How did early astronomers measure the stars and planets? Objective Have you ever wondered about how astronomers learned about the movement of Earth and the placement of constellations throughout the galaxy? Early astronomers used a tool called an astrolabe to measure the angle of stars in the night sky. This tool taught early astronomers many different things about our galaxy including the amount of time it takes for the Earth to revolve around the sun, the hours in a day, and how to track constellations and eclipses. Background Early astronomers used a tool called an astrolabe to measure the Duration: 60 Minutes Additional Standards: (HS-ESS1-4) Essential Question How did early astronomers measure the stars and planets? Objective Have you ever wondered about how astronomers learned about the movement of Earth and the placement of constellations throughout the galaxy? Early astronomers used a tool.