Eudoxus of Cyzicus was an ancient Greek mathematician and astronomer who lived in the 4th century BC. He is best known for his work on the theory of proportion, which was later developed by Euclid and Archimedes. He is also credited with introducing the idea of the celestial sphere, which was used by later astronomers to explain the motion of the planets.

Eudoxus was born in Cyzicus, a city on the southern coast of the Black Sea, in the 4th century BC. He was a student of the philosopher Plato, and was also influenced by the work of the mathematician Theaetetus. He is believed to have traveled to Egypt and Babylon, where he studied astronomy and mathematics.

Eudoxus is best known for his work on the theory of proportion, which he developed in his book On Magnitudes and Ratios. In this work, he proposed that any two magnitudes can be compared by dividing them into equal parts. This idea was later developed by Euclid and Archimedes, who used it to develop the theory of proportion.

Eudoxus is also credited with introducing the idea of the celestial sphere, which was used by later astronomers to explain the motion of the planets. He proposed that the stars and planets were embedded in a sphere that rotated around the Earth. This idea was later developed by Ptolemy, who used it to explain the motion of the planets.

Eudoxus also made important contributions to mathematics. He developed the method of exhaustion, which was later used by Archimedes to calculate the area of a circle. He also developed the theory of proportion, which was later used by Euclid and Archimedes.

Eudoxus is also credited with introducing the idea of the celestial sphere, which was used by later astronomers to explain the motion of the planets. He proposed that the stars and planets were embedded in a sphere that rotated around the Earth. This idea was later developed by Ptolemy, who used it to explain the motion of the planets.

Eudoxus also made important contributions to astronomy. He proposed that the planets moved in circles around the Earth, and that the stars were fixed in the sky. He also proposed that the Sun and Moon were spherical, and that the Moon's light was reflected from the Sun.

Eudoxus was an important figure in the development of mathematics and astronomy in the ancient world. His work on the theory of proportion and the celestial sphere were used by later astronomers and mathematicians to explain the motion of the planets. He is also credited with introducing the idea of the celestial sphere, which was used by later astronomers to explain the motion of the planets. His work was an important step in the development of mathematics and astronomy in the ancient world.