It is not an opinion that the earth is flat any more than it is the opinion that the moon is made of cheese.  :/

opinion (ə-pĭnˈyən)►

    n.
    A belief or conclusion held with confidence but not substantiated by positive knowledge or proof: "The world is not run by thought, nor by imagination, but by opinion” ( Elizabeth Drew).
    n.
    A judgment based on special knowledge and given by an expert: a medical opinion.
    n.
    A judgment or estimation of the merit of a person or thing: has a low opinion of braggarts.