I read somewhere (I can't remember the website) that when Cassini and Jean Richer measured the parallax of Mars, they used Jupiter's (or maybe Saturn's) moons to sync their time. How was this done?

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    $\begingroup$ This is certainly on-topic here, but just fyi there is also History of Science and Mathematics SE as well. If you don't receive an answer after some period of time, you might consider moving this question there. $\endgroup$ – uhoh Sep 11 at 9:20
  • $\begingroup$ Wild guess: the angular separation of these observers when, say, a Saturnian moon initates occultation is in the noise compared with their parallax when observing Mars. $\endgroup$ – Carl Witthoft Sep 11 at 14:57

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