Abell, George (1927-1983) - astronomer who created a category system for galaxy clusters, based on his studying the information obtained from the Palomar Sky Survey. He divided galactic clusters into two types: regular and irregular. Adams, John Couch (1819-1892) - American astronomer who independently from LeVerrier mathematically predicted the planet Neptune based upon the perturbations of the orbit of Uranus. Adams, Walter Sydney (1876-1956) - American astronomer who developed a technique to calculate the distances to stars using spectroscopic studies. Also, he identified Sirius B as a white dwarf star, the first known. Airy, Sir George Biddell (1801-1892) - seventh Astronomer Royal of England. He is known for updating and modernizing the Greenwich Observatory's equipment. He organized expeditions to study the transits of Venus in 1874 and 1882 in order to measure the size of the solar system. They were only partially successful. Angstrom, Anders Jonas (1814-1874) - Swedish scientist who in 1862 announced the discovery of hydrogen in the Sun. He took careful measurements of the wavelengths revealed in spectra, and the unit of measurement he used (ten billionth of a meter) is named in his honor. Apian, Peter (1495-1552) - also known as Petrus Apianus and Peter Bienewitz, this German astronomer was the first to describe the fact that a comets tail always pointed away from the Sun. Argelander, Friedrich Wilhelm August (1799-1875) - director of the Bonn Observatory in Germany, during his tenure the staff completed the Bonner Durchmusterung, a catalog of 324,000 stars listing their magnitudes and positions.