Page:Popular Science Monthly Volume 11.djvu/291

Rh earth facing the sun and moon at this point of her orbit must have an equally high tide at this time. The centripetal force here is produced by the gravity of both sun and moon acting jointly.

Let us now suppose the moon to be in quadrature, as at B. Then the two centrifugal forces, the one produced by revolution around the sun, the other by revolution around the centre of gravity of these two



bodies, do not act in the same line of direction, but at right angles with each other. The phenomena of solar and lunar tides are then about 90° apart; the solar being the smaller and the lunar the larger. Here the centres of both earth and moon are in the path described by their centre of gravity.

In the last place, let us suppose the moon and sun to be in opposition, as at C. Then, according to my theory, the earth feels, on her side farthest from the sun, an influence which diminishes the centrifugal force produced by her orbital revolution. For at this point the earth's centre is within and the moon's centre is without the elliptic path described by their centre of gravity. Here the revolution of the earth around this centre of gravity is contrary to her general motion around the sun. But what is thus lost in centrifugal force on her side turned away from the sun is more than made up by the