Tom Herring teaches the Modern Navigation course at MIT, showing students how to use modern GPS systems not just for finding direction but also for conducting scientific research. To make accurate maps of areas with lots of seismic activity, such as California's San Andreas fault, Herring uses laser altimetry: Satellites can fire lasers to specific points on Earth and measure how long it takes for the light to bounce back. A satellite fires thousands of beams per second in order to map terrain. But before Herring's students get to play with fancy technology, Herring makes them learn how to navigate the old-fashioned way--with the same techniques that Christopher Columbus used 516 years ago on his first voyage across the ocean blue.
Five centuries ago, sailors depended mostly on taking latitude readings to get where they were going. So Herring starts his students out with a sextant, an instrument used to divine one's latitude by using the position of the sun at high noon. To determine their distance from the equator, sailors used the ship's celestial charts to check the sun's height in the sky against the date. But compared to Columbus's crew, Herring's students have plenty of advantages. Fifteenth-century sailors couldn't double-check their results against the GPS receiver in the corner; they also would have had a more primitive version of the sextant called the quadrant, Herring says. To use the quadrant, one person pointed its sight at the sun while another took a reading of a plumb bob that showed the angle of elevation. Holding still on a bobbing ship was no easy task, and the person aiming the sight at the sun had to do so accurately but without looking for so long that he burned his eyes. This is partly why, Herring says, history is filled with tales of ships that traveled way off course.
Since the ancient Greeks first calculated the circumference of Earth, humans have known how to find latitude in this way. But longitude is another story. While zero latitude, the equator, has a natural basis to which the sun stays consistent, longitude's origin is totally arbitrary--prime meridian runs through Greenwich, England.
Before the 18th century sailors often couldn't get reliable longitude readings. For one thing, Herring says, "You just can't get the precision on a bucking ship." But the most important part of the problem was time. You can determine longitude by comparing your time to the time of day at a known location, say Greenwich, England. If you're 6 hours behind Greenwich time, you know that you're a quarter of the way around the world, at 90 degrees west longitude. But Columbus would've had no way to know what the time was back in Europe. The longitude problem meant that ships had to cross the sea to reach the appropriate latitude and then sail west or east until they hit land, Herring says. "It's not the most efficient way to sail."
Englishman John Harrison revolutionized navigation in the 1700s by inventing the marine chronometer, the first reliable timepiece that could withstand the grinding conditions of an ocean voyage and keep on ticking. Durable and accurate clocks allowed sailors to establish both their latitude and longitude at sea and to record the coordinates of places they visited so that they could find them again in the future. Reliable timekeeping may have been the most important invention in navigation history. It made modern navigation, along with its Earth-orbiting satellites and electronic measurement of distance, possible. It led to GPS, which can show a modern seafarer his position on Earth down to a few centimeters.
Still, Herring can't help but romanticize the bygone era of navigation, when instrument designers had to adjust to the imperfections in their machines, like compensating for the bias in a telescope rotation caused by the space in the gears that allowed for movement. "The way they designed in those days--software designers could take a lesson," he says with a laugh. "They just don't have the finesse today." Finesse aside, could Herring sail successfully with a sextant and some charts? "I think I could," he says, "But I'd want to take my GPS receiver."