Earth and Mars Share a Clock in the Sky ; on Sunday, a Sundial Will Land on Mars for the First Time. You, Too, Can Play a Role

Article excerpt

What time is it on Mars? Just as on Earth, the time of day on Mars depends on where you are on the planet. When NASA's two Mars Exploration Rovers, Spirit and Opportunity, land on the red planet a few days from now, they'll be able to tell us the time there using a system that is thousands of years old here on Earth: the sundial.

You'll be able to watch these first-ever "MarsDials," and many more sundials all across our own planet, thanks to a project called EarthDial. If you like, you can even join in by making your own sundial for everyone on the Web to see.

Project EarthDial is the work of astronomy professor Woodruff Sullivan, TV personality Bill Nye ("The Science Guy"), and The Planetary Society.

EarthDial's goal is to have schools, community groups, and individuals around the world build sundials and display them on the Internet. That way, you'll be able to compare the shadows cast by sundials throughout the northern and southern hemispheres, the equator - and Mars - at the same time.

All the elaborate sundials in the project, including the two MarsDials, display the motto "Two Worlds One Sun." It's the same sun creating light and shadows on Mars as it is on Earth.

All you need for a basic sundial is a surface on which to track the shadows cast by the sun and a gnomon (pronounced NO-mun). The gnomon is the part that casts the shadow.

The first gnomon was probably a person, who looked at his or her shadow on the ground. Your shadow can tell you whether it's morning, noon, or afternoon.

In the morning, as the sun rises in the east, your shadow will stretch to the west. As the sun climbs higher, your sha-dow shortens and - if you live in the Northern Hemisphere - swings toward the north. That's because the sun is now south. (In the Southern Hemisphere, your shadow would swing toward the south because the sun is north.)

Then in the afternoon, your shadow moves toward the east as the sun moves westward.

Thus, the shadows on sundials in the Northern Hemisphere move clockwise around the gnomon. (The shadows move counterclockwise in the Southern Hemisphere.)

Clocks go clockwise, too. Sundials were used most in the Northern Hemisphere, and so "clockwise" became the standard for clocks around the world.

At some point in history, before written records, people began to use sundials to tell time. Ancient Egyptians and Greeks used flat surfaces with upright gnomons, but developed other instruments as well. Some were curved, some hung on walls. As long as they could accurately track the movement of the sun by its shadow, they could tell time.

To make a good sundial, you also need to know your latitude, or distance from the equator. As you move north or south from the equator, the shadows cast by the sun change, and the angles on the sundial must change as well. You also have to make sure your sundial faces the right way. In the Northern Hemisphere, the shadow must point directly at celestial north at "noon." (Celestial north is a little different from geographic north, so a compass will be a little off. And "noon" here means the moment at which the sun is at its highest point in the sky.)

The length of the shadows on your sundial will change with each season. During the summer, as the sun moves higher in the sky, the shadows will grow shorter. Then, as the sun goes lower in the sky in winter, the shadows will grow longer.

Some sundials have an equinox line, which tracks the length of the shadow on the two days each year (spring and fall equinox) when the sun is directly over the equator.

If you stood on the equator at "noon" on one of these days, you'd have no shadow at all! But if you stood farther north or south, you'd begin to see a shadow, and it would grow longer the farther you went from the equator.

Greek mathematician Eratosthenes used this principle around 200 BC to figure out the size of Earth. …