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part of his work aside. Fortunately, before
Newton wrote the Principia the French as-
tronomer Picard made a new and correct
measure of an arc of the meridian, from
which he obtained an accurate value of the
earth’s diameter. Newton applied this value,
and found, to his great joy, that when the
distance of the moon is 60 times the ra-
dius of the earth she is attracted out of the
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