Scientists use the sundial to adjust the rovers' panoramic cameras, while students participating in NASA's Red Rover Goes to Mars program will monitor the dial to track time on Mars. Students worldwide will also have the opportunity to build their own Earth sundial and compare it to that on Mars.

"Sundials are so much a part of human history," said Bill Nye, one of the key figures in making the project possible. The Greek mathematician "Eratosthenes inferred the diameter of the Earth within perhaps four percent using the shadows of sticks".

The left image was captured near martian noon when the Sun was very high in the sky. The right image was acquired later in the afternoon when the Sun was lower in sky, casting longer shadows. The colored blocks in the corners of the sundial are used to fine-tune the panoramic camera's sense of color. Shadows cast on the sundial help scientists adjust the brightness of images.

After imposing hour markings on the face of the dial, local time on Mars can be determined. The position of the shadow of the sundial's post within the markings indicates the time of day and the season, which in this image is 12:17 p.m. local solar time, late summer. A team of 16 students from 12 countries were selected by the Planetary Society to participate in this program. This image was taken on Mars by the rover's panoramic camera.

Each sundial is inscribed with the words "Two Worlds, One Sun" and bears the name "Mars" in 17 languages, including Bengali, Inuktituk, Lingala and Malay-Indonesian, as well as ancient Sumerian and Mayan. At least three-quarters of the human population can read the inscription in their native language. Four gold panels along the sides of the sundials are inscribed with stick-figure drawings of people [called 'sticksters'], as well as a message to future Mars explorers.

Behind the idea for the martian sundial is the Cornell graduate and educator Bill Nye, whose contagious passion for science made him a television star. Nye is equally enthusiastic about the martian missions. Nye said: "I'm very proud of the sundial, and I'm honored to be part of the team...The shadow-casting post on the calibration target was an obvious opportunity for proselytizing, perhaps enriching the lives of Mars scientists."

"Jim Bell, the Cornell astronomy professor in charge of the Panoramic Camera (pancam), approached me on a flight to Ithaca, New York," Nye told Astrobiology Magazine about the idea's inception. "He asked me if I'd like to attend a meeting about the Mars Exploration Rovers, which were on the drawing boards in the early winter of 2000. I was thrilled.

"I attended the meeting in the Space Sciences building, where Carl Sagan had worked. I took one look at the "photometric calibration target," and exclaimed or quickly proposed with a weird urgency, 'That's got to be a sundial,' or something like that."

"For the next level of detail, I said, the guy for this is Woody Sullivan," said Nye.

Sullivan is a University of Washington professor, key advocate of the SETI@home screensaver, and an expert in sundial design. "Bill Nye got into sundials via his father originally", Sullivan told Astrobiology Magazine.

"Bill and I saw our common interest in sundials. When he came up with the calibration target on the Mars rover, he thought of me. He asked me if I wanted to put a sundial on Mars. I answered immediately in email: Are you kidding? Is the Pope Catholic? Does Ken Griffey play centerfield for the Seattle Mariners (true at the time)? Does it rain in Seattle? So we got started. Over six months, we turned it into a sundial."

"We had to get NASA and the Jet Propulsion Lab to approve the epoxy," said Sullivan. "We had to anodize the metal. We had to glue in the silicone rubber, which attaches the rings and color patches. It all had to be color-calibrated. This was all very interesting to me--to make something that would rest on another planet. And fortunately, except for a very few high, thin clouds, most days on Mars are sunny".

"But we had the big problem: a sundial on a moving vehicle", said Sullivan. "How do you know how the vehicle is oriented? We have now, because of this history, a sundial on a moving vehicle. We have to get the orientation from NASA. But NASA is going to get the vehicle's orientation by pointing the Pancam to find the sun in the martian sky. So ironically, they are going to use some of the same information that the sundial relies on for its shadow. Knowing the time, and seeing the shadow, we can find the same orientation".

Knowing whether the sundial is facing north or south is only half of the calibration, since the shadow length will vary not only with time of day, but also with where the rover landed.

"But still, what latitude is [the rover] going to be at?" Continued Sullivan. "NASA said: 'The landing site is going to be selected long after you have to deliver the dial'. So we came up with superimposing the dials' hour lines via the web. Electronically we can change the selected lines and what they mean. Sundials can show about a dozen things other than just the hour or time of day, like what is the Martian zodiac? "

As JPL Center Director, Dr. Charles Elachi, pointed out on the first night of the Spirit rover's landing, each day is a new landing if the rover can roam around to investigate new landscapes. The martian laboratory, and its sundial, are on wheels.

By using the internet to superimpose their daily clockface, Nye and Sullivan have turned the rover's mobility to the project's benefit: "In a reasonable sense", said Nye, "the invention of clocks has had a greater effect on our lives than the invention of wheels."

Sullivan concluded: "It is a great way to get people to think about their place in the cosmos."

"Now, two of these inscriptions are on their way to Mars," said Nye. "It gives me chills," he continued, quoting the inset inscription on each sundial.

People launched this spacecraft from Earth in our year 2003. It arrived on Mars in 2004. We built its instruments to study the martian environment and to look for signs of water and life. We used this post and these patterns to adjust our cameras and as a sundial to reckon the passage of time.

The drawings and words represent the people of Earth. We sent this craft in peace to learn about Mars' past and about our future. To those who visit here, we wish a safe journey and the joy of discovery.

Article is courtesy of NASA's Astrobiology Magazine team at Ames Research Center. This article is public domain and available for reprint with appropriate credit.

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