When they made the first clocks, how did they know whether they were accurate?

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They had nothing to compare the clocks against, and I don’t think the stars and the sun would have made an useful target for comparison.

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> I don’t think the stars and the sun would have made an useful target for comparison.

But they are. You can spot noon fairly easily by observing the sun. When the sun is in the highest point in the sky, then it is noon, and someone can mark the time. Instruments for determining the sun’s position in the sky have been around for ages.

With a stationary (and properly filtered!) telescope you can quite easily track the sun’s motion. A clock could be adjusted to show noon at the precise momemt the edge or middle of the sun touches the crosshair in a properly oriented telescope.

You could check again the next day without moving the telescope and compare times.

a large sundial can be surprisingly accurate. the noon mark changes very little from day to day. so if the clock is accurate within minutes for a day, you’re good to go.

How did they decide which direction was clockwise?

How accurate did they need to be?

You could probably point to multiple times in history where clocks finally became accurate enough to be used in certain applications [https://en.wikipedia.org/wiki/Longitude_(book)](https://en.wikipedia.org/wiki/Longitude_(book)) is a book about a timepiece being made accurate enough for navigation in the 18th century.

I can imagine they had multiple timekeeping breakthroughs in the 20th century.