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 ex

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.