antities are the mass of the earth and
the distance of the sun. The distance of the sun bears to a certain
distance (which can be calculated when we know the intensity of
gravitation at the earth's surface, the size of the earth and the length
of the year) the same proportion that the cube root of the sun's mass
bears to the cube root of that of the earth. There is no uncertainty
about this result, and the consequence is obvious. If we have the means
of weighing the earth in comparison with the sun, then the distance of
the sun can be immediately deduced. How are we to place our great earth
in the weighing scales? This is the problem which Le Verrier has shown
us how to solve, and he does so by invoking the aid of the planet Mars.

If Mars in his revolution around the sun were solely swayed by the
attraction of the sun, he would, in accordance with the well-known laws
of planetary motion, follow for ever the same elliptic path. At the end
of one century, or even of many centuries, the shape, the size, and the
position of that ellipse would remain unaltered. Fortunately for our
present purpose, a disturbance in the orbit of Mars is produced by the
earth. Although the mass of our globe is so much less than that of the
sun, yet the earth is still large enough to exercise an appreciable
attraction on Mars. The ellipse described by the planet is consequently
not invariable. The shape of that ellipse and its position gradually
change, so that the position of the planet depends to some extent upon
the mass of the earth. The place in which the planet is found can be
determined by observation; the place which the planet would have had if
the earth were absent can be found by calculation. The difference
between the two is due to the attraction of the earth, and, when it has
been measured, the mass of the earth can be ascertained. The amount of
displacement increases from one century to another, but as the rate of
growth is small, ancient observations are necessary to enable the
measures to be made with accuracy.

A remarkable occurrence which took place more than two centuries ago
fortunately enables the place of Mars to be determined with great
precision at that date. On the 1st of October, 1672, three independent
observers witnessed the occultation of a star in Aquarius by the ruddy
planet. The place of the star is known with accuracy, and hence we are
provided with the means of indicating the exact point in the heavens
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