What you may not know is that Jupiter's moons were discovered by Galileo and Simon Marius independently at the same time.

Jupiter has been observed since ancient times. The planet is visible to the naked eye. Jupiter is the fifth planet from the Sun which is 484 million miles or 778 million kilometers away from the Sun. It is the largest planet in the solar system in terms of size. I call it the king of the planets in the solar system. If you add up the masses of all the planets in the solar system except Jupiter, Jupiter's mass will be two and a half times greater. Jupiter has more mass than all the planets. The following chart will make it clear.

However, it is not known who discovered Jupiter. It is visible to the naked eye. So people already knew about it. On January 7, 1610, astronomer Galileo Galilei observed Jupiter using a small and low-power telescope. This day is very important in history. He found the strange fixed stars surrounding the planet and began to observe them. Galileo recorded the movements of the stars for the next few days in which there were four stars. Actually they were not stars. They were moons of Jupiter. He was wrong the first time. Discovered that the stars moved with Jupiter. He also found that the stars changed their position around the planet every night.

Before this, Galileo observed the Earth's moon. So he knew about the movement behavior of the moon. As a result, he thought these stars were not actually stars. He thought they were Jupiter's moons and that they revolved around Jupiter. He was right. Galileo's discoveries disproved the Ptolemaic system of astronomy. In the Ptolemaic system, each planet revolves similarly along a circular path, and Earth is considered the center of the solar system. If you want to know more about Ptolemaic system then click this link.

Thus Galileo discovered the four moons of Jupiter, Io, Europa, Ganymede and Callisto respectively. Galileo first saw them in December 1609 or January 1610. He then studied them and only a few months later, in March 1610, recognized them as satellites of Jupiter.

You will be surprised to know that these moons are among the largest objects in the solar system apart from the Sun and the eight planets. whose radius is larger than any dwarf planet such as Pluto. Jupiter's large moon Ganymede is the largest moon in the Solar System. Ganymede's radius is 2,634.1 km which is larger than Mercury. The Galilean moons are round but the other smaller moons are different. Jupiter's smaller remaining moons may vary in size. Small moons cannot hold their material in the same shape due to their weak self-gravity.

Although Jupiter can be seen with the naked eye, its moons cannot be seen. If the telescope had not been invented at that time, people of that time would not have known about Jupiter's moons. Galileo made improvements to his telescope in 1609. As a result, he was able to observe celestial objects more clearly than before. If he could not do that, he might not have created this history.

We today look for more powerful full telescopes in the modern era to learn more about the universe. Galileo's observations showed the importance of telescopes to astronomers. The telescope was just beginning its journey, at that time. They realized that there are objects in space that cannot be seen with the naked eye. The importance of telescopes for seeing unseen objects was easily understood.

Galileo was not alone in making this discovery. Simon Marius and Galileo independently discovered Jupiter's moons. Simon Marius claimed to have discovered the moons 5 weeks ago. Galileo initially named his discovery "Cosmica Sidera" or "Cosimore's Star". The names of the moons which prevailed were given by Simon Marius. He was a noted astronomer.

Marius' work is rarely discussed so we probably know very little about him. But he was a very good observer. He has many important tasks.
We need to know about his work.

• Work no. 1
  Many of you know about the Andromeda "nebula" in current science. He observed the Andromeda "nebula", although Astronomers knew about it at the time.
• Work no. 2
  Simon Marius, with his telescope, detected the false disk formed by the stars.
• Work no. 3
  Simon Marius made better observations of Jupiter's moons, called Galileo's moons, and discovered other orbital elements.
• Work no. 4
  Simon Marius observed the location of the Tycho Brahe supernova in 1572. At the same time find a star there. He considered this star to be slightly fainter than Jupiter's third moon.

Simon Marius was born on 10 January 1573 and died on 5 January 1625 at the age of 51. In 1586 , he entered the Margrave Academy of Heilsbronn and studied until 1601. It was during his studies that he published observations about a comet as well as astronomical tables. This contribution made him famous as a good astronomer and mathematician. Capra had a dispute with Galileo over the invention of the proportional compass. Marius was on his student's side in the argument. This proves that Galileo and Simon Marius were previously known to each other