Why did cow jump over the moon?

According to one theory, the rhyme is a lesson in astronomy, Hey diddle diddle, the Cat (Leo - the Lion) and the Fiddle (Lyra - the Lyre), the Cow (Taurus - the Bull) jumped over the Moon (the Moon); the Little Dog (Canis Minor - the Lesser Dog) laughed to see such sport, and the Dish (Crater - a dish shaped constellation) ran after the Spoon (Ursa Major - the Big Dipper).