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the relative direction from night to night.
Now, Kepler had twenty years of observa-
tions of Mars to deal with. This enabled
him to use a new method, to find the earth’s
orbit. Observe the date at any time when
Mars is in opposition. The earth’s position
E at that date gives the longitude of Mars
M. His period is 687 days. Now choose
dates before and after the principal date
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