At noon on the longest day the plane of longitude passing through Marseille is exactly on edge to the Sun. If the Earth's axis were not tilted toward the Sun, a vertical rod at the equator would have no shadow. A rod further north would have a north–south shadow, and as an elevation of 90° would be a zero latitude, the complement of the elevation gives the latitude. The Sun is even higher in the sky due to the tilt. The angle added to the elevation by the tilt is known as the obliquity of the ecliptic and at that time was 23° 44′ 40″. The complement of the elevation less the obliquity is 43° 13′, only 5′ in error from Marseille's latitude, 43° 18′.