Replying off the top of my head, but IIRC once astronomical phenomena became known and observable, these could provide constant "clocks" which could be verified at different points on the Earth's surface. In particular, transits/occlusions of planets, moons, or stars could be viewed from different points. While observations regarding the Earth's moon are subject to some parallax, by the time you're looking at other planets (or their lunar systems), the error is minimal.

We're talking Keplerian / Gallilean times onward (telescopic observation and orbital mechanics) for the most part, though some observations may have been possible / performed in earlier times. Turns out that Gallileo and Halley proposed such methods in 1612 and ~1683.

Accurate navigational chronometers came along in the late 18th century.