Astronomy Practice Exam 2026 – 400 Free Practice Questions to Pass the Exam

Galileo's observations greatly contributed to our understanding of the solar system and celestial phenomena. In addition to discovering the four largest moons of Jupiter—Io, Europa, Ganymede, and Callisto—he also made significant observations of the Sun. He notably documented sunspots, which are temporary phenomena on the Sun's photosphere that appear as spots darker than the surrounding areas. This discovery was groundbreaking because it provided evidence that the Sun is not a perfect, unblemished sphere and that it undergoes changes, which challenged existing views of celestial bodies as immutable and flawless.

Additionally, the study of sunspots indicated that the Sun itself was rotating, as Galileo noticed that these spots moved across the face of the Sun over time. This understanding was crucial in advancing heliocentric theories of the solar system, as it suggested a more dynamic solar structure than previously thought.

Galileo's work with sunspots helped lay the foundation for modern solar studies and enhanced the understanding of not only the Sun but also the nature of other celestial bodies.