This week we're running a series of posts by Matt Soniak about Abraham Lincoln's foray into forensic meteorology. If you missed the first, second, or third installments of the series, check them out.

Lincoln had secured an acquittal for Duff Armstrong largely by poking a hole in a witness’ testimony with the aid of an almanac. This tactic would come back to haunt him almost immediately during his Senate race against Stephen Douglas and again years later when he ran for President.

During the Senate campaign, Lincoln’s critics accused him of using an altered almanac in the Armstrong trial. It was a serious charge against anyone, but Lincoln especially, as “Honest Abe” had built a reputation around his integrity.

The claim is a little absurd, and Lincoln would have had to put a lot of effort into such a ruse, since he had three copies of the almanac in the courthouse. After cross-examining Allen, he gave one to the judge, one to the prosecutor, and one to the jury to inspect. The prosecutor even sent his assistant to the store to purchase more almanacs. The man returned with several copies, some published by different companies than the one Lincoln had used. They agreed with Lincoln's almanac on the position of the moon and the time of moonset down to a minute or two. If Lincoln was attempting to fool the court, he was involved in a massive conspiracy that went to the highest offices of several almanac publishers.

In the years following the trial, and even into the next century, various astronomers have calculated the lunar phase and time of moonset the night of the murder – often at the request of Lincoln historians seeking to squelch continuing criticisms of Lincoln’s ethics. Calculations were made in 1871 by Elias Loomis; 1909 and 1925 by Joel Stebbins; 1925 by the Yerkes Observatory; 1928 by the Harvard Observatory; and 1905, 1925, and 1976 by the U.S. Naval Observatory. Every time, the time of moonset was found to be at 12:03 or 12:04 a.m. In 1990, two astronomers from Texas State University calculated moonset time as 12:04 a.m. and the moon’s position at 11:00 p.m., the time of the assault, as only 8 degrees above the horizon.


Hannah Armstrong married Samuel Wilcox, and moved to Iowa, where she died August 15, 1890, at the age of 79.

Abe Lincoln lost the Senate seat to Stephen Douglas, but later beat Douglas in 1860 to become the sixteenth president of the United States. He was assassinated five years later.

Duff Armstrong and three of his brothers enlisted in the Union Army during the Civil War. In 1862, Duff became ill and was laid up in an army hospital. His mother wrote to President Lincoln, explaining her son’s condition and asking for his discharge so he could recover at home. Lincoln granted the discharge immediately and Duff returned to his mother, who nursed him back to health. He died in Illinois in May, 1899, at the age of 66.