SYZYGY

, a term equally used for the conjunction and opposition of a planet with the sun.

On the phenomena and circumstances of the Syzygies, a great part of the lunar theory depends. See Moon. For,

1. It is shewn in the physical astronomy, that the force which diminishes the gravity of the moon in the Syzygies, is double that which increases it in the quadratures; so that, in the Syzygies, the gravity of the moon is diminished by a part which is to the whole gravity, as 1 to 89.36; for in the quadratures, the addition of gravity is to the whole gravity, as 1 to 178.73.

2. In the Syzygies, the disturbing force is directly as the distance of the moon from the earth, and inversely as the cube of the distance of the earth from the sun. And at the Syzygies, the gravity of the moon towards the earth receding from its centre, is more diminished than according to the inverse ratio of the square of the distance from that centre.—Hence, in the moon's motion from the Syzygies to the quadratures, the gravity of the moon towards the earth is continually increased, and the moon is continually retarded in her motion; but in the moon's motion from the quadratures to the Syzygies, her gravity is continually diminished, and the motion in her orbit is accelerated.

3. Farther, in the Syzygies, the moon's orbit, or circuit round the earth, is more convex than in the quadratures; for which reason she is less distant from the earth at the former than the latter.—Also, when the moon is in the Syzygies, her apses go backward, or are retrograde.—Moreover, when the moon is in the Syzygies, the nodes move in antecedentia fastest; then slower and slower, till they become at rest when the moon is in the quadratures.—Lastly, when the nodes are come to the Syzygies, the inclination of the plane of the orbit is the least of all.

However, these several irregularities are not equal in each Syzygy, being all somewhat greater in the conjunction than in the opposition.