Astronomy Practice Exam 2025 – Complete Preparation Guide

Epicycles refer to small circular orbits described by planets in the context of the geocentric model of the universe, which posits that Earth is at the center of the universe and that all celestial bodies orbit around it. The concept of epicycles was developed to explain the apparent retrograde motion of the planets, where they appear to move backward in the sky relative to the background stars. By introducing these smaller circular motions, or epicycles, astronomers like Claudius Ptolemy could make more accurate predictions of planetary positions.

In contrast, the other choices describe different astronomical concepts. Stellar bodies outside the solar system would pertain to stars and galaxies beyond our own solar system, which does not relate to the orbital mechanics of planets. Planets that emit light describes luminescent celestial bodies, but in general, planets do not emit their own light; they reflect the light of their star. Natural satellites of planets, such as moons, are distinct objects that orbit planets and are not part of the epicyclic motion described in the geocentric model.