Upvote:3
Not sure if they were the first, but ancient Greeks clearly had the idea based on Wikipedia's history of longitude
Eratosthenes in the 3rd century BC first proposed a system of latitude and longitude for a map of the world. [...] By the 2nd century BC Hipparchus [...] proposed a method of determining longitude by comparing the local time of a lunar eclipse at two different places, to obtain the difference in longitude between them. This method was not very accurate, given the limitations of the available clocks, and it was seldom done - possibly only once, using the Arbela eclipse of 330.
If anyone noticed much earlier than this, I would suspect the Babylonians, but I am not finding any direct reference to support that they did.