Thomas Jefferson and the 1811 Constitution Day Eclipse

This post is by Lee Ann Potter, the Director of Educational Outreach at the Library of Congress.

As Americans anxiously await next week’s total eclipse of the sun, many are making plans not only to observe it, but also to record their observations in order to calculate their longitude.

Or maybe not…But in 1811, when the solar eclipse that occurred on Constitution Day was visible in central Virginia, that is exactly what Thomas Jefferson did.

Thomas Jefferson, President of the United States, Rembrandt Peale, 1801

On September 17, he diligently recorded his observations in his weather journal. He noted the times when the moon first “contacted” the sun, when the annulus (ring shape) formed, when the annulus broke, and when the contact ended.  He also indicated the central time of the contact and the central time of the annulus. According to Jefferson’s observations, the entire event lasted 3 hours, 15 minutes, and 34 seconds.

In addition to his journal entry – and also among his papers in the Manuscripts Division at the Library of Congress – are many letters about the astronomical event. Over the span of nearly a year, from March 1811 to early January 1812, Jefferson exchanged letters about the eclipse with:

  • Robert Patterson (the vice president of the American Philosophical Society),
  • John Payne Todd (President James Madison’s stepson),
  • Henry A.S. Dearborn (son of Henry Dearborn, who served as the secretary of war during Jefferson’s presidency),
  • Bishop James Madison (President of the College of William and Mary, and cousin of President James Madison), and
  • William Lambert, (mathematician and astronomer from Virginia, whom Jefferson first met in the early 1790s when Lambert served as a State Department clerk).

Before the eclipse, the letters were about Jefferson’s plans and his wish for a better timepiece; after the eclipse, the friends shared their observations and calculations.  By mid-November, Lambert had compared Jefferson’s observations to observations he had made in Washington, DC, and calculated Monticello’s longitude to be 78° 35′ 10.950″ W (it is actually 78° 27′ 11.5″ W).

Jefferson was clearly impressed by Lambert’s mathematical skills.  In a humble letter of appreciation to him on December 29th  Jefferson wrote,

“I am very thankful for your calculations on my observations of the late Solar eclipse. I have for some time past been rubbing off the rust of my mathematics contracted during 50. years engrossed by other pursuits, and have found in it a delight and a success beyond my expectations. I observed the eclipse of the sun with a view to calculate my longitude from it, but other occupations had prevented my undertaking it before my journey; and the calculations you have furnished me with shew it would have been more elaborate than I had expected, and that most probably I should have foundered by the way.”

Invite your students to explore the Thomas Jefferson Papers, read his journal entry, and read some of the letters related to the eclipse.  Ask them what this episode in Jefferson’s life reveals about him that they might not have known.

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