References are numbered in [brackets], which are listed here. A number after the dot gives the page in the source.

Last updated: 2008 June 28.

start-1899	1900-1949	1950-1964	1965-1969	1970-1974	1975-1979	1980-1984	1985-1989	1990-1994	1995-1999
2000-end

1900 May 28 Solar eclipse occurs. [1] June 4 M Wolf and A Schwassmann discovers asteroid #456 Abnoba. [1] December 17 First prize of 100,000 francs offered for communications with extraterrestrials. Martians excluded-considered too easy. [1] December 20 Giacobini discovers a comet (will be first comet visited by spacecraft). [1] 1901 June 7 M Wolf discovers asteroid #471 Papagena. [1] July 11 L Carnera discovers asteroid #472 Roma. [1] 1902 June 26 M Wolf and L Carnera discovers asteroid #488 Kreusa. [1] June 30 S I Bailey discovers asteroid #504 Cora. [1] July 9 L Carnera discovers asteroid #487 Venetia. [1] 1903 June 23 M Wolf discovers asteroid #512 Taurinensis. [1] 1904 July 7 A Charlois discovers asteroid #537 Pauly. [1] July 18 P Gotz discovers asteroid #538 Friederike. [1] 1905 January 2 Elara, a satellite of Jupiter, discovered by Perrine. [1] January 5 Charles Perrine announces discovery of Jupiter's 7th satellite, Elara. [1] July 26 P Gotz discovers asteroid #568 Cheruskia. [1] July 27 J Palisa discovers asteroid #569 Misa. [1] July 30 M Wolf discovers asteroid #570 Kythera. [1] 1906 June 14 J H Metcalf discovers asteroid #600 Musa. [1] 1907 June 9 K Lohnert discovers asteroid #635 Vundtia. [1] July 19 K Lohnert discovers asteroid #639 Latona. [1] 1908 January 27 Pasiphaë, a satellite of Jupiter, discovered by Melotte. [1] June 24 A Kopff discovers asteroids #663 Gerlinde and #664 Judith. [1] July 22 W Lorenz discovers asteroid #665 Sabine. [1] July 23 A Kopff discovers asteroids #666 Desdemona and #667 Denise. [1] July 27 A Kopff discovers asteroid #668 Dora. [1] 1909 June 17 A Kopff discovers asteroid #682 Hagar. [1] July 23 M Wolf discovers asteroid #683 Lanzia. [1] 1910 April 19 Halley's comet seen by naked eye first time this trip (Curacao). [1] April 20 Halley's Comet passes 29th recorded perihelion at 87.9 million km. [1] May 10 Halley's Comet closest approach to Earth in 1910 pass. [1] May 18 Passage of Earth through tail of Halley's Comet causes near-panic. [1] June 5 J Helffrich discovers asteroids #699 Hela and #700 Auravictrix. [1] July 10 Johann Galle discoverer of Neptune with telescope, dies. [1] July 12 J Helffrich discovers asteroid #701 Oriola. [1] July 16 J Helffrich discovers asteroid #702 Alauda. [1] 1911 July 30 J Palisa discovers asteroid #716 Berkeley. [1] 1912 January 4 Smallest earth-moon distance this century, 356,375 km center-to-center. [1] 1914 June 24 G Neujmin discovers asteroid #789 Lena. [1] June 29 G Neujmin discovers asteroid #791 Ani. [1] 1915 March 19 Pluto photographed for first time (although unknown at the time). [1] 1916 July 30 G Neujmin discovers asteroid #951 Gaspra. [1] 1917 July 22 M Wolf discovers asteroids #879 Ricarda, #880 Herba and #881 Athene. [1] 1918 June 4 M Wolf discovers asteroid #894 Erda. [1] June 8 Nova Aquila, brightest nova since Kepler's nova of 1604, discovered. [1] July 11 M Wolf discovers asteroid #895 Helio. [1] 1919 July 4 M Wolf discovers asteroid #914 Palisana. [1] 1922 June 29 K Reinmuth discovers asteroid #979 Ilsewa. [1] July 30 K Reinmuth discovers asteroid #983 Gunila. [1] 1923 June 8 S Belyavskij discovers asteroid #995 Sternberga. [1] July 12 K Reinmuth discovers asteroids #997 Priska and #3682. [1] 1924 June 6 S Belyavskij discovers asteroid #1031 Arctica. [1] June 23 V Albitzkij discovers asteroid #1022 Olympiada. [1] June 25 K Reinmuth discovers asteroid #1023 Thomana. [1] July 7 E Hertzsprung discovers asteroid #1702 Kalahari. [1] 1925 January 24 Moving picture of a solar eclipse taken from dirigible over Long Island. [1] June 15 B Jekhovsky discovers asteroid #1093 Freda. [1] June 22 G Shajn discovers asteroid #1058 Grubba. [1] July 19 V Albitzkij discovers asteroid #1059 Mussorgskia. [1] November 11 Robert Millikan announces discovery of cosmic rays. [1] 1928 July 10 H E Wood discovers asteroid #3300. [1] July 19 H E Wood discovers asteroid #1305 Pongola. [1] July 21 H E Wood discovers asteroid #1096 Reunerta. [1] 1929 June 11 G Neujmin discovers asteroid #1147 Stavropolis. [1] July 5 A Deutsch discovers asteroid #1148 Rarahu. [1] 1930 January 23 Clyde Tombaugh photographs planet Pluto. [1] February 18 US astronomer Clyde Tombaugh