet. Hementions that he had seen the spots in the month of February, 1666;but the earliest observation contained in his paper is dated the 3rd of thefollowing month. Having found that the spots returned at regular inter-vals to the same position on the disk, he inferred that the planet accom-plishes a rotation on an axis in a period amounting to 12 or 24 hours *.The contemporaneous observations of Cassini in Italy conducted that astro-nomer to a result of greater precision. He concluded, from the intervalwhich elapsed between the return of the spots to the same position on thedisk, that the planet performs a complete revolution round its axis in 24 h40 m . This result received a satisfactory confirmation from the researchesof the elder Maraldi. In 1704 that astronomer, having watched the changesin the positions of the spots, inferred a period of rotation equal to 24 h 39 m .In the autumn of 1719, there occurred a peculiarly favourable opportunityfor verifying this conclusion. When the planet arrived in opposition onthe 27th August of that year it was only 2^° distant from the perihelion,and on account of its proximity to the earth it shone with such uncommonsplendour that many persons supposed it to be a comet or a new star. Onthe 19th August, Maraldi, having examined the planet with a telescope34 feet long, perceived two obscure bands on the disk, forming with eachother an obtuse angle, which exhibited a very conspicuous point f. On the25th September, he again observed the planet, and perceived the angularpoint in the same position on the disk. During the interval of 37 days,which had elapsed between the two observations, the planet had thereforecompleted 36 rotations on its axis. This gave 24 h 40 m for the period of rota-tion, a result agreeing exactly with that arrived at by Cassini. In theyears 1777 and 1779, Sir William Herschel carefully observed the changesin the appearance of the planet, and concluded from them that the periodof rotation amounted to 24 h 39 m 21 s -67 j. He also found that the inclina-tion of the planet’s equator to the ecliptic amounts to 28° 42', and that itsascending node is situate in 19° 28' of Sagittarius §. The recent researchesof MM. Beer and Madler make the period of rotation to be 24 h 37 m 23 s .
Mars being endued with a rotation on a fixed axis, it is reasonable toconclude that the figure of the planet will be flattened at the poles. Itwas reserved for Sir William Herschel to establish this interesting factupon the basis of observation. By micrometrical measurement he deter-mined the lengths of the equatorial and polar axes to be as 1355 to 1272,or as 16 to 15 nearly ||. This indicates an ellipticity considerably greaterthan what appears necessary to counterbalance the centi'ifugal force of theequatorial parts of the planet, if its density be supposed homogeneous.The discordance, however, in this as well as in all similar instances, maybe explained by an adequate supposition respecting the internal structureof the planet.
The earlier observers of Mars had remarked the appearance of very brightspots at the poles of his equator. Maraldi, who devoted much attentionto these phenomena, suspected that they w r ere the indications of physicalchanges occurring on the surface of the planet. In 1777-79, Sir WilliamHerschel examined the telescopic appearance of the planet with instru-
* Phil. Trans., No. 14. f Mem. Acad, des Sciences, 1720.
\ Phil. Trans. 1781, p, 134. § Phil. Trans., 1784, p. 273. || Ibid.