If he made the most accurate clock in the world, how would he know?
The prize here was for most accurate at sea, so compare with more accurate land clocks. But in general?
If salt should lose its savour, with what shall it be salted?
... tried feeling bits of himself to see where he might be hurt. Wherever he touched himself, he encountered a pain. After a short while he worked out that this was because it was his hand that was hurting.a
I think he watched for a particular star to disappear behind his neighbour's chimney, from a fixed vantage, on the understanding that this should always take exactly 1 day, and by measuring how many seconds have elapsed on your clock you know how well your clock has kept time over the previous day.
The prize here was for most accurate at sea, so compare with more accurate land clocks. But in general?
If salt should lose its savour, with what shall it be salted?
... tried feeling bits of himself to see where he might be hurt. Wherever he touched himself, he encountered a pain. After a short while he worked out that this was because it was his hand that was hurting.a