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Name: Frederick G.
Status: other
Age: 50s
Location: N/A
Country: N/A
Date: Saturday, April 20, 2002 8:17:44 PM

Is the center of earth intersected by a plumb bob? What means are there for measuring direction of the center of earth? How does the direction of gravity vary with that? What are the most reliable electronic means of measuring the direction of gravity?

If I wanted to measure the angle from the horizon to a star, but could see the horizon, would an angle from the direction of gravity be just as accurate?

These are several questions:

1. A plumb bob is attracted by the force of gravity beneath the bob. It points approximately toward the center of the earth, not exactly.

2. There are several ways to define the "center" of the earth and they do not coincide with one another. Several definitions: a.) the center of mass, which does not lie exactly on the polar -- polar axis, but actually oscillates because of ocean tides, so it kind of wobbles. b.) the axis of rotation also processes on a daily, monthly and annual basis depending upon the relative location of the sun, moon and earth. c.) there exist very sensitive instruments called gravitometers that measure the gravitational field at a particular location. These instruments give various numbers depending upon the altitude of the instrument, the density of the rock and crust below the instrument and other factors as well.

3. I do not understand what the "star" question is asking.

Vince Calder


The answers to your questions depend on two things. First, do you mean the geometric center of the Earth or the center of mass of the Earth? It is possible, due to the distribution of water and land on the Earth being asymmetric, for the Earth's center of mass to be in a slightly different location than the geometric center. If geometric center and center of mass happen to be at the same place on a certain day, a plumb bob will point toward both. If they are not in exactly the same location, a plumb bob will point somewhere between them. When the size of the Earth is considered, the plumb bob is always a good approximation.

Gravitational force at a specific location will always pull an object in the direction of the plumb bob. Changing the object being pulled may change the size of the gravitational force, but not the direction.

I am not familiar with any electrical devices for gravity. The major problem with such a device would be orientation. The direction cannot be defined numerically without first having a reference direction to call zero degrees. Other than gravity, there is no constant direction due to the rotational motion of the Earth. Most directions are defined with gravity as the reference. Gravity cannot be used as its own reference.

With the question regarding horizon, gravity is more precise. Where you see the horizon depends on the lay of the land. If the land is perfectly horizontal, angle with respect to gravity will differ from angle above the horizon by 90 degrees. If the land has a slope, the difference may be greater than 90 degrees when looking uphill and less when looking downhill.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College

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