At the top center of the mosaic is a sundial. It plays a key role in locating the height of the sun and thus the latitude of the physical locations within the Roman Empire.
The city of Alexandria, Egypt, after Alexander the Great founded it at the mouth of the Nile River in 331 BC, became the location for the greatest library in the world. This library was the center of intellectual activity for the Western world for millennia. There is a commemorative sundial (lat/lon 31.2064, 29.9148) 517 meters 117° from the center of the modern Library of Alexandria.
It was near this library that in 240 BC Eratosthenes measured the sun’s angle above the horizon and compared it to the report of a well near Aswan on the Nile River.
He knew approximately how far south along the Nile River that well was located and by comparing the distance to the angle of the sun at Alexandria on the same date he was able to calculate the circumference of the Earth. Kom Ombo was a fortified location at the south-east corner of the Roman Empire at the time the Seven Sages mosaic was created.
These lines are precisely parallel at 31.2° above horizontal and flipped horizontally to also be precisely parallel at 31.2°. The lines were drawn through eyes, fingertips, toe tips, scroll tips and horizontal and vertical lines defining the center of the picture.
Alexandria is at the north end of the Nile River. It is between here and Kom Ombo at the southern end of this picture that the circumference of the spherical Earth was first calculated and demonstrated.
The Seven Sages of Greece mosaic demonstrates that the Romans knew the Earth was a sphere.