Astronomy Practice Exam 2026 – Complete Preparation Guide

Galileo's most significant contribution to astronomy was the discovery of the four largest moons of Jupiter, commonly referred to as the Galilean moons: Io, Europa, Ganymede, and Callisto. This discovery was groundbreaking as it provided direct observational evidence that not everything orbits the Earth, challenging the geocentric model that placed Earth at the center of the universe. By using a telescope, Galileo was able to observe these moons orbiting Jupiter, which supported the heliocentric model proposed by Copernicus. This observation also laid the groundwork for understanding that celestial bodies could exist in orbit around other planets, revolutionizing the field of astronomy and our understanding of the solar system.

The other choices do not accurately reflect Galileo's contributions. While the notion of Earth's rotation was discussed in the works of earlier astronomers, Galileo focused primarily on observational science rather than theoretical or experimental foundational work of gravity. The development of a new theory of gravity is primarily attributed to Newton much later, and the concept of black holes was not introduced until centuries after Galileo's time. Therefore, the discovery of Jupiter's moons stands out as his major achievement in the field.