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O

THE

CAMBRIDGE COURSE

OF

ELEMENTARY PHYSICS.

PART THIRD.

ASTRONOMY.

By W. J. ROLFE AND J. A. GILLET,

TEACHERS IN THE HIGH SCHOOL, CAMBRIDGE, MASS.

BOSTON:

CROSBY AND AINSWORTH.

NEW YORK: O. S. FELT.

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Entered according to Act of Congress, in the year 1867, by

CROSBY AND AINSWORTH,

in the Clerk's Office of the District Court of the District of Massachusetts.

UNIVERSITY PRESS: WELCH, BIGELOW, & Co.,

CAMBRIDGE.

PREFACE.

ASTRONOMY naturally falls into three divisions. The first treats of the motions and distances of the heavenly bodies; the second, of their physical features; and the third, of gravity, or the force by which they act upon one another.

In the first part, the authors have endeavored to show how we know that the earth rotates on its axis; that the earth and planets with their satellites revolve in elliptical orbits about the sun; and that the sun and the stars are moving through space, or about other stars.

They have also endeavored to show how, by measuring a line of a few miles in length on the surface of the earth, and a few angles, we are able to find the size of the earth, and to pass out into space and measure the distance from the earth to the sun, from the sun to the planets, and from the earth to the fixed stars, - a distance so vast that the velocity of light is the only unit suitable for expressing it.

To complete the picture of the magnitude of the universe, they have added a short account of the nebulæ,

The unequal lengths of the solar days, the varying length of day and night, and of twilight, and the change of seasons, - all of which are due to the motion of the earth on its axis and around the sun, are naturally explained in this part.

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