1897
Height of the great airship wave in the midwestern United States.
See The Great Airship of 1897 

1906
Charles Fort begins writing about unexplained events he gleans from newspaper reports. Many of these are about mysterious flying craft.
See Amazing Connections 

1924
August 23
David Peck Todd listens for radio signals from Mars. See Whispers in the Dark, Part 1. 

1927
Long-distance echoes of radio signals noticed. See Echoes. 

1932
Karl Jansky invents the radio telescope. 

May 3
Death of Charles Fort. 

1937
Grote Reber invents the parabolic dish antenna. 

1943
October - November
The Philadelphia Experiment supposedly took place at the Philadelphia Naval Yards.
See A Twisted Tale 

1944 - 1945
Foo fighters are seen by many American pilots. These are unidentified lights or metallic spheres. See Foo Fighters and Ghost Rockets and Feuerball 

Raymond Palmer, editor of Amazing Stories, publishes Richard Shaver's letters about aliens. See Amazing Connections 

1945
December 5
Five Avenger Torpedo bombers vanish in the Bermuda Triangle. seePilot Error 1946
Ghost rockets seen in the skies of Sweden. See Foo Fighters and Ghost Rockets 

May
A UFO supposedly crashes on the Norwegian island of Spitzbergen 

1947
Rear Admiral Roscoe H. Hillenkoetter is appointed first Director of the Central Intelligence Agency. James V. Forrestal is Secretary of Defense. Rear Admiral Sidney Souers becomes the first Executive Secretary of the National Security Council. 

Marcel Griaule is told of the secret Dogon cosmology by the Dogon tribe of Mali 

June 21
The Maury Island event supposedly occurs. See Maury Island 

June 24
Kenneth Arnold sights nine UFOs near Mt. Rainer in Washington State. See Kenneth Arnold 

July 2 - 4
Mac Brazel discovers the debris of the Roswell Incident out on the ranch.
See Roswell, Part 1 

July 7
Major Jesse Marcel and Captain Sheridan Cavitt drive out to the debris site and collect the debris, carrying it back to Roswell Army Air Force Base. See Roswell, Part 2 

September 23
General Nathan F. Twining, Commander of the Air Materiel Command, issues a report on the current knowledge of UFOs and recommends that a permanent project be set up to study them. 

December
A commission to study UFOs, called Project Sign is ordered to be set up by the Air Force.
See Project Sign 

1948
January 7
F-51 pilot Captain Thomas Mantell crashes while chasing a UFO near Fort Knox, Kentucky.
See The Curious Case Of Captain Mantell 

January 22
Project Sign formally established. 

March 25
Supposed date of a UFO crash at Aztec, New Mexico according to one source.
See The Aztec Crash 

July 24
Commercial airliner pilots Chiles & Whitted see UFO near Montgomery, Alabama.
See The Eastern Airlines Sighting 

August - September
Project Sign issues a formal Estimate of the Situation. It concludes that flying saucers are real and that they probably come from outer space. 

October 1
F-51 pilot Lt. George Gorman has a "dogfight" with a UFO at Fargo, North Dakota. 

October
Air Force Chief of Staff General Hoyt S. Vandenburg rejects the "Estimate of the Situation", saying there is not enough evidence to support its conclusions about UFOs. 

1949
Gordon Gray is Secretary of the Army.
Secretary of Defense James V. Forrestal has a mental breakdown and commits suicide at Bethesda Naval Hospital. 

April 27
Project Sign issues its final report and becomes Project Grudge. The name change is reflective of a change in attitude towards UFOs by the Air Force.
See Project Grudge 

April 30
An article by Sidney Shallett called What You Can Believe About Flying Saucers is published in the Saturday Evening Post 

June
Reports of green fireballs sighted in the skies of New Mexico began pouring in last December. In February, a meeting of experts such as Dr. Lincoln La Paz was held to discuss them. In June, Project Twinkle is set up to study them, but it accomplishes nothing due to underfunding. See Project Twinkle 

December
True magazine prints Major Donald Keyhoe's article about UFOs titled "The Flying Saucers Are Real!" 

1950
January
The Air Force issues The Grudge Report. Of 273 sightings investigated, it concluded 23% were "unexplained". 

1951
The flying saucer movie The Day the Earth Stood Still is released. 

October
Project Grudge becomes Project Bluebook, headed by Captain Edward Ruppelt. 

1952
The biggest year for UFO reports.
See: Case files

January
APRO (Aerial Phenomena Research Organization) founded by Jim & Coral Lorenzen 

June
A UFO supposedly crashes on the Norwegian island of Spitzbergen 

July
Peak of the 1952 "flap".
See Summer of '52 

July
The skies over Washington D.C. become filled with unidentified radar blips. A few of these are confirmed by visual sightings. No satisfactory explanation is ever found, although some blame "temperature inversions". 

September 12
A very strange creature is seen in West Virginia in the town of Flatwoods 1953
January
The CIA has become interested in UFOs as a matter of national security. The spy group sponsors The Robertson Panel to study the matter. The Panel is a respected group of scientists whose final recommendation is for a policy of "debunking" UFOs.
See Project Bluebook 

November 23
An F-89 piloted by Lt. Felix Moncla Jr., with radar operator R. R. Wilson, disappears while chasing a UFO over Lake Superior. 

1954
March
Project Bluebook is reorganized under Captain Charles Hardin. The 4602nd Air Intelligence Service Squadron is trained as field investigators.
See Project Bluebook