Extraterrestrial life is hypothetical life which may occur outside of Earth and which did not originate on Earth. Such life might range from simple prokaryotes to beings with civilizations far more advanced than humanity. The Drake equation speculates about the existence of intelligent life elsewhere in the universe. Unidentified flying object is the popular term for any aerial phenomenon that cannot immediately be identified. Most UFOs are identified on investigation as conventional objects or phenomena. The term is widely used for claimed observations of extraterrestrial spacecraft. The Moon is an astronomical body that orbits Earth as its only natural satellite. It is the fifth-largest satellite in the Solar System, and the largest among planetary satellites relative to the size of the planet that it orbits. Just 100 years ago, the nature of the Milky Way — and the universe itself — was still a matter of debate. On April 26, 1920, astronomer Harlow Shapley claimed our galaxy was the entire universe. Astronomer Heber Curtis countered that the blurry star clusters seen with a telescope were “island universes,” separate from, but analogous to, the Milky Way.