Comprehension
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 somewhat, 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 (twinkling); 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 (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 from high-altitude aircraft, balloons, rockets, space probes, and artificial satellites. From stations all or most of the atmosphere, gamma rays and X-rays-that is, high-energy radiation at extremely short wave-lengths and far-ultraviolet rays and far-infrared radiation, all completely absorbed by the atmosphere at ground level observatories 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 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 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 uncertainties in the rate of these motions can lead to quite small but important uncertainties in measurements of stellar movements.
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 slight shift in the direction so that the object appears in a slightly different place; (3) scintillation (twinkling); 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 (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 from high-altitude aircraft, balloons, rockets, space probes, and artificial satellites. From stations all or most of the atmosphere, gamma rays and X-rays-that is, high-energy radiation at extremely short wave-lengths and far-ultraviolet rays and far-infrared radiation, all completely absorbed by the atmosphere at ground level observatories 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 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 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 uncertainties in the rate of these motions can lead to quite small but important uncertainties in measurements of stellar movements.
Question: 1
One of the types of radiation that cannot pass through the atmospheric 'windows' without distortion is:
One of the types of radiation that cannot pass through the atmospheric 'windows' without distortion is:
Show Hint
When answering questions on atmospheric transparency, remember that short-wavelength high-energy radiation (UV, X-rays, gamma) is blocked, while optical, near-infrared, and much of the radio spectrum can pass through.
Updated On: Aug 5, 2025
- near infra-red spectrum
- far-ultraviolet spectrum
- optical band in the spectrum
- radio band in the spectrum
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The Correct Option is B
Solution and Explanation
The passage explains that only certain wavelength regions — known as "windows" — allow radiation to pass through the atmosphere with little absorption or distortion.
It specifies that the atmosphere is relatively transparent in the optical, near-infrared, and radio bands of the electromagnetic spectrum.
However, it also states that high-energy radiation at extremely short wavelengths, such as far-ultraviolet and gamma rays, is completely absorbed by the atmosphere.
This means far-ultraviolet light cannot pass through these atmospheric windows and must be observed from outside the Earth's atmosphere using space-based instruments.
Option (a) near infra-red spectrum — passes through relatively well and is within one of the atmospheric windows, so it is incorrect.
Option (c) optical band — this is the main region where Earth's atmosphere is most transparent, so it can pass with minimal distortion; hence incorrect.
Option (d) radio band — most of this band passes through the atmosphere, except for some very low frequencies affected by the ionosphere; so this is not the correct answer.
Therefore, the far-ultraviolet spectrum is the radiation type that cannot pass through without distortion, making option (b) correct.
It specifies that the atmosphere is relatively transparent in the optical, near-infrared, and radio bands of the electromagnetic spectrum.
However, it also states that high-energy radiation at extremely short wavelengths, such as far-ultraviolet and gamma rays, is completely absorbed by the atmosphere.
This means far-ultraviolet light cannot pass through these atmospheric windows and must be observed from outside the Earth's atmosphere using space-based instruments.
Option (a) near infra-red spectrum — passes through relatively well and is within one of the atmospheric windows, so it is incorrect.
Option (c) optical band — this is the main region where Earth's atmosphere is most transparent, so it can pass with minimal distortion; hence incorrect.
Option (d) radio band — most of this band passes through the atmosphere, except for some very low frequencies affected by the ionosphere; so this is not the correct answer.
Therefore, the far-ultraviolet spectrum is the radiation type that cannot pass through without distortion, making option (b) correct.
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Question: 2
One of the atmospheric effects on Earth-based experiments that is not mentioned in the passage is:
One of the atmospheric effects on Earth-based experiments that is not mentioned in the passage is:
Show Hint
For “NOT mentioned” questions, carefully check each option against the list in the passage. Even if something is possible in real life, it’s only correct here if the passage does not mention it.
Updated On: Aug 5, 2025
- twinkling
- refraction
- image movement
- clouds from volcano eruptions
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The Correct Option is D
Solution and Explanation
The passage lists multiple atmospheric effects that impact astronomy:
1. Absorption — dimming radiation even in clear sky.
2. Refraction — slight shift in apparent position of celestial objects.
3. Scintillation (twinkling) — fluctuations in brightness due to atmospheric turbulence.
4. Image movement — spread of a star’s image due to turbulence ("bad seeing").
5. Background light — from the night sky that interferes with observations.
Option (a) twinkling is explicitly mentioned as scintillation, so it is in the passage.
Option (b) refraction is also mentioned directly, so it is included.
Option (c) image movement is described as part of “bad seeing” and thus is mentioned.
Option (d) clouds from volcano eruptions are never mentioned in the passage — while such clouds can affect observations in reality, they are not part of the atmospheric effects listed in this text.
