Observatories, Ground
Astronomers study the universe by measuring electromagnetic radiation—gamma rays, X rays, optical and infrared radiation, and radio waves—emitted by planets, stars, galaxies, and other distant objects. Because Earth's atmosphere is transparent to optical and infrared radiation and to radio waves, these types of radiation can be studied from ground-based observatories. Astronomers must launch telescopes into space in order to study X rays, gamma rays, and other radiation that is blocked by absorption in Earth's atmosphere.
Astronomers make use of ground-based observatories whenever they can. It is about 1,000 times cheaper to build a telescope of a given size on the ground than to launch it into space, so it is much more economical to operate on the surface of Earth.
A telescope can be thought of as a bucket that collects light or radio waves and brings them to a focus. More light can be gathered with a larger bucket. Since most astronomical sources of light are very faint, it is desirable to build telescopes as large as possible. Given current technology, we can build much larger telescopes on the ground than we can in space, which is another reason that ground-based observatories remain very important.
