Unit: Motion in a Plane
Year: 1991 Question#: 66
Question: The shape of the path of the Earth about the sun is
(1 ) a circle with the sun at the center
(2 ) an ellipse with the sun at one focus
(3 ) an ellipse with the moon at one focus
(4 ) an ellipse with nothing at either focus
What is this question really asking?
Explanation by: Conrad C.
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Answer 1
a circle with the sun at the center
INCORRECT
Since this is a knowledge question it should be noted that only ideal orbits are perfectly circular. In the case of the earth about the sun the actual orbit is an ellipse and not a circle. Therefore question answer number 1 (a circle) is wrong. return to top
Answer 2
An Ellipse with the sun at one focus
CORRECT
The geometry of an elliptical orbit says that the sun is at the focus of the ellipse. This is the correct answer and is knowledge based.
This above picture is an accurate picture of the Sun-Earth system. Note the elliptical orbit and the sun located at the focus. return to top
Answer 3
An ellipse with the moon at one focus
INCORRECT
The moon's effect on the sun-Earth system is small. The moon orbits the Earth, meaning that it is quite impossible for it to sit at one focus of the ellipse while the Earth moves around in its orbit. If this was the case then the moon would not be considered a satellite of the Earth. . return to top
Answer 4
An ellipse with nothing at either focus
INCORRECT
If this was the case than an elliptical orbit would not be stable. The sun does sit at one of the foci, and therefore answer number 4 is wrong. This is basically something that must just be memorized as a basic fact of movements of bodies in motion. return to top
What's this question really asking?
The question is really asking if you have memorized the basic ideas of how the sun, Earth, and moon system operates. The information for this question is contained in Kepler's law of ellipses. return to top