Hence, option (d) is correct.
1. Absorption — dimming radiation even in clear sky.
2. Refraction — slight shift in apparent position of celestial objects.
3. Scintillation (twinkling) — fluctuations in brightness due to atmospheric turbulence.
4. Image movement — spread of a star’s image due to turbulence ("bad seeing").
5. Background light — from the night sky that interferes with observations.
Option (a) twinkling is explicitly mentioned as scintillation, so it is in the passage.
Option (b) refraction is also mentioned directly, so it is included.
Option (c) image movement is described as part of “bad seeing” and thus is mentioned.
Option (d) clouds from volcano eruptions are never mentioned in the passage — while such clouds can affect observations in reality, they are not part of the atmospheric effects listed in this text.
Hence, option (d) is correct.
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Question: 3
The purpose of telescope mounting is to neutralize:
The purpose of telescope mounting is to neutralize:
Show Hint
Always match the function of the instrument or technique with the specific problem it solves; telescope mounting directly addresses Earth's rotation, not long-term orbital changes.
Updated On: Aug 5, 2025
- atmospheric interference
- the effect of precession
- the effect of nutation
- the effect of diurnal spinning
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The Correct Option is D
Solution and Explanation
The passage explains that the Earth’s daily rotation (diurnal spin) causes an apparent motion of the stars from east to west in the sky.
To counteract this motion and allow telescopes to keep a celestial object fixed in the field of view, special mountings are used.
Ground-based telescopes use equatorial mountings or computerized altazimuth mountings to track objects continuously and compensate for Earth's spin.
Option (a) atmospheric interference — this is mitigated by location and altitude, not by mounting, so it is incorrect.
Option (b) precession — a very slow movement over thousands of years; mounting is not designed to neutralize it during observations, so it is incorrect.
Option (c) nutation — another long-term wobble in Earth's axis; again, telescope mounting is not meant to cancel it in short-term observations.
Option (d) diurnal spinning — this is the main reason for mounting, as it keeps the telescope aligned with the apparent movement of stars, making it the correct choice.
To counteract this motion and allow telescopes to keep a celestial object fixed in the field of view, special mountings are used.
Ground-based telescopes use equatorial mountings or computerized altazimuth mountings to track objects continuously and compensate for Earth's spin.
Option (a) atmospheric interference — this is mitigated by location and altitude, not by mounting, so it is incorrect.
Option (b) precession — a very slow movement over thousands of years; mounting is not designed to neutralize it during observations, so it is incorrect.
Option (c) nutation — another long-term wobble in Earth's axis; again, telescope mounting is not meant to cancel it in short-term observations.
Option (d) diurnal spinning — this is the main reason for mounting, as it keeps the telescope aligned with the apparent movement of stars, making it the correct choice.
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Question: 4
The precession period of Earth is:
The precession period of Earth is:
Show Hint
Precession is a slow movement of Earth's rotational axis; remember its period is thousands of years, not hours or days.
Updated On: Aug 5, 2025
- 24 hours
- 365.25 days
- 18.6 years
- 26,000 years
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The Correct Option is D
Solution and Explanation
In the passage, precession is described as a very gradual movement of the Earth's axis, caused by the gravitational forces of the Sun and the Moon acting on the Earth's equatorial bulge.
It states that precession causes the Earth’s axis to slowly trace a circular path on the celestial sphere in a period of about 26,000 years.
Option (a) 24 hours refers to Earth’s daily rotation (diurnal spin), not precession.
Option (b) 365.25 days refers to Earth's revolution around the Sun, defining a year, not precession.
Option (c) 18.6 years refers to the nutation cycle, another wobble in Earth's axis, not precession.
Therefore, option (d) 26,000 years is correct as it matches the precession period mentioned in the passage.
It states that precession causes the Earth’s axis to slowly trace a circular path on the celestial sphere in a period of about 26,000 years.
Option (a) 24 hours refers to Earth’s daily rotation (diurnal spin), not precession.
Option (b) 365.25 days refers to Earth's revolution around the Sun, defining a year, not precession.
Option (c) 18.6 years refers to the nutation cycle, another wobble in Earth's axis, not precession.
Therefore, option (d) 26,000 years is correct as it matches the precession period mentioned in the passage.
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Question: 5
Gravitational action of the Sun and the Moon on Earth causes:
I. Diurnal spinning
II. Precession
III. Nutation
Gravitational action of the Sun and the Moon on Earth causes:
I. Diurnal spinning
II. Precession
III. Nutation
I. Diurnal spinning
II. Precession
III. Nutation
Show Hint
Differentiate between Earth's own rotational motion (diurnal spin) and long-term axis changes (precession and nutation) when identifying causes.
