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at intervals of 687 days and its multiples.
Mars is in each case in the same position.
Now for any date when Mars is at M and the
earth at Eˆa the date of the year gives the
angle EˆaSM. And the observation of Tycho
gives the direction of Mars compared with
the sun, SEˆaM. So all the angles of the tri-
angle SEM in any of these positions of E
are known, and also the ratios of SEˆa, SEˆa,
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