On January 7, 1610, Italian astronomer Galileo Galilei discovered, using a homemade telescope, four moons orbiting the planet Jupiter. Looking at what he thought were a group of stars, he realized the objects appeared to move in a regular pattern. These objects moved in the "wrong direction," according to the understanding of nature at the time. After a few weeks, Galileo determined that he was observing not stars, but objects in orbit around Jupiter. Today, Jupiter’s four largest satellites—Io, Europa, Ganymede, and Callisto—are named the Galilean Moons in honor of their discoverer.

Galileo’s discovery provided evidence for the Copernican understanding of the universe. This was the idea that everything in existence did not, indeed, move around the Earth. His discoveries would lead to the development of modern astronomy.

astronomer
Noun

person who studies space and the universe beyond Earth's atmosphere.

astronomy
Noun

the study of space beyond Earth's atmosphere.

determine
Verb

to decide.

Noun

natural satellite of a planet.

Verb

to move in a circular pattern around a more massive object.

Noun

large, spherical celestial body that regularly rotates around a star.

satellite
Noun

object that orbits around something else. Satellites can be natural, like moons, or made by people.

star
Noun

large ball of gas and plasma that radiates energy through nuclear fusion, such as the sun.

telescope
Noun

scientific instrument that uses mirrors to view distant objects.

universe
Noun

all known matter, energy, and space.

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