Hun Kal is a small (about 1.5 km in diameter) crater on Mercury that serves as the reference point for the planet's system of

longitude Longitude (, ) is a geographic coordinate that specifies the east– west position of a point on the surface of the Earth, or another celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek lette ...

. The longitude of Hun Kal's center is defined as being 20° W, thus establishing the planet's

prime meridian A prime meridian is an arbitrary meridian (a line of longitude) in a geographic coordinate system at which longitude is defined to be 0°. Together, a prime meridian and its anti-meridian (the 180th meridian in a 360°-system) form a great ...

. The name "Hun Kal" means '20' in the language of the Maya. 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. The prime meridian of Mercury is thermocentric, it runs through the point on the equator where the planet is hottest (due to the planet's rotation and orbit, the sun briefly retrogrades at noon at this point during

perihelion An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any elli ...

, giving it more sun).Davies, M. E., "Surface Coordinates and Cartography of Mercury," Journal of Geophysical Research, Vol. 80, No. 17, June 10, 1975. The large crater in which Hun Kal lies is not named. To the northwest is Al-Jāhiz crater, and to the west is Lu Hsun crater.

References

Impact craters on Mercury Prime meridians {{Mercury-planet-stub