Most children are not taught that the Earth is round until they are about 10 years old and in the fourth grade, according to a survey conducted for the National Science Teachers Association. As a result, they rely on their own perceptions and assume the Earth is flat.

The study found that 95 percent of second graders thought the Earth was flat. The proportion who knew the Earth to be round did not exceed 50 percent until the fourth grade.

The study, conducted in 65 classrooms in the United States and Israel, also revealed that teachers grossly overestimated their students' knowledge on this point. Second-grade teachers, for example, estimated that 95 percent of their students knew the Earth was round. The tests put the actual figure at 5 percent.

The study was conducted by Alan Lightman, a research physicist, and Philip Sadler, a former science teacher, both at the Smithsonian Astrophysical Observatory in Cambridge, Mass. It was published in the current issue of Science and Children, a journal of the science teachers' association.

The authors said their findings show that children learning science must struggle with contradictions "between what they are told about their world and what they see with their own eyes. Children's deep-seated impression that the Earth is flat is . . . what science educators call a naive theory. Such theories, based on common-sense experiences from an early age, are very difficult to give up and can strongly interfere with any learning that challenges them."

A comparable "naive theory" held by many adults is that heavier objects fall faster than lighter objects in a vacuum. Without wind resistance, all objects fall at the same rate.