development of astronomy

  I am going to write about the development of astronomy.
    
     First of all, what is an astronomy? If I answer this question with the technical terms, astronomy is a field of natural science  observing the heavenly bodies and astronomical phenomenon and discovering a law. If I use my words, astronomy is charming study watching the beautiful sky.

    
     This study started 2000 B.C.  A Solar calendar was used in Egypt, and a lunar calendar was used in Mesopotamia then. When humankind started farming, they needed to grasp a change of season. To change season every a year is a cycle changing a positon of stars and sun.
    
     In the two century B.C., Hipparchus,an astronomer in Greece, classified the brightness of the stars into six ranks. These ranks are used as an apparent magnitude now. (But,these were revised.)

     Nicolaus Copernicus advanced the heliocentric system and Galileo Galilei completed it in 1632. This theory is that the earth is moving and it is against the Ptolemaic theory.
     Galileo Galilei is one of the earliest pople using telescope. When he looked at the moon through the telescope, he found that there is a black part and irregularities on the moon. In modern times, these irregularities are called craters. Then, because the moon is perfectly spherical, these irregularities were what people could not explain. He also discovered four moons of Jupiter. Currently, these satellites are called Galileo satellites. This discovery was announced. But this discovery was a disadvantage because people believed in the Ptolemaic system at the time. So he was involved in controversy but he got a worldwide reputation.

     In 1666, Isaac Newton thought up the law of universal gravitation.This law is that theobject on the ground is attracted to the earth and  all objects in the universe affect each other everywhrer by gravity.

     Albert Einstein published special relativity in 1905 and he also published general theory of relativity in 1910. Special relativity is a theory which treats inertial system without a gravitational field. General relativity is a theory that describes space-time distortion by the gravitational field using the Riemannian geometry. These theories are that there is an assumption that only immutable thing is the speed of light.
     Einstein had a lot of achievements so he is called the father of modern physics and the greatest physicist in twentieth century. He was awarded the Nobel Prize in Physics in 1921 by theoretical elucidation of the photoelectric effect based on the photon hypothesis.

     In 1920, Edwin Powell Hubble discovered that the universe was expanding. Because he discovered that the light from the galaxies redshifted, he found an expansion of the universe. He is one of the representative modern astronomers and he laid the foundation of modern cosmology.  

     Hubble Space Telescope was launched in1990. This is a space telescope to orbit about 600km above the earth. Because it is not affected by the atmosphere and weather, we are able to observe celestial objects with high accuracy than on the ground. We can see a lot of beautiful pictures of the universe thanks to this telescope.


     I think the first article tried to end. Thanks for reading to the end.