discovers Pluto. [1] March 13 Clyde Tombaugh announces discovery of Pluto at Lowell Observatory. [1] March 18 Pluto discovered by Clyde Tombaugh (US). [1] March 24 Planet Pluto named. [1] May 10 First US planetarium opens (Adler-Chicago). [1] May 31 Comet 73P/1930 (Schwassmann-Wachmann 3) approaches 0.0617 astronomical units (AUs) of Earth. [1] June 27 P Parchomenko discovers asteroid #1166 Sakuntala. [1] July 22 G Neujmin discovers asteroid #1306 Scythia. [1] H Van Gent discovers asteroids #1666 van Gent, #1752 van Herk. [1] 1931 June 6 G Neujmin discovers asteroid #1210 Morosovia. [1] June 9 C Jackson discovers asteroid #1197 Rhodesia. [1] 1932 June 11 E Delporte discovers asteroid #1222 Tina. [1] July 8 G Neujmin discovers asteroid #1255 Schilowa. [1] July 23 C Jackson discovers asteroid #1246 Chaka. [1] July 30 G Van Biesbroeck discovers asteroid #2253 Espinette. [1] 1933 April 27 Karl Jansky reports reception of cosmic radio signal in Washington DC. [1] June 15 C Jackson discovers asteroids #1278 Kenya and #1279 Uganda. [1] July 1 G Neujmin discovers asteroid #1590 Tsiolkovskaja. [1] July 24 K Reinmuth discovers asteroids #1645 Waterfield, #1668 Hanna. [1] July 27 G Van Biesbroeck discovers asteroid #1312 Vassar. [1] K Reinmuth discovers asteroid #1284 Latvia. [1] 1934 June 4 C Jackson discovers asteroid #2066 Palala. [1] June 13 C Jackson discovers asteroid #1349 Bechuana. [1] June 15 C Jackson discovers asteroid #1324 Knysna. [1] K Reinmuth discovers asteroid #1322 Coppernicus. [1] July 3 C Jackson discovers asteroid #1367 Nongoma. [1] July 14 C Jackson discovers asteroids #1325 Inanda and #1326 Losaka. [1] July 16 K Reinmuth discovers asteroid #1334 Lundmarka. [1] December 2 5.08-m (200") Mount Palomar Observatory mirror is cast. [1] 1935 May 14 Los Angeles' Griffith Planetarium opens, third in US. [1] June 30 C Jackson discovers asteroid #1784 Benguella. [1] July 2 C Jackson discovers asteroid #1357 Khama. [1] July 21 C Jackson discovers asteroid #1358 Gaika. [1] July 22 C Jackson discovers asteroids #1359 Prieska and #1360 Tarka. [1] July 25 C Jackson discovers asteroid #1641 Tana. [1] July 28 G Neujmin discovers asteroid #1386 Storeria. [1] October 2 New York Hayden Planetarium, the fourth in the US, opens. [1] November 11 Explorer 2 balloon sets altitude record of 72,000 feet over South Dakota. [1] 1936 June 12 C Jackson discovers asteroid #1394 Algoa. [1] June 14 C Jackson discovers asteroid #1490 Limpopo. [1] June 26 L Boyer discovers asteroid #2021 Poincare. [1] July 8 C Jackson discovers asteroid #1949 Messina. [1] July 16 K Reinmuth discovers asteroids #1395 Aribeda and #1402 Eri. [1] July 25 G Neujmin discovers asteroid #3761. [1] 1937 June 5 A Bohrmann discovers asteroid #1455 Mitchella. [1] July 2 C Jackson discovers asteroids #1429 Pemba and #1456 Saldanha. [1] July 5 C Jackson discovers asteroids #1428 Mombasa and #1430 Somalia. [1] 1938 June 24 A 450 metric ton meteorite strikes the earth in an empty field near Chicora, Pennsylvania. [5] July 23 C Jackson discovers asteroid #1468 Zomba. [1] July 28 K Reinmuth discovers asteroid #1485 Isa. [1] July 30 C Jackson discovers asteroid #1467 Mashona. [1] 1939 June 20 C Jackson discovers asteroid #1817 Katanga. [1] September 1 Physical Review publishes first paper to deal with "black holes". [1] 1945 May 25 Arther C Clark proposes relay satellites in geosynchronous orbit. [1] 1946 January 10 US Army establishes first radar contact with Moon, Belmar New Jersey. [1] March 22 First US rocket to leave the Earth's atmosphere (50 miles up). [1] June 4 Largest solar prominence (300,000 mi/500,000 km) observed. [1] 1947 February 12 Daytime fireball and meteorite fall seen in eastern Siberia. [1] 1948 February 16 Miranda, famous moon of Uranus, photographed for first time. [1] April 14 A flash of light is observed in the crater Plato on the Moon. [1] June 3 200" (5.08 m) Hale telescope dedicated at Palomar Observatory. [1] July 5 E L Johnson discovers asteroid #1618 Dawn. [1] July 17 C A Wirtanen discovers asteroid #1685 Toro. [1] December 28 US announced a study to launch an Earth satellite. [1] December 29 US State Department announces work on placing objects into Earth orbit. [1] 1949 February 24 V-2/WAC-Corporal first rocket to outer space, White Sands New Mexico, 400 km. [1] May 1 Gerard Kuiper discovers Nereid, (second satellite of Neptune). [1] June 26 Walter Baade discovers asteroid Icarus inside orbit of Mercury. [1]