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would eliminate errors from the theory of the orbit of the Earth. Between 1700 and 1760, a number of attempts were made to measure the solar parallax using Cassini's method but, even though observational techniques improved, the accuracy of the results did not and, while then it was accepted that the correct value was less than 15'', no one could lay claim to an accurate direct measurement.

The fact that transits of the inferior planets could be used to determine the solar parallax was pointed out by the mathematician James Gregory in 1663. He wrote:

This problem has a very beautiful application, although perhaps laborious, in observations of Venus or Mercury when they obscure a small portion of the sun; for by means of such observations the parallax of the sun may be investigated.

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