[Friends_of_SSASTROS] Alvan Clark

[LOUIS GENTILE]

Today is the anniversary of the birth, at Fall River, Massachusetts on 10 Jul 1832, of the American astronomer and telescope maker Alvan Graham Clark, pictured here with his father Alvan Clark (centre) and brother George Bassett Clark (right). Founded in 1846, Alvan Clark & Sons manufactured lenses for some of the largest refracting telescopes of the nineteenth and twentieth centuries, including the 36-inch (91-centimetre) refractor at Lick Observatory in 1887 and the 40-inch (1-metre) Yerkes Observatory refractor (pictured) in 1897. In January 1862, while testing a new 18.5-inch (470 mm) aperture great refractor telescope in Cambridgeport, Massachusetts, Alvan Graham Clark discovered the companion star to Sirius, the first ever observation of a white dwarf star, which had previously been predicted but until then never sighted. This discovery of Sirius B, or affectionately ‘the Pup’, proved an earlier hypothesis (Friedrich Bessel in 1844) that Sirius, the brightest star in the night sky with an apparent magnitude of −1.46, had an unseen companion disturbing its motion. The telescope that Clark was using was the largest refracting telescope lens in existence at the time, and the largest telescope in the United States. It was subsequently delivered to the Dearborn Observatory of Northwestern University in Evanston, near Chicago, Illinois, where it is still being used today.
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