200 years later, geologist re-orients Lewis and Clark’s compass readings

Lewis and Clark and Criss, oh my!

Lewis and Clark, meet Robert Criss.

Virtual explorer Robert Criss, Ph.D., professor of earth and planetary sciences in Arts & Sciences at Washington University in St. Louis, has teamed up with Lewis and Clark to provide the oldest determinations of the magnetic declination of America’s interior.

Robert Criss
Robert Criss

Declination is the horizontal angle between true north and magnetic north. This difference arises because a compass needle aligns with local magnetic north instead of with Earth’s spin axis. Criss determined the deviations of Lewis and Clark’s compass needle from true north, which is possible because the famous explorers made positional measurements with precise nautical instruments. There are plenty of data of this type from ships at sea, but before 1850 practically no data were available for the North American interior.

When Meriwether Lewis and William Clark explored the Louisiana Territory in 1804-6, they frequently determined their position by using a sextant to read the altitude of the Sun and the North Star. When these data are combined with their compass measurements, the difference between true north and magnetic north can be calculated.

Maps need to be referred to true north because the direction of magnetic north not only varies from place to place, but it also fluctuates with the movement of molten iron in the Earth’s core. In contrast, the geographic north pole is constant. Criss gave the exploration journals a close geographer’s reading, gleaning data on the altitude and compass direction of the Sun and North Star. He used tables to determine the true location of the Sun and star on various dates in the early 1800s. He then compared the two data from each location to determine how much the compass needle deviated from true north.

Meriwether Lewis and William Clark
Meriwether Lewis and William Clark

He found that the magnetic declination near St. Louis has changed from an azimuth of 7.7 degrees east of north to 0 degrees today. The azimuth is a horizontal angle measured from a vertical plane. He found that the magnetic pole was close to 19.4 degrees east at Cape Disappointment, Wash., where the expedition turned back, and it is still around 18.2 degrees east today.

Criss also used the declination data to correct some of the explorers’ maps and found many to be remarkably accurate, once they are rotated.

“Lewis and Clark were topnotch scientists who were versed in surveying and celestial navigation” Criss said. Criss, a geochemist and hydrologist, often makes Lewis and Clark references in his classes. He had long been a fan of the journals. The closer he read, the more obvious the question became: Did Lewis and Clark correct their compass readings for the declination?

“I think it became too difficult for them to do, what with gathering other data, coping with unruly and uncharted waters, and just plain surviving,” he said. “It’s important to have the data corrected because they can help geologists improve models of the variation of Earth’s magnetic field over time.”

Criss’s paper, “Mid-Continental magnetic declination: A 200-year record starting with Lewis and Clark,” is the cover article of the October 2003 GSA Today, a publication of the Geological Society of America.