The MarsDial is a sundial that was devised for missions to Mars. It is used to calibrate the Pancam cameras of the Mars landers. MarsDials were placed on the Spirit and Opportunity Mars rovers, inscribed with the words "Two worlds, One sun" and the word "Mars" in 22 languages. The MarsDial can function as a type of sun-dial known as a gnomon, in modern times this is the stick part of a sundial. A gnomon type sundial is basically a "stick in the ground" design, but by looking at the length and direction of the shadow the time of day can be calculated. The sundial can also be used to tell which way is North, and to overcome the limitations of a magnetic north different from a true north.