Theon Senior is a lunar impact crater that is located to the northwest of Delambre crater, and south of D'Arrest crater. It forms a matching pair with the Theon Junior crater, about 2 crater diameters to the south-southeast. The satellite crater Theon Senior A can be found to the North. Theon Senior is named for Theon of Smyrna, a 1st century Greek mathematician and philosopher.
This crater is circular and bowl-shaped, with only a small floor at the center of the steeply sloping interior walls. The crater appears relatively young, as it shows little sign of wear due to impacts. The crater is about 18 kilometers in diameter, and the distance from its rim to the floor is 3470 meters. Theon Senior is from the Eratosthenian period, which lasted from 3.2 to 1.1 billion years ago.
|A||0.2° S||15.4° E||5 km|
|B||0.2° N||14.1° E||6 km|
|C||1.4° S||14.5° E||6 km|