Many lights and objects in the sky are misunderstood, and often completely unrecognised by observers. These range from rainbows and meteors, to the unusual: green flash and ball lightning. Many are classed as "UFOs," a description which has long connoted mysterious, extraterrestrial visitors. The first part of this book is an identification guide offering quick and structured clues to known day or nighttime aerial phenomena. Objects thus found are referenced to the second part of the book, which gives a full description, physical explanation, and relevant notes on observing and photographing various phenomena. Some require optical aids such as binoculars or telescopes, but the main thrust of the book is identification and explanation rather than imaging. The final chapter approaches UFOs from a scientific standpoint, with a short section on "extraterrestrial UFOs," emphasising burden of proof and touching on scientific theories of life on other worlds, and the improbability of visitors.