The early Greeks are credited with many valid concepts in astronomy. Some of their theories were correct; some were later proven incorrect. One theory was that Earth was the center of the universe and that other planets circled Earth. The Greeks thought Earth did not move because its movement was not obvious from the surface of the planet. The Greeks also believed that an invisible sphere surrounding our planet contained the stars. This sphere rotated, explaining the apparent movement of constellations over time. Early Greeks also developed theories about the relationship between the moon and Earth. Why is the same side of the moon always visible from Earth?

The questions have been asked in several important state and regional science exams in the United States. For example: TAKS-Texas Assessment of Knowledge and Skills, New York State Educational Department Regents exams, California Standards Test, etc.