Word Finder - perigee

definition

The point, in an orbit about the Earth, that is closest to the Earth: the periapsis of an Earth orbiter.

definition

(more generally) The point, in an orbit about any planet, that is closest to the planet: the periapsis of any satellite.

definition

(possibly obsolete outside astrology) The point, in any trajectory of an object in space, where it is closest to the Earth.

An Anomalistic month is the time in which the moon passes from perigee to perigee, &c.

The problem of determining an orbit may be regarded as coeval with Hipparchus, who, it is supposed, found the moving positions of the apogee and perigee of the moon's orbit.

In the case of the motion of the moon around the earth, assuming the gravitation of the latter to be subject to the modification in question, the annual motion of the moon's perigee should be greater by I 5" than the theoretical motion.

A line joining the moon in perigee and in apogee is called the " line of apsides."

It may be considered as arising from a semi-annual variation in the eccentricity of the moon's orbit and the position of its perigee.

This is a cycle of 18 years II days, or 223 lunations, discovered at an unknown epoch in Chaldaea, at the end of which the moon very nearly returns to her original position with regard as well to the sun as to her own nodes and perigee.

Thus the apogee and perigee became two definite points of the orbit, indicated by the variations in the angular motion of the moon.

His most important contribution to the subject consisted in working out by extremely elegant mathematical processes the method of determining the motion of the perigee.

The first of the outstanding gravitational problems with which they grappled was the unaccountably rapid advance of the lunar perigee.

The eccentricity of the ellipse is in the general average about 0.055, whence the moon is commonly more than i' further from the earth at apogee than at perigee.

The Babylonians knew of the inequality in the daily motion of the sun, but misplaced by to' the perigee of his orbit.

Then, by an obvious law of kinematics, the angular motion round the earth would be most rapid at the point nearest the earth, that is at perigee, and slowest at the point most distant from the earth, that is at apogee.

Assuming the mean motion of the moon to be known and the perigee to be fixed, three eclipses, observed in different points of the orbit, would give as many true longitudes of the moon, which longitudes could be employed to determine three unknown quantities - the mean longitude at a given epoch, the eccentricity, and the position of the perigee.

We may conclude the ancient history of the lunar theory by saying that the only real progress from Hipparchus to Newton consisted in the more exact determination of the mean motions of the moon, its perigee and its line of nodes, and in the discovery of three inequalities, the representation of which required geometrical constructions increasing in complexity with every step.

By taking three eclipses separated at short intervals, both the mean motion and the motion of the perigee would be known beforehand, from other data, with sufficient accuracy to reduce all the observations to the same epoch, and thus to leave only the three elements already mentioned unknown.