Hun Kal (crater)

Hun Kal is a small crater on Mercury that serves as the reference point for the planet's system of longitude. The longitude of Hun Kal's center is defined as being 20° W, thus establishing the planet's prime meridian.
Hun Kal was chosen as a reference point since the actual prime meridian was in shadow when "Mariner 10" photographed the region, hiding any features near 0° longitude from view.
Hun Kal is about 1.5 km in diameter.
The name "Hun Kal" means '20' in the language of the Maya.