In fact they knew it.
Some where around 200BC for sure and probably before. They had even figured out how big it was. Their calculations were close enough for government work.
The irregular English suggests the writer of the piece operates under ESL.
As HTB notes, the spherical structure of the Earth and Moon was understood by Aristotle's time, and before the middle of the 3rd c BC, Eratosthenes, devised and conducted an experiment to calculate the circumference of the Earth (25,000 miles, a bit higher than the actual figure).
‘Draco’ that old serpant,the Dragon.
.
The Gospel in the Stars is a
Fascinating read.