By Alyson Foster
Content & Collections Specialist, National Geographic Library

Welcome to #tbt—ThrowBack Thursday! Today we’re getting geographic (and National Geographic) with a blast from the polar past.

Say you’re a sailor or a pilot and you want to get to the North or South Pole. How do you find your way there without getting lost?

For those of us living in an age of GPS, the question sounds obvious, but a hundred years ago this was a serious logistical issue for explorers traveling in high latitudes. Take a look at how the U.S. Coast Guard deals with the issue today—and why they’re happy to have gyroscopes and GPS!

Magnetic compasses (one of the key pieces of equipment used by navigators) rely on horizontal lines of force in order to work. At the Poles, the magnetic lines of force enter the Earth at steep angles. This means that people near the North or South Poles can find the needles of their compasses acting strangely—moving sluggishly, for example, or pointing toward the ground.

In 1925, the American aviator Richard Byrd was preparing to fly a plane to the far northern part of the globe. He wanted to survey one of the few unexplored areas left in the world—a region of more than a million square kilometers lying between Alaska and the North Pole.

Byrd was considering this compass dilemma. He knew the trip was going to be a challenging and dangerous one. He would be soaring above a region with no landmarks to guide him. Below his plane would be hundreds of kilometers of icy, inhospitable terrain. He couldn’t afford to miss his mark or get lost.

Fortunately for Byrd, he was about to receive some help from Albert Bumstead, the chief cartographer at the National Geographic Society. Bumstead attended a lecture where he heard Byrd describing the challenges of polar navigation, and he offered to build Byrd something he thought might help.

That something was called a sun compass and it was based on the idea that the sun’s shadow can be used to determine geographic directions—a concept that sailors had been leveraging for centuries to help them navigate in high latitudes.

To make the compass, Bumstead mounted a watch above the face of a standard compass. Not the kind of standard two-handed watch that can be found on a wristband, but one with a single hand that completed a rotation every twenty-four hours. The hand of the watch was equipped with a pin that stuck straight up and cast a shadow.

Once the watch was set to local time and inclined at an angle appropriate for the latitude, its hand would follow the motion of the sun, and Byrd could use the shadow to determine which direction was north. In essence, it worked like the opposite of an old-fashioned sundial—instead of using the shadow’s direction to determine the time, Byrd used his knowledge of the time to figure out which way was north!

Of course, as with a sundial, this assumed that the sky would be clear enough for the sun to cast a shadow. In spite of poor weather conditions that plagued Byrd’s 1925 expedition, the compass worked well enough that he continued to use it on his later expeditions to the polar regions—including the first flight over the South Pole in 1929, perhaps the accomplishment for which he is best remembered.

To show his appreciation, Byrd named a mountain in Antarctica Mount Bumstead, after the cartographer who had helped him find his way. And he returned a sun compass to Bumstead as a thank-you gift. When Bumstead received the compass it had a new inscription on it:

To Albert H. Bumstead

For Getting Us There

From Richard E. Byrd, Jr.

Want to learn more about compasses? Who doesn’t?! Take a look at our encyclopedic entry on compasses for more information on how to build your own solar compass.