The Mars Exploration Rovers each carry an identical sundial, approximately three inches square. Space artist Jon Lomberg (a Planetary Society Advisor) designed the face of the MarsDial, and Louis Friedman, Executive Director of The Planetary Society, coined the MarsDial's motto: Two Worlds, One Sun. Their primary function is as calibration targets for the high-resolution Panoramic Cameras aboard each rover, so they are imaged frequently over the course of the mission. But these thousands of images of the MarsDials with their moving shadows calso serve to remind the public that Mars and Earth truly are two worlds with one Sun.
The idea of using the calibration target as a Martian sundial was a brainstorm of Bill Nye the Science Guy, then a Planetary Society board member.
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