Second, from the Earth's surface these arc viewed through a thick atmosphere that completely absorbs most radiation except within certain "windows", wavelength regions in which the radiation can pass
through the atmosphere relatively freely in the optical, near-infrared, and radio bands of the electromagnetic spectrum: and even in these windows the atmosphere has considerable effects. For light, these atmospheric effects arc as follows:
(1) some absorption that dims the radiation somewhat, even in a clear sky;
(2) refraction, which causes slight shin in the direction so that the object appears in a slightly different place;
(3) scintillation (twinkling): Le., fluctuations in brightness of effectively
point-like sources such as stars, fluctuations that are, however, averaged out for objects with larger images, such as planets (the ionosphere, an ionised layer high in the atmosphere, and interplanetary medium have similar effects on radio sources):
 (4) image movement because of atmospheric turbulence
("bad seeing") spreads the image of a tiny point over an angle of nearly one arc second or more on the celestial sphere (one arc second equals 1/3,600 degrees); and
(5) background light from the night sky The obscuring effects of the atmosphere and its clouds are reduced by placing observing stations on mountains, preferably in desert regions (e.g., southern California and Chile), and away from city lights.

The effects arc eliminated by observing from high-altitude aircraft, balloons, rockets, space probes, and artificial satellites. From stations outside all or most of the atmosphere, gamma rays and X-rays -- that
is, high-energy radiation at extremely short wave-lengths and far-ultraviolet and. far-infrared radiation, all completely absorbed by the atmosphere at ground level observatories, can be measured. At radio
wave-lengths between about one centimetre and 20 meters, the atmosphere (even when cloudy) has little effect, and man-made radio signals arc the chief interference

Third, the Earth is a spinning, shifting, and wobbling platform. Spin on its axis causes alternation of day and night and an apparent rotation of the celestial sphere with stars moving from east to west.Ground-based telescopes use a mounting that makes it possible to neutralize the rotation of Earth relative to the stars: with an equatorial mounting driven a proper speed, the direction of the telescope tube can be kept constant for hours while the Earth turns under the mounting. Large radio telescopes usually have vertical and horizontal axes (altazimuth mounting), with their pointing continuously controlled by a computer. In addition to the daily spin, there are much more gradual effects, called precession and nutation

Gravitational action of the Sun and Moon on the Earth's equatorial bulge causes the Earth's axis to process like a top or gyroscope, gradually tracing out a circle on the celestial sphere in about 26,000 years, and also to nutate or wobble slightly in a period of I 8.6 years. The Earth's rotation and orbital motion provide the basic standard of directions of stars, so that uncertainties in the rate of these motions can lead to quite small but important uncertainties in measurements of stellar movements