Updated On: Aug 5, 2025
- I only
- I and II only
- II and III only
- I, II and III
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The Correct Option is C
Solution and Explanation
The passage explains that the gravitational forces of the Sun and Moon cause two major slow changes in Earth's axis: precession and nutation.
Precession is the slow circular movement of Earth's axis with a period of about 26,000 years.
Nutation is a smaller oscillation superimposed on precession, with a period of about 18.6 years.
These two are directly linked to the gravitational pull of the Sun and Moon.
Option I — diurnal spinning — is caused by Earth's own rotation on its axis, not by gravitational forces of the Sun and Moon, so it is not included.
Thus, option (c) II and III only is correct because both precession and nutation are caused by the Sun and Moon's gravitational influence.
Precession is the slow circular movement of Earth's axis with a period of about 26,000 years.
Nutation is a smaller oscillation superimposed on precession, with a period of about 18.6 years.
These two are directly linked to the gravitational pull of the Sun and Moon.
Option I — diurnal spinning — is caused by Earth's own rotation on its axis, not by gravitational forces of the Sun and Moon, so it is not included.
Thus, option (c) II and III only is correct because both precession and nutation are caused by the Sun and Moon's gravitational influence.
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Question: 6
The orbital motion of the Earth:
The orbital motion of the Earth:
Show Hint
When distinguishing between Earth's different motions, note that diurnal spin is daily, revolution is yearly, precession is ~26,000 years, and nutation is 18.6 years.
Updated On: Aug 5, 2025
- is partly caused by the moon.
- can have uncertain rates.
- has a periodicity of 18.6 years.
- is neutralized by telescope mounting.
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The Correct Option is B
Solution and Explanation
The passage explains that the Earth's orbital motion can have small but important uncertainties in its rate due to variations in precession and nutation.
These uncertainties occur because the gravitational influences of the Sun and Moon change the orientation of Earth's axis over long periods, which slightly affects positional measurements.
Option (a) is incorrect — while the Moon influences precession and nutation, the Earth's orbital motion around the Sun is primarily caused by gravitational attraction to the Sun, not the Moon.
Option (c) is incorrect — the 18.6-year periodicity refers to nutation, not to the Earth's orbital motion.
Option (d) is incorrect — telescope mounting compensates for diurnal spinning, not for the Earth's orbital motion.
Therefore, the correct answer is (b) — the Earth's orbital motion can have uncertain rates due to long-term gravitational effects.
These uncertainties occur because the gravitational influences of the Sun and Moon change the orientation of Earth's axis over long periods, which slightly affects positional measurements.
Option (a) is incorrect — while the Moon influences precession and nutation, the Earth's orbital motion around the Sun is primarily caused by gravitational attraction to the Sun, not the Moon.
Option (c) is incorrect — the 18.6-year periodicity refers to nutation, not to the Earth's orbital motion.
Option (d) is incorrect — telescope mounting compensates for diurnal spinning, not for the Earth's orbital motion.
Therefore, the correct answer is (b) — the Earth's orbital motion can have uncertain rates due to long-term gravitational effects.
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Question: 7
The man-made radio signals have wavelengths of:
The man-made radio signals have wavelengths of:
Show Hint
Always match the exact numeric range mentioned in the passage to the options; wavelength ranges are precise in astronomy and physics questions.
Updated On: Aug 5, 2025
- more than 20 meters.
- less than one centimeter.
- between one centimeter and 20 meters.
- gamma rays.
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The Correct Option is C
Solution and Explanation
The passage states that at radio wavelengths between about one centimeter and 20 meters, the Earth's atmosphere has little effect, and man-made radio signals are the chief source of interference.
Option (a) is incorrect — while some natural radio waves can have wavelengths greater than 20 meters, man-made interference specifically mentioned falls in the one centimeter to 20 meters range.
Option (b) is incorrect — wavelengths less than one centimeter fall into the microwave region and are not identified as the primary interference range in the passage.
Option (d) is incorrect — gamma rays are high-energy electromagnetic waves, not radio waves, and are completely absorbed by Earth's atmosphere.
Thus, the correct answer is (c) — man-made radio signals have wavelengths between one centimeter and 20 meters.
Option (a) is incorrect — while some natural radio waves can have wavelengths greater than 20 meters, man-made interference specifically mentioned falls in the one centimeter to 20 meters range.
Option (b) is incorrect — wavelengths less than one centimeter fall into the microwave region and are not identified as the primary interference range in the passage.
Option (d) is incorrect — gamma rays are high-energy electromagnetic waves, not radio waves, and are completely absorbed by Earth's atmosphere.
Thus, the correct answer is (c) — man-made radio signals have wavelengths between one centimeter and 20 meters.
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