- 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 Charles Perrine. [1]
- January 5
- Charles Perrine announces discovery of Jupiter's 7th satellite, Elara. [1]
- July 26
- P Gotz discovers asteroid #568 Cheruskia. [1]
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- 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, is 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 (in Curacao). [1]
- April 20
- Halley's Comet passes 29th recorded perihelion at 87.9 million km. [1]
- May 10
- Halley's Comet makes closest approach to Earth in 1910. [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 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 (planet) is photographed for first time (although unknown at the time). [1] [5]
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, is discovered. [1]
- July 11
- M Wolf discovers asteroid #895 Helio. [1]
1919
- July 4
- M Wolf discovers asteroid #914 Palisana. [1]
1920
- December 13
- F Pease's interferometer measures first stellar diameter (Betelgeuse). [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]
- October 5
- Edwin Hubble identifies Cepheid variable star. [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]
- August 23
- Mars makes closest approach to Earth since the 10th century. [1]
1925
- January 1
- American astronomer Edwin Hubble announces the discovery of galaxies outside the Milky Way. (December 31, 1924 [1]) [5]
- January 24
- Moving picture of a solar eclipse taken from dirigible over Long Island, New York, USA. [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 planet Pluto. [1]
- March 13
- Clyde Tombaugh announces discovery of Pluto at Lowell Observatory, USA. [1]
- May 1
- The dwarf planet Pluto is officially named. [5]
- 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
- May 27
- Belgian scientist Auguste Picard launches hydrogen gas-filled balloon carrying himself and an assistant in a 300-pound, 82-inch diameter aluminum gondola. The balloon rises to a record 51,775 feet, and lands safely. [1] [985.340]
- 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]
- September 30
- USSR launches SSSR balloon (with 860,000 cubic-foot capacity) with 3-man aluminum gondola; it reaches a record 60,695 feet (19 km), landing safely. [985.341]
- November 20
- In Akron, Ohio, USA, the balloon Century of Progress is launched, with 7-foot diameter gondola, balloon capacity 600,000 cubic feet, rising to a record 61,238 feet. [985.341]
1934
- January 30
- In Moscow, Russia, the USSR launches the Osoaviakhim balloon, reaching record 72,178 feet, but crashes on landing 70 miles from Moscow, killing the crew. [985.341]
- March 8
- Edwin Hubble photo shows as many galaxies as Milky Way has stars. [1]
- 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]
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
- National Geographic Society-funded American Explorer II balloon sets altitude record of 72,395 feet over South Dakota. [1] [985.342]
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
- 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]
1942
- February 27
- J S Hey discovers radio emissions from the Sun. [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, from 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 miles / 500,000 km) observed. [1]
1948
- February 16
- Miranda, famous moon of Uranus, is photographed for first time. [1]
- April 14
- A flash of light is observed in the crater Plato on the Moon. [1]
- June 3
- 200-inch (5.08m) 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 20
- James Thomas Mangan of Chicago, Illinois, USA declares creation of the Nation of Celestial Space. [1045.211]
- December 29
- US State Department announces work on placing objects into Earth orbit. [1]
1949
- February 1
- 200-inch (5.08-metre) Hale telescope first used. [1]
- February 24
- V-2/WAC-Corporal first rocket to outer space, White Sands, New Mexico, 400km. [1]
- May 1
- Gerard Kuiper discovers Nereid, (second satellite of Neptune). [1]
- June 26
- Walter Baade discovers asteroid Icarus inside orbit of Mercury. [1]
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