You bet it's not! My teacher made that very clear the first day of geometry class in grade school, yeah, grade school. Those were her exact words "a sphere is not a circle", she said it rather sternly and she pointed her finger at us for emphases.
Then I went to the Kingdom hall and some guy from the stage said that the bible calling the earth a circle was proof that it was inspired.
So, now, as an adult, I realize why my teacher used such emphasis, apparently, lot's of people don't know this simple fact.