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A long time ago, during the time of the Ancient Greeks, people thought that the way things worked in the sky was different from the way things worked on Earth. They also thought that they could not study how things in the sky worked. This was because they could not do experiments with things in the sky, but they could do experiments with things that are on the surface of the Earth.
When people realized that, by looking at the sky, they were able to work out how the planets moved, the science of astrophysics was born. The first people to write books on how they thought the planets moved were Nicolaus Copernicus, Galileo, and Johannes Kepler.
Sir Isaac Newton realized that the same rules of mechanics that he had found on the surface of the Earth also could be used to predict how the planets moved. He said, "As above, so below." By this, he meant that we can study how things work on this planet to find out how things work in space.
Later scientists found that by looking at the light from stars, they were able to work out what they were made from. This process is called spectroscopy.
Types of astrophysics
There are two main types of astrophysics: observational astrophysics and theoretical astrophysics.
Observational astrophysics is like astronomy. Like astronomers, observational astrophysicists use telescopes to study the Universe, but observational astrophysicists study the physics of what they see to explain the Universe.