File:Hun Kal crater on Mercury.jpg

English: A fresh new crater in the center of an older crater basin provides a landmark for the tiny crater named Hun kal- the Mayan number 20 - which is the basis for positioning the longitudes on Mercury. By definition, the 20° meridian passes through the center of this small crater. Assuming that the spin axis of Mercury is perpendicular to its orbital plane, the latitude of Hun Kal is 0.23°S. This picture, which covers an area of 130 by 170 km ( 90 by 105 mi), was taken from a distance of about 20,700 km (12,860 mi), a half-hour before Mariner made its first close flyby of Mercury, March 1974.