Throughout sci fi books/movies and even "UFO sightings" a common theme is alien ships being the flying saucer shape. When and why did this become the stereotypical shape? Assuming little green men hevent visited is what is the origin of this trope?
FYI, there's actually a flaired user here specializing in UFOs: /u/dute. Here are a few of his/her comments relating to "flying saucers"
Wednesday AMA: I research the history of UFO reports and investigations. Ask me anything! - introduces the origin of the saucer image
Were there any recorded UFO sighting prior to the early 1900s? - a longer discussion of early UFO images
Were UFO theories as widespread in the Soviet Union? Did similar types of conspiracy theories occur? - more on early UFO theories
All of these posts have been archived by now, so if you have follow-up questions, just them here & mention the user's name to notify them