Stick-in-the-mud science: you'll need your brain and plenty of patience—but not much more—to take the measure of the Earth and its motions

Natural History, March, 2003 by Neil deGrasse Tyson

For a century or so, various blends of high technology and clever thinking have driven cosmic discovery. But suppose you have no technology. Suppose all you have in your backyard laboratory is a stick. What can you learn? Plenty.

With patience and careful measurement, you and your stick can glean an outrageous amount of information about our place in the cosmos. It doesn't matter what the stick is made of. And it doesn't matter what color it is. The stick just has to be straight.

Hammer the stick firmly into the ground where you have a clear view of the horizon. Since you're going low-tech, you might as well use a rock for a hammer. Make sure the stick isn't floppy and that it stands up straight. Your caveman laboratory is now ready.

On a clear morning, track the length of the stick's shadow as the Sun rises, crosses the sky, and finally sets. The shadow will start long, get shorter and shorter until the Sun reaches its highest point in the sky, and finally lengthen again until sunset. Collecting data for this experiment is about as exciting as watching the hour hand move on a clock. But since you have no technology, not much else is competing for your attention. Notice that when the shadow is shortest, half the day has passed. At that moment--called local noon--the shadow points due north or due south, depending which side of the equator you're on.

You've just made a rudimentary sundial. And if you want to sound erudite, you can now call the stick a gnomon (I still prefer "stick"). Note that in the Northern Hemisphere, where civilization began, the stick's shadow will revolve clockwise around the base of the stick as the Sun moves across the sky. Indeed, that's why the hands of a clock turn "clockwise" in the first place.

If you have enough patience and cloudless skies to repeat the exercise 365 times in a row, you will notice that the Sun doesn't rise from day to day at the same spot on the horizon. And on two days a year the shadow of the stick at sunrise points exactly opposite the shadow of the stick at sunset. When that happens, the Sun is rising due east, setting due west, and daylight lasts as long as night. Those two days are the spring and fall equinoxes (from the Latin for "equal night"). On all other days of the year the Sun rises and sets elsewhere on the horizon. So the adage that the Sun always rises in the east and sets in the west was invented by somebody who never paid attention to the sky.

If you're in the Northern Hemisphere while you're tracking the points on the horizon where the Sun rises and sets, you'll see that those spots inch north of the east-west line after the spring equinox, eventually stop, and then inch south for a while. After they cross the east-west line again, the southward inching eventually slows down, stops, and gives way to the northward inching once again. The entire cycle repeats annually.

All the while, the Sun's trajectory is changing. On the summer solstice (Latin for "stationary Sun"), the Sun rises and sets at its northernmost point along the horizon, tracing its highest path across the sky. That makes the solstice the year's longest day, and the stick's noontime shadow on that day the shortest. When the Sun rises and sets at its southernmost point along the horizon, its trajectory across the sky is the lowest, creating the year's longest noontime shadow. What else to call that day but the winter solstice?

For 60 percent of the Earth's surface and about 75 percent of its human inhabitants, the Sun is never, ever directly overhead. For the rest of our planet, a 3,200-mile-wide belt around the equator, the Sun climbs to the zenith only two days a year (OK, just one day a year if you're smack on the tropic of Cancer or the tropic of Capricorn). I'd bet the same person who professed to know where the Sun rises and sets on the horizon also started the adage about the Sun always being directly overhead at high noon.

So far, with a single stick and herculean patience, you have identified the cardinal points on the compass and the four days of the year that mark the change of seasons. Now you need to invent some way to time the interval between one day's local noon and the next. An expensive chronometer would help here, but one or more well-made hourglasses will also do just fine. Either timer will enable you to determine, with great accuracy, how long it takes for the Sun to revolve around the Earth: the solar day. Averaged over the entire year, that time interval is equal to twenty-four hours--exactly--though this doesn't include the leap second added now and then to account for the slowing of the Earth's rotation by the Moon's gravitational tug on Earth's oceans.

Back to you and your stick. We're not done yet. Establish a line of sight from its tip to a spot on the sky, and use your trusty timer to mark the moment a familiar star from a familiar constellation passes by. Then, still using your timer, record how long it takes for the star to realign with the stick from one night to the next. That interval, the sidereal day, lasts twenty-three hours, fifty-six minutes, and four seconds. The almost-four-minute mismatch between the sidereal and solar days forces the Sun to migrate across the patterns of background stars, creating the impression that the Sun visits the stars in one constellation after another throughout the year.