Capella is significant as its heliacal rising took place within a day of the Sun passing directly overhead over Monte Albán. Multiple status Professor William Wallace Campbell of the Lick Observatory announced that Capella was binary in 1899, based on spectroscopic observations—he noted on photographic plates taken from August 1896 to February 1897 that a second spectrum appeared superimposed over the first, and that there was a doppler shift to violet in September and October and to red in November and February—showing that the components were moving toward and away from the Earth (and hence orbiting each other). Almost simultaneously, British astronomer Hugh Newall had observed its composite spectrum with a four prism spectroscope attached to a telescope at Cambridge in July 1899, concluding that it was a binary star system. Many observers tried to discern the component stars without success. Known as "The Interferometrist's Friend", it was first resolved interferometrically in 1919 by John Anderson and Francis Pease at Mount Wilson Observatory, who published an orbit in 1920 based on their observations. This was the first interferometric measurement of any object outside the Solar System. A high-precision orbit was published in 1994 based on observations by the Mark III Stellar Interferometer, again at Mount Wilson Observatory. Capella also became the first astronomical object to be imaged by a separate element optical interferometer when it was imaged by the Cambridge Optical Aperture Synthesis Telescope in September 1995. In 1914, Finnish astronomer Ragnar Furuhjelm observed that the spectroscopic binary had a faint companion star, which, as its proper motion was similar to that of the spectroscopic binary, was probably physically bound to it. In February 1936, Carl L. Stearns observed that this companion appeared to be double itself; this was confirmed in September that year by Gerard Kuiper. This pair are designated Capella H and L. X-ray source Two Aerobee-Hi rocket flights on September 20, 1962, and March 15, 1963, detected and confirmed an X-ray source in Auriga at RA Dec , identified as Capella. Stellar X-ray astronomy started on April 5, 1974, with the detection of X-rays from Capella. A rocket flight on that date briefly calibrated its attitude control system when a star sensor pointed the payload axis at Capella. During this period, X-rays in the range 0.2–1.6 keV were detected by an X-ray reflector system co-aligned with the star sensor. The X-ray luminosity (Lx) of ~1024 W (1031