A simulated image of Sirius A and B using Celestia
In 1844 the German astronomer Friedrich Bessel deduced from changes in the proper motion of Sirius that it had an unseen companion.[56] Nearly two decades later, on January 31, 1862, American telescope-maker and astronomer Alvan Graham Clark first observed the faint companion, which is now called Sirius B, or affectionately "the Pup".[57] This happened during testing of an 18.5-inch (470 mm) aperture great refractor telescope for Dearborn Observatory, which was the largest refracting telescope lens in existence at the time, and the largest telescope in the United States.[58] Sirius B sighting was confirmed on March 8 with smaller telescopes as well.[59]