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# 2.15 The earth-moon system

Frank L. Preuss

Now follows an extract from the book "The A - Z of Astronomy" by Patrick Moore:

Barycentre. The centre of gravity of the Earth-Moon system. The Earth is 81 times more massive than the Moon, so that the barycentre lies inside the Earth's gloobe (Fig. 4).

And now follow extracts from thne book "Stars, Planets, and Galaxies":

Measurements to the Moon and The Earth-Moon System

"Measurements to the Moon
The distance between the earth and the moon can be determined by observing the moon from two points or stations on the earth's surface. This method, called triangulation, is also used by surveyors to measure distances on the earth. Since the moon is much nearer to the earth than the stars, each station will see the moon in a different star field. The amount of angular displacement of shift of the moon's position among the stars is called parallax. This parallactic displacement decreases as the distance of the object in space increases. For exmaple, the stars are too far away to show a measurable parallax from two points on the suface of the earth. Stellar parallax requires the diameter of the earth's orbit for a baseline. (See page 111.) The parallactic displacement of the moon among the stars is equal to the angle made by the two stations on the earth as seen from the moon. When the radius of the earth is the baseline the displacement is called the moon's horizontal parallax.
Parallax is used to find the diameter of the moon. Once its distance has been calculated, the moon's angular diameter can be converted to linar measure. At perigee (nearest to the earth) the moon will appear larger than at apogee (most distant from earth). The mean value of the angular diamenter is 31' 7" of arc or about one half of a degree. At the moons's mean distance from the earth this angular measure is equivalent to a distance of 2,160 miles."

"The Earth-Moon System
The gravitational effect of the moon on the earth is exemplified in the periodic rise and fall of the ocean tide. The gravitational attraction of the earth is said to keep the moon in an orbit around the earth. Strictly speaking, the earth and the moon revolve about a common center of gravity called the barycenter, which is located about 1,000 miles below the earth's surface.
The distance between the center of the moon and the barycenter is 81 times greater than the distance from the barycenter to the center of the earth. Since the barycenter is the center of mass, then the earth is 81 times more massive than the moon. The barycenter is located by observing the nearby planets. For example, Mars oscillates against the background stars in a period of a sideral month. This motion of Mars is not real but the effect of the earth's center revolding about the barycenter. The amount of the displacement of Mars is a measure of the distance between the observer on the surface and the barycenter.
The earth and moon revolve about the sun in one year and about the barycenter in one sideral month. Yet the orbits of the earth and moon remain concave to the sun. Relative to the earth, the moon revolves in an apparent elliptical orbit with the earth at one focal point. Relative to the sun, the earth and moon revolve around the barycenter, which revolves in an apparant elliptical orbit around the sun."

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This is the end of "2.15 The earth-moon system"
To the German version of this chapter: 2.15 Das Erde-Mond-System

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