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The 1769 transit of Venus was observed and coordinated by over one hundred astronomers around the world. How did they measure time so accurately, key to the observations having any scientific value? I find little information online regarding 18th century timekeeping devices. I assume that they were mostly using clocks intended for nautical navigation, as I think those were the most accurate of the era.

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    $\begingroup$ I thought (but not sure enough to answer) that the transit was measured primarily by latitude differences (easy to know) and transit durations (easy to measure locally), rather than longitude differences and absolute times. That is, you measure how the chord of the transit shifts vertically across the surface of the Sun with a known vertical shift in observation location. $\endgroup$
    – user10851
    Nov 1 '15 at 2:57
  • $\begingroup$ @ChrisWhite - you could do either lat or long differences to triangulate. I don't know which they used more. You have a bigger potential range in Long (without going into the polar bad weather) $\endgroup$ Nov 1 '15 at 18:50
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They couldn't carry sufficently accurate time from London with portable clocks.

But they were able to use clocks to time measure the time between the sun crossing and the transit of stars the night before and after. The absolute transit time of stars can be trivially obtained if you know the site's longitude.

cook's portable observatory - with clock

If you are on land and have an observatory to hand you can use the lunar method to determine the longitude. Determining the latitude at sea needed accurate portable clocks that came a little later

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  • $\begingroup$ @ChrisWhite oops ! $\endgroup$ Nov 1 '15 at 14:50
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Since pre-historic times, it is the motion of stars which were used to measure time. Astronomers were well-aware of their motions and their relation to time. And as far as my knowledge goes, they had angle measuring instruments similar to modern day sextants by 18th century.

If you refer to any textbook in astronomy, one of the most important things they teach is how to keep time using motion of stars. This used to be every day(or night) practice in observatories.

There were assistants in observatories to calibrate time. It was a very important job in those times. This calculation had to be accurate. You can get the importance of this practice by your question.

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  • $\begingroup$ There is a 100% degeneracy between the time and the longitude - watching a star won't tell you whether it's 17:00 GMT at one longitude or 18:00 GMT 15 degrees further west. $\endgroup$
    – user10851
    Nov 1 '15 at 2:54
  • $\begingroup$ @ChrisWhite In fact, now I know better than before that it was not at all an easy job because there was no way of knowing longitude at that time too. The Royal Observatory had something called a Longitude Prize worth 20 times the annual salary of the Royal Astronomer for discovering a quicker method. It took days of iterative observations to get time and longitude, the details of which I am not aware of. $\endgroup$
    – Cheeku
    Nov 1 '15 at 14:31
  • $\begingroup$ Thee were several ways of knowing longitude at the time, they just weren't as easy reliably usable on shipboard. Observing the eclipses and emergences of the moons of Jupiter was in may ways the best, but it required descent telescope, a stable observing platform, extended periods of clear skies, and several fixes stars. This is the means that was used to establish the longitudes of the places at which chronometers were checked for the prize. $\endgroup$ Nov 1 '15 at 15:28
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For short periods of time very accurate time could be kept by using pendulums.

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    $\begingroup$ Your answer seems woefully incomplete and little more than a comment. $\endgroup$ Nov 1 '15 at 1:54

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