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The book Transits -- When planets cross the Sun, Maunder & Moore, page 44 states that

Legentil [was] one of the first to realise that transits of Mercury could never be observed with sufficient accuracy to make them really valuable in measuring the length of the astronomical unit.

Is this still the case? With modern instrumentation, can the transit of Mercury still not be measured with enough precision to derive the distance to the Sun? If not, why not? Has Mercury never been used to determine the distance between the Earth and the Sun?

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When transits of Mercury and Venus occur, amateur groups do try to observe those transits so as to measure the astronomical unit. To do so, observations of a transit must be made at widely separated points on the Earth. The separated observers will see slightly different versions of a transit, with the differences eventually yielding an estimate of the astronomical unit.

Observations of transits inevitably yield less accurate estimates than do modern estimates based on radar observations of the distances to other planets and on even more accurate distance measurements based on probes sent to other planets. The radar and communications measurements are extremely accurate, twelve places of accuracy or more. Modern transit-based estimates using accurate clocks and GPS are lucky to get better than four places of accuracy.

Transit-based estimates are less accurate than radar-based and communications-based estimates because of the very small parallax involved in Earth-based observations of a transit. Observations of transits of Mercury yield even less accurate results than do observations of transits of Venus because Mercury is closer to the Sun / further from Earth than is Venus during its transits.

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  • $\begingroup$ Also the "black drop" effect limits the timing precision of transits. As the article discusses, this is not (just) an effect of the planet's and Earth's atmosphere. $\endgroup$ – astrosnapper Oct 3 '20 at 18:57

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