Page 205 - 89 history of astronomy_opt
P. 205

SEˆa, SEˆa to SM and to each other.
                                            For the orbit of Mars observations were
                                         chosen at intervals of a year, when the earth
                                         was always in the same place.
                                            [Illustration]
                                            But Kepler saw much farther than the
                                         geometrical facts. He realised that the or-
                                         bits are followed owing to a force directed
                                         to the sun; and he guessed that this is the
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