I believe that the distortion introduced to a rainbow by the light source being reflected from the spherical surface of the moon could be measured to determine the length of time required for light to traverse half the distance of the moon. Since they appear to have known the diameter of the moon it was a simple computation to determine the speed of light in a vacuum (the ether). In other words they knew the speed of light by observing the differences between rainbows and moonbows.

This means could be considered to be suggested by the ancient literature.

Ancient science was cool.