Page:Popular Science Monthly Volume 39.djvu/207

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195
QUESTIONS CONCERNING THE MINOR PLANETS.

formed, we can imagine how complete is the transformation to which the sorts of wool are subjected in the carding process. We can thus readily understand how much more perfect is the carding operation, as now performed by machinery, than anything that was possible under hand manipulation.


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QUESTIONS CONCERNING THE MINOR PLANETS.

By M. F. F. TISSERAND, of the Institute of France.

KEPLER, having found a break in the continuity of the mean distances of the planets from the sun, boldly filled it by supposing a new planet between Mars and Jupiter. The publication of Bode's empiric law in 1772 helped confirm the ideas of Kepler, and fixed the distance of the hypothetical planet at 2·8 times that of the earth. A new authority was given to this conclusion after the discovery of Uranus by William Herschel in 1781. The calculations of Lexell and Laplace showed in fact that Uranus's distance might have been furnished in advance, with a near approach to exactness, by Bode's law. At a conference. held in Gotha in 1796, Lalande and De Zach proposed to search for the unknown planet, and to divide the labor among twenty-four astronomers, each of whom should examine an hour of the zodiac.

On the first day of this century — that is, January 1, 1801 — Piazzi discovered at Palermo a star which he took at first for a little comet and observed several times till the 11th of February following, when illness stopped his work. Bode was the first to recognize that the star could not be a comet, and thought that in Ceres Piazzi had found the planet suspected by Kepler. When Piazzi had become well again, he did not know where to look for Ceres. It was to be sought for toward the end of the year, after coming out from the glare of the sun, but no data were at hand for determining its position except the geocentric arc of 3° which it had traversed during the forty days it had been under observation.

Here Gauss came to the rescue; he was then twenty-four years of age, and had had little or nothing to do with astronomical calculations, having been occupied chiefly with the higher arithmetic. In less than a month he invented an admirable method for calculating the elements of the elliptical orbit of Ceres and an ephemeris, by means of which Olbers found the star again on the first day of January, 1802. The mean distance of Ceres from the sun is 2·77. It corresponds exactly with Bode's law, and fills the gap, but with a very modest planet, having a diameter of only about 200 miles.