For most of history, people studied the sky with the naked eye alone. They could record where the Sun, Moon, planets and stars were, but not see fine detail. In the early 1600s Galileo pointed an optical telescope at the sky. It used lenses to collect visible light, and he saw mountains on the Moon and moons orbiting Jupiter. Later telescopes used curved mirrors, which can be made much larger than lenses, so they collect more light and show fainter objects.
Visible light is only one part of the electromagnetic spectrum. Stars and galaxies also give out radio waves, infrared, ultraviolet, X-rays and gamma rays, and each type of radiation tells us something different, such as the temperature of a gas cloud or the activity near a black hole. In the 20th century astronomers built radio telescopes and detectors for the other parts of the spectrum. Photographic film and then digital cameras replaced the eye, because they can collect light over a long exposure, and computers now process the data.
The atmosphere is a problem for some of this work. It absorbs most X-rays and gamma rays, so they are absorbed before they reach the ground. It also absorbs much of the ultraviolet and infrared, so these cannot be detected well from the ground. Telescopes for them must be carried above the atmosphere on satellites. Even for visible light, the moving air causes blurring, which is why stars appear to twinkle. The Hubble Space Telescope orbits above the atmosphere and so gives much sharper images.
Telescopes on the ground are still useful and are usually built on high mountains in dry places, where the air is thin, clear and free from light pollution. Today many telescopes around the world and in space are used together, and their data is shared.