The words loon, lune sound the same but have different meanings and spellings. Why do loon, lune sound the same even though they are completely different words?
The answer is simple: loon, lune are homophones of the English language.
Any of several fish-eating, diving birds of the genus Gavia of northern regions, having a short tail, webbed feet, and a laughlike cry.
Informal One who is crazy or deranged.
A crescent-shaped portion of a plane or sphere bounded by two arcs of circles.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.