Astronomy is one of the oldest scientific fields, with a history spanning thousands of years. Early understanding of the heavens and celestial bodies paved the way for a series of important discoveries about humanity’s place in the universe. This article will look at the pioneering astronomers who have completely changed the way we see the universe, from ancient times to modern times, and their great discoveries.

## Nicolaus Copernicus (1473 – 1543)

“Nicolaus Copernicus”, a Polish astronomer, was the first to propose the “heliocentric” theory, which changed the concept of the universe. Before Copernicus, the “geocentric” model, with the Earth at the center of the universe, was widely accepted. In 1543, he published the book “De Revolutionibus Orbium Coelestium” (On the Revolutions of the Heavenly Bodies), which asserted that the Sun was the center of the universe and that the planets, including the Earth, revolved around the Sun.

This discovery was met with strong opposition from the church and scholars of the time, but it paved the way for the development of modern astronomy. Copernicus’s model was the basis for much later research and was the basis for other important discoveries in the history of science.

## Galileo Galilei (1564 – 1642)

“Galileo Galilei”, an Italian scientist and astronomer, made great contributions to the development of astronomy through the improvement of telescopes and sky observations. He was the first person to use a telescope to observe celestial bodies in detail, something that no one had done before.

### Discoveries Through the Telescope

Using a telescope, Galileo observed the four largest moons of Jupiter, now known as the Galilean moons (Ganymede, Io, Europa, and Callisto). This discovery disproved the notion that all celestial bodies orbited the Earth. He also observed the phases of Venus, sunspots on the Sun, and the dimpled surface of the Moon, observations that strongly supported Copernicus’s “heliocentric” model.

Galileo’s discoveries brought him into conflict with the Catholic Church, and he was charged with heresy. Although he spent the rest of his life under house arrest, Galileo continued to research and write, contributing to the advancement of science and astronomy.

## Johannes Kepler (1571 – 1630)

“Johannes Kepler”, a German astronomer, is famous for his “laws of planetary motion”. Kepler studied and built mathematical models of how the planets move around the Sun, based on observations made by his predecessor Tycho Brahe.

### Three Laws of Planetary Motion

Kepler discovered that the planets do not revolve around the Sun in circular orbits, but in elliptical orbits. He also proposed three laws of planetary motion:

- Law of Orbits: Planets move in elliptical orbits, with the Sun located at one of the two foci of the ellipse.
- Law of Area: A straight line joining a planet to the Sun sweeps out equal areas in equal time intervals, meaning that a planet moves faster when closer to the Sun and slower when farther from the Sun.
- Orbital-period law: The square of a planet’s orbital period around the Sun is proportional to the cube of the planet’s mean distance from the Sun.

These laws laid the foundations of celestial mechanics and were a major turning point in the history of science, helping us better understand the motions of celestial bodies in the Solar System.

## Isaac Newton (1642 – 1727)

“Isaac Newton”, an English physicist, mathematician and astronomer, was one of the greatest scientists in history. He is best known for his work on the “law of universal gravitation” and his book “Principia Mathematica” (Mathematical Principles of Natural Philosophy), published in 1687.

Newton’s law of universal gravitation explains that all objects in the universe have an attractive force toward each other, and this force is directly proportional to the masses of the objects and inversely proportional to the square of the distance between them. This discovery has explained not only the motion of the planets around the Sun but also many other astronomical phenomena.

### Contributions to Astronomy

Newton’s work provided a comprehensive and scientific explanation of why the planets move in the orbits that Kepler had discovered. The combination of Newton’s laws and Kepler’s laws of planetary motion provided the mathematical basis for accurately predicting the motion of celestial bodies in the universe. This was one of the greatest contributions to the development of astronomy and modern science.

## William Herschel (1738 – 1822)

“William Herschel”, a German-born British astronomer, is known for discovering “Uranus” in 1781. It was the first planet to be discovered using a telescope, and this discovery greatly expanded our understanding of the Solar System.

Herschel also discovered many other celestial bodies, including the moons of Uranus and nebular clouds, and developed advanced observational techniques with telescopes. He was also the first to use large reflecting telescopes to observe the heavens, contributing greatly to the development of observational astronomy.

## Edwin Hubble (1889 – 1953)

“Edwin Hubble”, an American astronomer, changed our understanding of the universe with his discovery of “cosmic expansion”. In the 1920s, Hubble used the Hooker telescope at Mount Wilson Observatory to observe distant galaxies and discovered that they were moving away from us, proving that the universe is expanding.

This discovery led to the “Big Bang” theory, the theory of the formation and expansion of the universe, and confirmed that the universe was not static as previously thought. Hubble is also named after the famous “Hubble Space Telescope”, one of the most important astronomical instruments in the history of space exploration.

## Astronomy

Pioneering astronomers such as Copernicus, Galileo, Kepler, Newton, Herschel, and Hubble completely transformed our understanding of the universe and humanity’s place in it. Their great discoveries not only expanded humanity’s view of planets and celestial bodies, but also laid the foundation for the development of modern space science and technology.

With constant advances in technology and science, future generations of astronomers will continue the work of the pioneers, uncovering more mysteries of the universe and bringing us closer to answering big questions about the origin and nature of the universe.