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.
Al-Battani, Abu Allah Mohammed ibn Jabir (850-928) - also known as Albategniuis, showed that the distance from the Earth to the Sun changes during the year. Using methods of Ptolemy, he created astronomical tables of the positions of the stars, adjusted for precession and calculated a length of the solar year.
Al-Biruni, Abu ar-Rayhan Mohammed ibn Ahmad (973-1048) - completed extensive studies of equinoxes and latitudes.
Al-Farghani, Muhammed ibn Ketir (d. 840) - also known as Alfraganus, is best known for Elements of Astronomy, a book which examines the work of Ptolemy and influenced European astronomers for centuries.
Al-Khayyami, Ghiyath al-Din Abu'l-Fath Umar ibn Ibrahim al-Nisburi (1048-1131) - also known as Omar Khayyam, he was a legendary writer along with being an astronomer. He compiled astronomical tables and helped complete calendar reform. He also calculated one of the most accurate measurements of the length of the year.
Al-Khujandi, Abu Mahmud Hamid ibn al-Khidr (940-1000) - using one of the largest mural sextants of his time, he attempted to calculate the obliquity of the ecliptic from observations taken of a series of meridian transits of the Sun during the solstices.
Al-Khwarizmi, Abu Jafar Mohammed ibn Musa (780-850) - creator of algebra and the oldest surviving Arabic astronomical tables. He also introduced the Indian style of numbers into Arabic culture replacing the Bedouin and Greek numbers in use.
Al-Sufi - created a book on astronomy where he adjusts their positions relative to those stated by Ptolemy by 12 degrees. Also included in the book were magnitudes of the stars.
Al-Zaraqala, Abu Ishaq Ibrahim ibn Yahya (1029-1087) - also known as Alzachel, he published the Toledo Tables of astronomical data taken in Spain, within the tables are descriptions of the instruments used including the astrolabe.
Anaxagoras (c. 500-428 B.C.) - Greek philosopher/scientist. He was the first to clearly state that the moon shines by reflected light and lunar eclipses occur when the Earth blocks the sunlight.
Anaximander (611-546 BC) - Greek philosopher/scientist believed that a cylindrical Earth floated free in space, and the Sun, Moon, and stars are hollow, fiery wheels.
Anaximenes (570-500 BC) - the first Greek philosopher to clearly distinguish the differences between stars and planets.
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.
Archimedes (287-212 BC) - Greek scientist whose astronomical influence includes an attempt to calculate the size of the universe by using the studies of Aristarchus. In his work The Sand-Reckoner he estimates the size of the universe to be 100 million Earth-diameters.
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.
Aristarchus (c. 310-230 BC) - Greek astronomer who was the first to propose the heliocentric theory, that the earth and other planets orbited the sun.
Aristotle (384-322 BC) - Greek philosopher who wrote about many subjects. In astronomy he agreed with the views of Plato and Eudoxas, but increased the number of spheres surrounding the earth to 54. He also believed that the world was round. His views on science and logic became preeminent in Europe during the twelfth and thirteenth centuries.