The four largest well-preserved impact basins in the solar system, Borealis, Hellas, and Utopia on Mars, and South Pole–Aitken on the Moon, are all significantly elongated, with aspect ratios >1.2. This population stands in contrast to experimental studies of impact cratering that predict <1% of craters should be elliptical, and the observation that ~5% of the small crater population on the terrestrial planets is elliptical. Here, we develop a simple geometric model to represent elliptical crater formation and apply it to understanding the observed population of elliptical craters and basins. A projectile impacting the surface at an oblique angle leaves...