Paleocartography? Longitued BCE?

[icosahedron]

Hi and welcome Greenman.

Primarily, this forum is populated by people who like to draw maps - largely, but not exclusively, maps of fictional realms.
However, cartographical interests do intersect.

Longitude determination by ancient civilizations was an interest of mine, back in the days when I had time for interests, though I'm no expert.
I am aware, however, that contrary to Mathus' thoughts, it is not entirely a matter of geometry, certainly not when you have several landmasses separated by sea.

Like Relativity, Longitude requires a definition of simultaneity before you can begin your calculations, and there you open a writhing can of worms.

I, too, independently figured out a solution, only to find later that it was a known technique, and I pretty much lost interest from thereon.

Certainly, it was possible. The only question is about the degree of accuracy obtainable, and whether anyone actually did it. The Romans could have built steam engines, but they didn't...

Edit: If you haven't already, I would recommend you read 'Longitude' by Dava Sobel. Essentially, it's Harrison's story, but it includes a description of the problem and a number of previous solutions. You might find your solution is there, though I hope with you that it isn't.