Management Aptitude Test Sep 2020 Paper

Compared with other experimental sciences, astronomy has certain limitations. First, apart from meteorites, the Moon, and the nearer planets, the objects of study are inaccessible and cannot be manipulated, although nature sometimes provides special conditions, such as eclipses and other temporary effects. The astronomer must content himself with studying radiation emitted or reflected from celestial bodies. Second, from the Earth's surface these are 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 are as follows: (1) some absorption that dims the radiation, even in a clear sky; (2) refraction, which causes slight shift in the direction so that the object appears in a slightly different place; (3) scintillation i.e., 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; (4) image movement because of atmospheric turbulence which 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 are eliminated by observing from high-altitude aircraft, balloons, rockets, space probes, and artificial satellites. From stations all or most of the atmosphere, gamma rays and X-rays, high-energy radiation of extremely short wave-lengths and far-ultraviolet rays and far-infrared radiation, can be measured, At radio wave-lengths between about one centimeter and 20 meters, the atmosphere (even when cloudy) has little effect, and man-made radio signals are 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 neutralise the rotation of Earth relative to the stars; with an equatorial mounting driven at a proper speed, the direction of the telescope tube can be kept constant for hours while the Earth turns under the mounting.
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 18.6 years. The Earth's rotation and orbital motion provide the basic standard of directions of stars, so that the uncertainties in the rate of these motions can lead to quite small but important uncertainties in measurements of stellar movements.