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
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
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
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
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. …