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.
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.
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.
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.
Dava Sobel’s book Longitude is a great story of how accurate ship chronometers were devised. Accurate clocks were needed to help in navigation on the world’s oceans.
I don’t know exactly what they did but you can always compare multiple clocks against each other.
If you make several clocks, you can set them running and see if they still agree with each other after some time. If you are happy with the accuracy of the type of clocks you can then set it against some known period of time
They just knew cause of measuring it based on the shadow made on the ground.
The sundial is just a translated version of your own shadow.
They didn’t need to be super accurate, because time as we know it wasn’t fixed. The first mechanical clocks were used by monasteries to tell them when to go pray. They were required to say prayers at particular times of day and night. the concept of an hour was not a fixed period of time, but a roughly equal division of the time between dawn and dusk. They didn’t need to count minutes or seconds, they just needed a point on the sundial halfway between dawn and noon. Sundials are fine if there’s sunshine, but if it’s night, or raining, it doesn’t work, which is why the mechanical clocks were invented, and they only needed to be *kinda okay* at dividing up the day and night into four parts.
Only with observation of shadows, then with what are today basic math, geometry and tools you can go a long way. I think stonehenge shows precisely where the sun will set on the day of the solstice? Given that the clock has to show the same time in 1 year from now, you’ll have to divise time in a way to mark each year, each midday, each midnight, why not ad month to it, and prove its accuracy through tests. Basic math but some commitment I guess
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