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Sundials on Earth and Mars Mark the Spring Equinox


Mat Kaplan
Phone: +1-626-793-5100

An unexpected spinoff from the high-technology, high-precision Mars Exploration Rover mission is an Earthly educational project – EarthDials around the world.

On March 20th, The Planetary Society invites people worldwide to mark Earth’s Spring Equinox with a vicarious trip around the globe to see these special sundials, called EarthDials.

The EarthDial Project is a growing worldwide network of sundials to show the passage of time the old-fashioned way by tracking the motion of the Sun across the sky. Volunteers at 14 sites in six countries have created their own sundials in conjunction with the two sundials ("MarsDials") currently traversing the Martian terrain on NASA's twin rovers Spirit and Opportunity. The EarthDial Project is a partnership between The Planetary Society (the world’s largest space interest group); Bill Nye, the Science Guy®; and Professor Woody Sullivan at the University of Washington.

“Seeing live sundials around the world makes the variations of time on our planet more real in the minds of kids and adults, and does it in a fun way,” said Planetary Society Director of Projects Bruce Betts. “Time zone maps can’t convey Earth time the way EarthDials can.”

The gently curved path that the Sun traces is quite different from England to Malaysia, and so, the angles and lines on every EarthDial are different. But on the Spring Equinox, the shadows of objects all over the Earth, including those cast on the EarthDials, will trace a straight line as the Sun rises exactly in the east and sets exactly in the west. Equinoxes occur twice a year in Spring and Autumn.

In principle, we might expect exactly 12 hours of daylight and 12 hours of darkness on the Equinox. However, the Earth’s atmosphere scatters and bends sunlight, so that we get a bit more than 12 hours of daylight. For North America, the moment of the Spring Equinox occurs at 1:49 AM (March 20) on the east coast and 10:49 PM (March 19) on the west coast.

Bill Nye said, "Most days, the shadows of flagpoles, trees, and posts track graceful hyperbolic curves, here on Earth and on Mars. But on the day of an Equinox, they become straight lines. On the 19th and 20th, we'll all be able to watch those straight shadow paths on EarthDials around the world."

Updated images of each EarthDial are posted on The Planetary Society’s website every 5-10 minutes. One gains a tangible sense of Earth’s rotation by watching shadows travel across the face of each EarthDial and dissolve into darkness as that corner of the world rotates away from the Sun. At any given moment, EarthDials on half of the planet are in darkness.

EarthDials are located in the United States (Virginia, Maryland, New Jersey, Ohio, Wisconsin, California, and Washington state), Honduras, Malaysia, England, Spain and Antarctica, but this last one is now entering six months of darkness so is only barely visible. However, you can still see time-lapse movies of this and other stations on the website.

While the EarthDials are all made to a common pattern, each is decorated in the language and cultural motifs of its host. They are visually reminiscent of the MarsDials aboard NASA's Spirit and Opportunity rovers, which arrived on Mars in January 2004. Both the EarthDials and the MarsDials bear the common motto "Two Worlds One Sun," because the shadows on both worlds are cast by light from the same star, our Sun.

The EarthDial Project will run at least for the duration of the rovers' missions on the surface of Mars, which is expected to be 3-6 months. More EarthDials will be added in the coming months.

The team that developed the MarsDials for rovers Spirit and Opportunity included Nye and Sullivan as well as Steve Squyres, the principal investigator for the rover missions; Louis Friedman, Executive Director of The Planetary Society; James Bell, lead researcher for the high-resolution stereo panoramic cameras carried by both rovers; Tyler Nordgren, University of Redlands in California; and Jon Lomberg, artist and creative consultant to the Mauna Kea Center for Astronomy Education, University of Hawaii at Hilo.

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