34
THE BEAUTY OF THE HEAVENS.
tances from Mars ; as, for instance, in one part of her orbit, the earth is hut I50 millions of miles distant from Mars , while, at the opposite point of her Iorbit, she is 240 millions of miles from him. The difference in his apparent Isize is nearly as three to one. Mars performs a revolution about the sun in I1 year, 321 days, \7\ hours, of our time. The telescopic spots on Mars Ihave enabled us to ascertain his rotation on his axis to be made in twenty-1four hours and thirty-nine minutes. This rotation of the planet being accu-1rately established, it was expected that, in conformity with the laws of gravity,it should have a spheroidal form, of similar character to that of the earth.The gibbous appearance of Mars occasioned considerable difficulty in takingaccurate measures of his equatorial and polar diameters. Dr. Herschel,however, succeeded in the attempt, and determined the figure of Mars to be anoblate spheroid, whose equatorial diameter is, to the polar diameter, nearly inthe proportion that sixteen bears to fifteen. It will, doubtless, be recollected,that the oblate spheroid is the figure we assigned to the earth, when we con-sidered her form and dimensions.
The planets next in succession to Mars , as respects distance from the sun,are the small ones called Vesta , Juno, Ceres , and Pallas : these four planetswere by Herschel termed asteroids (from two Greek words, signifying—in theforms of stars, or having the appearance of stars) ; they were discovered insuccession, within a few years of each other, at the beginning of the presentcentury.
The planet Vesta was discovered on the 29th of March, 1807, by Dr. Olbers , an eminent astronomer , at Bremen , in Lower Saxony . Vesta has theappearance of a very small star, and may be seen in a clear evening by thenaked eye. Her light is very pure, white, and intense. Her distance from thesun is computed to be 225 millions of miles ; her diameter is unknown : thetime in which she performs a revolution about the sun is 3 years and 274 days.
The Planet Juno.
This planet, the next beyond Vesta , was discovered by Mr. Harding, a 1his observatory near Bremen , on the 1st of September, 1804. Her distancefrom the sun is computed to be 254 millions of miles ; her diameter 1,425miles ; her revolution about the sun is performed in 4 years and 128 days.Juno appears of a reddish colour, and is always of a uniform brightness. Thediurnal rotation of this planet is unknown.
The Planet Ceres.
Ceres , the next planet beyond Juno, w'as discovered at Palermo in Sicily ,on the 1st of January, 1801, by Piazzi. Her diameter, as computed by