Earth and Mars share a clock in the sky

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. He compared the length of the shadows at noon in Alexandria, Egypt, and in Syene, which was directly to the south, on the same day. From the length of the shadows he also determined their angle to the sun. Since he knew exactly how far it was to Syene from Alexandria, he used simple geometry to determine the diameter of Earth. His measurement of 7,850 miles is very close to the current measurement of about 7,900 miles.

We have records showing that Egyptians used giant stone obelisks as gnomons around 1450 BC. The Chinese had determined the farthest northern and southern lengths of shadows by the 23rd century BC. And by 500 BC, for some reason a uniform height for all Chinese gnomons was determined by law, with severe penalties for offenders.

As civilization advanced, it became more important to track time carefully. Sundials grew in popularity. They were made of stone, wood, ceramic, or metal, and adorned with carvings or pictures. They could be small enough to be carried in a pocket, as early Greeks did, or as large as a building.

A cathedral in Florence, Italy, has a hole in the center that lets in the light at noon. A dial at Queens College in Cambridge, England, features charts that track the moon's monthly orbit, making it a moondial that can tell time at night.

In early America, towns often had a large sundial set in a public place to track the time. In fact, every town had its own time zone, based on the sun's position over that town. Once railway lines were established, this created much confusion. Suddenly, people could move from town to town much more quickly. How could a printed railway schedule account for a different time in every town in which it stopped?

In 1837 the newly invented telegraph was used to send a time signal to railway stations in London so that the stations could all keep the same time. By 1847, all of Britain had adopted a single time, based on railway time.

The United States was too wide to work on a single time. Noon in New England is just after breakfast in California. By 1800, there were more than 100 regional times in the US. In 1883, the railroads established four "time zones" across America, each spanning 15 degrees of the Earth's circumference - the length over which the sun's shadow moves in one hour. In 1908, Congress adopted this as the national standard.

The sundials and MarsDials you may see during Project EarthDial will show the different positions of the shadows for each location on the Earth and Mars. For more information on the project, go to www.planetary.org/mars/earthdial.html. The rover Spirit is scheduled to land on Mars on Jan. 4, 2004. Opportunity is due Jan. 24. To track their status, go to www.planetary.org/mars/tracking.html.

As sundials became common around the world, they also became works of art. The sundials, or 'MarsDials,' on the two soon-to-land Mars Exploration Rovers have followed this tradition. The MarsDials include designs, ideas, and illustrations submitted by students in schools across the United States. (See the photo of a MarsDial, below, along with the illustration that shows where it's located on the Mars rover).

On the MarsDial, the rings around the center post, or 'gnomon,' symbolize the orbits of Earth and Mars. The word Mars is displayed in 17 different languages. Small inscriptions tell why humans made the journey to Mars. (A main reason is to research the history of water on the planet.)

The MarsDials do more than just tell Martian time, though. The Mars rovers use panoramic cameras, or 'Pancams,' to send images of Mars back to Earth. If the camera is not adjusted correctly, the images received won't show us the true colors. Scientists will use the colored blocks, located on the corners of the MarsDials, to calibrate the color of the images being sent by Pancams. Since the scientists know what the exact colors of the blocks are, they can adjust the camera until it's sending images in which the colors of the blocks are exactly right. That tells them that they are getting images of the true colors of the Martian landscape. And when the gnomons cast shadows on the rings, scientists will be able to properly adjust the brightness of the Pancam images.

The Pancams will show images of the MarsDials many times during their exploration of Mars, which is expected to last at least until April 2004. Some of these pictures will be put together into a movie to show time passing on the red planet.

Question: Since a Martian day is 24 hours, 37 minutes long, will shadows on MarsDials move more quickly or more slowly than those on EarthDials?

Answer: Mars rotates more slowly than Earth. The shadows will move more slowly as well.

You can make your own simple sun clock! All you need is a pencil, a compass, a pair of scissors, a sunny day, and the diagrams shown here.

First, cut out the sundial and grid as indicated (diagram No. 1). Then determine which letter in the grid represents the city you live in (or the city you live closest to). Find your letter again on the sundial, and notice which way its arrow points.

Next, put a compass on the ground and find north. Now put the sundial on the ground and rotate it until your city's arrow aligns with north (see No. 2). Stand a pencil in the center slot next to today's date (No. 3). Look at the shadow to find out the time.

SOURCE: The Exploratorium museum in San Francisco. This project, and an explanation of the science behind it, are at: www.exploratorium.com/ science_explorer/ clock_diagram.html.