Ancient Astronomers' Timeline

Hipparchus

Hipparchus most notably discovered processional movements of equinoxes. He was also ground-breaking because he used a number of instruments to make scientific observations based on numbers and factual evidence. He used mathematical calculations to determine his predictions.

Hipparchus's work was mostly documented in a book known as "Almagest," which was published by Ptolemy. His writings were primarily in a part called "Strabo of Amaseia," which was published in 21 AD.

Overall, he was a very early astronomer but made great strides during his time.

Ptolemy

Ptolemy was an ancient astronomer who formulated the geocentric view of the solar system. That meant that he thought the Earth was the center, and the sun and planets revolved around it. He came to this conclusion through extending the work of Hipparchus.

Ptolemy contributed an important astronomy book known as "Almagest." This work contained 13 books in it and had a great deal of information compiled about the stars and other objects in our solar system. Ptolemy spent time plotting the courses of heavenly bodies in the universe, and this information was included.

Nicolaus Copernicus

Copernicus established that the sun was the center of the universe in an essay called "Commentariolus" that was written somewhere between 1510 and 1514. This became known as the heliocentric model.

Tycho Brahe

Instrument design by Tycho

Tycho Brahe was notable because he created many instruments that revolutionized astronomy. Tycho's instruments allowed him to make precise astronomical measurements. He would also regularly calibrate his instruments to be sure they were accurate.

Tycho also observed a new star known as Cassiopeia in 1572 and a comet in 1577. His observations caused him to conclude that comets moved through celestial spheres.

Galileo Galilei

Galileo contributed the telescope to astronomy. He heard about the spyglass in 1609, which he improved into the telescope. This allowed Galileo along with future astronomers to view celestial bodies up close and with more detail than possible with the naked eye.

He also discovered four of Jupiter's moons and was the first astronomer to make solid observations about the Earth's moon. He saw that the moon was bumpy and full of mountains rather than smooth as was previously thought.

Galileo also confirmed Copernicus's theory about the sun being the center of the solar system rather than the Earth.

Johannes Kepler

Kepler worked as Tycho Brahe's assistant in the beginning of his career as an astronomer. Using Tycho's observations, Kepler discovered was that the orbit of Mars was an ellipse.

One of Kepler's greatest achievements was his laws of planetary motion. The first two were published in 1609 in "Astronomia Nova."

In his most influencial work called "Epitome Astronomiae" (published in 1621) he explained the heliocentric model in a methodical way.

Sir Isaac Newton

Newton's biggest contribution to astronomy was his 3 fundamental laws of motion, which were stated in his book "Principia Mathematica." This book was published in 1686.

From his 3 laws of motion, he also derived the universal law of gravity, which shapes our understanding of how objects act and interact with one another within the universe.

Edmond Halley

Edmond Halley's most well-known contribution was predicting the movement of a comet that would orbit every 75-76 years. This comet was later named Halley's comet after the influencial astronomer.

Halley also predicted and calculated the movements of a number of other comets.

He invented an instrument called the diving bell to supply fresh air underwater. Another invention of Halley was an early working model of the magnetic compass.

Edwin Hubble

The Hubble Space Telescope

Edwin Hubble is known for making the discovery of the universe's expansiveness in 1923. This came about when he noticed a celestial object that turned out to be a Cepheid star. He calculated the distance to this star as being a million light-years away using a method devised by another astronomer named Shapley. No object had been discovered that far away before, so his discovery greatly expanded the known universe.

Later, the Hubble telescope was named after him. This telescope helps scientists today discover even more deep space objects in the universe.

Carl Sagan

Carl Sagan was an astronomer popular for writing the 1980s television series, "Cosmos: A Personal Voyage." The contribution of his TV show was that it educated the public about astronomy.

Carl Sagan also discovered some important things about a number of planets within our solar system. He figured out that Venus has a very high surface temperature and helped in our understanding of the atmospheres of Jupiter and Venus. He also researched seasonal changes on Mars.