Astronomy is a branch of physics that studies celestial bodies and the universe as a whole, or simply put, it is the study of the planets, solar systems, and the universe. There have been many famous astronomers, who have helped in the development and progress of this field. The most famous of them are listed below.
Eratosthenes (276 - 194 BC)
Eratosthenes, nicknamed as Beta was an all-round Greek scholar, who made significant contributions to various fields including astronomy, geography, mathematics, as well as music and literature. He was the first person, who attempted to determine the earth's circumference. He did this by comparing the midsummer's noon shadow in deep wells in Syene and Alexandria, and made assumptions that the sun's rays are virtually parallel and knowing the distance between the two locations, he calculated the circumference of the earth in a unit called stadia. He also made an attempt to determine the measurement for the tilt of the earth's axis, and the distance of the earth from the sun and the moon. Although his accuracy about these findings is uncertain, he laid the groundwork, which was useful for others who later researched on similar lines.
Nicholas Copernicus (1473 - 1543)
Nicholas Copernicus was a famous polish astronomer, who is well-known for his Copernican theory, which states that the sun is the center of the universe and the earth revolves around it, and completes one revolution in a year. Although having studied subjects in areas as varied as liberal arts, law, and medicine; it was his interaction with a mathematician Domenico Maria de Novara, that sparked his interest in astronomy. He was fascinated after experiencing eclipse by the moon of the star Aldebaran in the year 1497. He has authored several books on astronomy. His major theory was published in the book, De revolutionibus orbium coelestium (On the Revolutions of the Celestial Spheres).
Claudius Ptolemaeus (83 - 161 AD)
Claudius Ptolemaeus, also known as Ptolemy, was an ancient astronomer, mathematician, geographer as well as an astrologer. He has proposed several theories in his first treatise, now known as the Almagest. Ptolemy gave a useful tool for astronomical calculations in his Handy Tables, which tabulated all the data needed to compute the positions of the Sun, Moon, and planets. It also elaborated the rising and setting of the stars, and the solar and lunar eclipses. He was also the creator of a parapegma, which is a star calendar based on the appearances and disappearances of stars, over the course of the solar year. Although, some of his theories were later proved to be incorrect, he played an instrumental role in laying down the foundation for future researchers. His theories dominated the scientific field until the 16th century.
Tycho Brahe (1546 - 1601)
Tycho Brahe was a Danish researcher, who is supposed to have made the most accurate observations about measurements of the solar system and over 700 other stars. He was the first one to make these observations, and to catalog the planets and stars with adequate accuracy, so as to determine whether the Ptolemaic or Copernican system was more valid in describing the universe or the planetary systems. In 1572, he found a supernova near the Cassiopeia constellation. In 1576, he built the astronomical observatory, which is now known as the Castle of Uranienborg. He worked to combine the geometrical benefits of the Copernican system with the philosophical benefits of the Ptolemaic system, only to create his own model of the universe ― the Tychonic system. From 1600 until his death in 1601, Johannes Kepler assisted him. Kepler later used Tycho's work to develop his own theories.
Galileo Galilei (1564 - 1642)
Galileo Galilei was an Italian physicist and an astronomer as well. His achievements include improvements to the telescope, noteworthy astronomical observations, and support for Copernicanism. He has been called the 'father of modern observational astronomy'. In 1589, he became the professor of mathematics at Pisa and supposedly taught theories that contradicted Aristotle's theories, and as a result, his contract was not renewed in 1592. The same year, he was appointed as the chair of mathematics at the University of Padua, where he invented a 'calculating compass' for solving mathematics problems. In December 1609, he had built a telescope twenty times stronger than the first, with the help of which, he was able to view craters on the moon, stars in the Milky Way, the four largest satellites of Jupiter, and the phases of Venus. In 1613, he published a book about sunspots. In the year 1624, he wrote Dialogue on the Tides in which he discussed the Ptolemaic and Copernican theories. Galileo's championing of Copernicanism was controversial within his lifetime, since that view had been dominant since the time of Aristotle, and the controversy that emerged due to Galileo's opposition to this view, resulted in the Catholic Church's prohibition of heliocentrism. He was forced to recant his theory and had to spend the last years of his life under house arrest on orders of the Inquisition.