February 13: Guido Horn D'Arturo

Guido Horn D'Arturo

February 13, 1879 - April 1, 1967

Guido Horn D'Arturo was an Austrian astronomer who spent much of his career working in Italy. In 1920 he became director of the Bologna University Observatory. His research included positional astronomy, moving clusters, comets, solar eclipses, and history of astronomy; optics including geometric, instrumental, physiologic, & unusual optical instruments. 

Guido Horn d’Arturo designed and built the first “multi mirror” telescope, and the resulting 1.2-m transit instrument was operational at the University of Bologna for almost a decade.

Horn maintained a large number of connections with astronomers throughout the world and intense activity in order to enhance the international role of Bologna Observatory . Between 1912 and 1939 Horn studied the "Black drop" phenomenon and the flying shadows. 

The black drop effect is an optical phenomenon visible during a transit of Venus and, to a lesser extent, a transit of Mercury. Just after second contact, and again just before third contact during the transit, a small black "teardrop" appears to connect Venus' disk to the limb of the Sun, making it impossible to accurately time the exact moment of second or third contact. This led to the failure of the attempts during the 18th century transits of Venus to establish a truly precise value for the astronomical unit.

The black drop effect was long thought to be due to Venus' thick atmosphere, and indeed it was held to be the first real evidence that Venus had an atmosphere. However, it is now thought by many to be an optical effect.

In 1926 D'Arturo made an expedition to Somalia, to observe a solar eclipse. In 1931 he founded the magazine "Coelum" and started the project for setting up a telescope in Loiano. He also devised a method of measuring the density of stellar tracks on photographic plates using diffraction.