I have seen other similar questions and someone mentioned radar measurements made in the 1960's. Who did that and when and where? I am not interested in ancient measurements. Only the currently accepted measurement, And when it was made.
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$\begingroup$ -1 someone mentioned... You ought to provide details (eg a link) to what you have read. $\endgroup$– sammy gerbilCommented Mar 25, 2018 at 1:34
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$\begingroup$ One answer that asserts radar measurements from the '60s is for "How is distance between sun and earth calculated?" $\endgroup$– outisCommented Aug 9, 2023 at 15:06
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$\begingroup$ Previous answers that radar measurements to the sun have never been done are incorrect. Here is a link to the experiment. science.org/doi/10.1126/science.131.3397.329 $\endgroup$– DarkBownCommented Aug 13, 2023 at 4:00
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1 Answer
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There never has been a direct measurement of the distance between the Earth and the Sun. The best we can do is measure the distance between the Earth and the other planets, and from that infer the distance between the Earth and the Sun.
Today's estimates aren't based on radar pings from Venus. That is so last millennium! Early 1960s, to be precise. That said, estimates of the astronomical unit made possible by those 1960s radar measurements of the distance between the Earth and Venus were much more precise than were prior estimates based on parallax, reducing the uncertainty in the astronomical unit from 16000 km to less than 1000 km.
In the 50+ years since that time, space agencies have sent spacecraft to all of the other seven planets in the solar system (other than the Earth, of course), plus Pluto, plus some asteroids, plus some orbiting the Sun. The telemetry from those spacecraft, augmented by very long baseline interferometry (VLBI), have made those measurements from the 1960s look rather imprecise. The latest work was in 2009 by Pitjeva and Standish, who stated the uncertainty in the astronomical unit as being 3 meters. No work has been done on measuring the AU since then because the astronomical unit is now a defined quantity, exactly 149597870700 meters (the value stated by Pitjeva and Standish).
answered Jan 8, 2017 at 6:23
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$\begingroup$ What were the results of the 1960s and today? :) $\endgroup$ Commented Jan 8, 2017 at 21:18
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$\begingroup$ Do you know the name of the paper in which Pitjeva and Standish published their measurement of the AU? $\endgroup$– Will TCommented Jan 14, 2017 at 10:41
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1$\begingroup$ Thanks - great answer. It is amazing how many websites claim that the AU has been measured directly with radar. I wonder how can the Sun's distance be inferred when we don't know it's size? $\endgroup$– Will TCommented Jan 14, 2017 at 10:45
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1$\begingroup$ @WilliamClifford-Banks : Pitjeva, E. V., and E. M. Standish. "Proposals for the masses of the three largest asteroids, the Moon-Earth mass ratio and the Astronomical Unit." Celestial Mechanics and Dynamical Astronomy 103.4 (2009): 365-372. $\endgroup$ Commented Jan 15, 2017 at 5:17
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1$\begingroup$ @JEB - I agree with David, but just to clarify, EM waves can reflect off of plasmas if the plasma frequency is higher than the incident EM wave. This is done in the ionosphere (can't remember if it's AM or FM that works better for this). $\endgroup$ Commented Dec 5, 2018 at 17:39