Ernst Karl Abbe Charles Greeley Abbot Niels Henrik Abel Antonio Abetti Giorgio Abetti Abu Abdullah al-Bakri Abū al-Wafā' al-Būzjānī Charles Hitchcock Adams John Couch Adams Walter Sydney Adams Agatharchides Agrippa Pierre d'Ailly George Biddell Airy Robert Grant Aitken Ajima Naonobu Harold Alden Kurt Alder Buzz Aldrin Nikolai Alekhin Alexander the Great Alfonso X of Castile Alfraganus Dinsmore Alter Florentino Ameghino Giovanni Battista Amici Ammonius Saccas Guillaume Amontons Roald Amundsen Anaxagoras Anaximander Anaximenes of Miletus Karel Anděl William Anders John August Anderson Leif Erland Andersson Aleksandr Andronov Anders Jonas Ångström Ansgar Eugène Michel Antoniadi Dmitry Nikolayevich Anuchin Jean Baptiste Bourguignon d'Anville Petrus Apianus Apollonius of Perga Edward Victor Appleton François Arago Aratus Archimedes Archytas Friedrich Wilhelm Argelander Aristarchus of Samos Aristillus Aristotle Franciszek Armiński Neil Armstrong Christoph Arnold Svante Arrhenius Lev Artsimovich Aryabhata Arzachel Goryu Asada Giuseppe Asclepi Joseph Ashbrook Francis William Aston George Atwood Autolycus of Pitane Arthur Auwers Adrien Auzout Oswald Avery Avicenna