Identifying Unidentified Flying Objects is a difficult task due to the normally poor quality of the evidence provided by those who report sighting the objects. Most officially investigated UFO sightings, such as from the U.S. Air Force's Project Blue Book, have been identified as being due to honest misidentifications of natural phenomena, aircraft, or other everyday explanations. UFOs that can be explained are sometimes termed IFOs or Identified Flying Objects. Dr. Menzel developed this diagram to offer various explanations for UFO sightings. Donald Howard Menzel (April 11, 1901 - December 14, 1976) was one of the first theoretical astronomers and astrophysicists in the US. He discovered the physical properties of the solar chromosphere, the chemistry of stars, the atmosphere of Mars, and the nature of gaseous nebulae. addition to his academic and popular contributions to the field of astronomy, Menzel was a prominent skeptic concerning the reality of UFOs. All of his UFO books argued that UFOs are nothing more than misidentification of prosaic phenomena such as stars, clouds and airplanes; or the result of people seeing unusual atmospheric phenomena they were unfamiliar with. He often suggested that atmospheric hazes or temperature inversions could distort stars or planets, and make them appear to be larger than in reality, unusual in their shape, and in motion. No creation date appeared with chart.