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When I saw the first reports of this on Twitter I thought it was a joke. A sundial on Curiosity? Just in case the computer packs up they can still tell the time? I thought some wag was pulling our collective legs with a photo of one of the rover’s antennae.

But, turns out this is something like a sundial. Its a Marsdial – actually a calibration target enabling photographs to be corrected for colour. BIll Nye, from the Planetary Society, describes its role in Curiosity’s Marsdial is on Mars!

“As I’m sure you’re aware, geologists love rocks, and they especially love the rocks on Mars. The first thing they all want to know about a rock is what’s it made of. For that, it’s good to just take a look at the color of the rock surface. When everything is being done on the alien landscape of another world, it’s easy enough to electronically get the color wrong, or not quite right. To that end, artists, photographers, and a few scientists have noticed that by looking at the color of a shadow on a neutral white or gray background, you can infer the color contributed to the scene by the sky.

On Earth, shadows take on a sky blue tinge (what I like to call “cerulescence”). On Mars, it’s a salmon color (what I like to call “arangidescence”). And so, the MarsDials bear a small metal post that casts a shadow onto some white and gray rings of known value or grayness.”

The NASA animation above is made up from four Mastcam images of the calibration target — the Marsdial. They were taken on Curiosity’s sol 3 (August 9, 2012) over a period of about 8 minutes. In that time, the shadow moved slightly, marking time on Mars with a sundial. (You may need to click on the photo to see the animation).

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