Comprehension

Directions for Questions: Read the passage carefully and answer the given questions accordingly.

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 eclipse 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 somewhat, even in a clear sky; (2) refraction, which causes a slight shift in the direction so that the object appears in a slightly different place; (3) scintillation (twinkling); i.e., fluctuation in the 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 ionized 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 are eliminated by observing 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 wavelengths 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 spinning, shifting, and wobbling platforms. Spin-on its axis causes the 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 at 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 precess 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 uncertainties in the rate of these motions can lead to quite small but important uncertainties in measurements of stellar movements.

CAT/1994

Question . 171

The man-made radio signals have wave-lengths of