U.S. Exploring Expedition

Ship Model of USS Porpoise from the National Museum of American History, Smithsonian Institution

Let's start our journey with early navigation in a fleet of ships

In 1838, six ships set sail on the U.S. Exploring Expedition under the command of Lt. Charles Willkes. One of those ships was the USS Porpoise, which a model of is shown here. Want to learn more about the other ships? Check this digital collection from the Smithsonian out!

Charles Wilkes

Charles Wilkes was passionate about navigation technology, heading the Navy’s Department of Charts and Instruments efforts to synchronize marine chronometers. Much of the mission of the U.S. Exploring Expedition was uncharted. The primary instruments used for the expedition were 28 chronometers, 12 sextants, and a library. Other astronomical and meteorological instruments were also used.

Marine Chronometer by Molyneaux & Son No. 1436 from the National Museum of American History, Smithsonian Institution

Chronometers

Do you know what a chronometer is? It's simply an instrument to measure time. The chronometers used for the U.S. Exploring Expedition were from the finest English Makers and were similar to the the one shown here.

Why is it important to have chronometers when navigating? What do you think people did before having chronometers? Hint: it may have something to do with longitude. Explore how the longitude problem was solved by virtually visiting the Time and Navigation virtual gallery.

Sextants

Sextants, similar to this one made by Jesse Ramsden, were used to help with navigation at sea.

What components do you notice on this sextant? How do you think this instrument works?

Ramsden Sextant from the National Museum of American History, Smithsonian Institution

Image showing the method for using a sextant to locate position in the open ocean

How does a sextant work?

The Ramsden Sextant is from the last quarter of 18th century. The sextant became the symbol of navigation. The instrument is named for its scale—60 degrees or 1/6th of a circle—and can measure even greater angles than the octant.

Look at this graphic showing celestial navigation at sea. How might a sextant be helpful to navigators at sea? Do you think it may lead to accurate conclusions? Why or why not?

Within the sextant, there are two mirrors, which are used to locate the horizon as well as the object that you are trying to measure the angle of with relation to the horizon. By using these measurements along with clocks and accurate charts, location can be determined.

Who was Jesse Ramsden?

Jesse Ramsden was a celebrated instrument maker whose honors included fellowship in the Royal Society of London, membership in the Imperial Academy of St. Petersburg, and appointment as optician to King George III.

He built a circular dividing engine in the early 1770s in order to produce the graduated arcs for sextants and octants. This machine permitted the automatic and highly accurate division of a circle into degrees and fractions of degrees of arc. This ultimately led to mass production of precision octants and sextants and gave British manufacturers dominance in the field of marine instruments for decades.

Ramsden’s invention was so valuable to the nation’s maritime interests that he received a share of the Longitude Prize.

Dividing Engine from the National Museum of American History, Smithsonian Institution

While in most travels you attempt to follow a map, the US Exploring Expedition aimed to accurately remap their destinations. They circumnavigated the globe in 4 years, covering nearly 87,000 miles (140,000 km)! How far is that exactly? Well, the circumference of earth is only 24,901 miles (40,075 km). That’s only ⅓ of the distance they traveled! Take a look at their route using both the official map and a hand-drawn sketch since the official map is hard to see.

What locations do you recognize on these maps? Why do you think they traveled so far? Why may they have chosen this route? Notice how some of the continents (especially Antarctica!) may look skewed. Why is this?