““Eastern” relative to what?”
Planets generally orbit the sun in the plane of the ecliptic.
Earth’s north is “above” this plane (and tilted 23.5 degrees).
Consider the north half of the sun to be in the same region above the plane as earth’s north.
From earth and facing the sun with the north oriented on top, the eastern edge would be on the right side of the disk.
Similarly, if you look at a satellite view of the earth, the east is on the right, as long as you have the top (north) oriented correctly.
Keeping in mind, of course, that originally to “orient” a map one put the east, where the sun rose and where The Garden was believed to be, at the top.
Therefore, from a European/Mediterranean viewpoint the top of a T-O map was properly oriented thataway...