When time was based on earth rotation, astronomers used “transit instruments” to observe when certain “clock stars” passed directly overhead. The clock stars ha

When time was based on earth rotation, astronomers used “transit instruments” to observe when certain “clock stars” passed directly overhead. The clock stars had accurately known positions, so if you routinely record the time they pass overhead according to your observatory’s clock, then you can work out how accurate your clock is. Newcomb’s data would have been accurately timed observations, as many as he could get hold of, going back about two and a half centuries.