How long does it take for the Earth to go around the Sun: one day, one month, or one year?
On a science literacy survey of 1574 adults, 54% answered this question correctly.
But don’t worry—this number jumped to a whopping 66% when it included only those who considered themselves part of an attentive public, i.e., people very interested in the subject who incidentally consider themselves very well informed and happen to be regular readers of (e.g.) a newspaper or national magazine.
(NSF survey PDF via Political Animal)