Astronautics Questions and Answers – Earth-Centered to Sun-Centered Universe

This set of Astronautics Multiple Choice Questions & Answers (MCQs) focuses on “Earth-Centered to Sun-Centered Universe”.

1. Who among the following great minds believed that Earth was the center of the universe?
a) Ptolemy
b) Copernicus
c) Aristarchus
d) Galileo
View Answer

Answer: a
Explanation: Ptolemy was a strict advocate of the geocentric model of the universe. He, in fact, came up with a ‘circles-on-circles’ model (known as the ‘Deferent Epicycle’) in order to explain the observed motions of the heavenly bodies. When observations did not agree with geocentricity, Ptolemy added more and more complexity to the model to reconcile the two.

2. Which of the following observations can be explained using the geocentric model of the universe?
a) The phases of Venus
b) The apparent retrograde motion of Mars against the background of stars
c) The apparent motion of the stars
d) The apparent retrograde motion of the outer planets beyond Mars
View Answer

Answer: c
Explanation: The phases of Venus and the seemingly ‘backward’ drift of Mars (as well as the other planets beyond Mars) are difficult to explain using geocentricity. As far as Venus is concerned, the planet is usually fully lit up when it appears close to the sun in the sky. But according to the Earth-centered universe, where Venus orbits closer to Earth than the sun, this is impossible (due to the relative geometry between our planet, Venus and the sun). Geocentricity dictates that Venus would shine brightly as viewed from the Earth only when there is considerable angular separation between the sun and Venus. But in reality, this never happens.
Mars’ apparent retrograde motion is an illusion that occurs due to the fact that Mars, along with Earth, orbits the sun, but in an orbit that lies farther out than ours. Since our planet lies closer in towards the sun than Mars, Earth moves with a higher orbital velocity and races ahead of Mars, which makes the red planet appear to trace a retrograde path (similar to how a car seems to lag behind when a faster car overtakes it). This is near-impossible to justify in the geocentric model, according to which Mars should always seem to move prograde.
The motion of the stars is perhaps one of the few observations that go well with the Earth-centered Universe. Stars are frighteningly distant and, unlike the planets that lie in our near vicinity, remain fixed in the background. For this reason, their apparent circular motion around us, which occurs due to Earth’s rotation, does not change whether we live in a heliocentric or geocentric universe.

3. Who among the following finally erased the geocentric model of the universe and firmly established heliocentricity backed up by experimental data as well as mathematical laws?
a) Tycho Brahe
b) Johannes Kepler
c) Isaac Newton
d) Nicolaus Copernicus
View Answer

Answer: b
Explanation: Johannes Kepler, using observational data gathered by Tycho Brahe, restored faith in the sun-centered model and formulated three laws, known as Kepler’s laws of planetary motion, which are fundamental to the field of astrodynamics and spaceflight.

4. Kepler’s first law states that _________
a) all planets revolve in circular orbits with the sun at the center
b) all planets revolve in elliptical orbits with the sun at the center
c) all planets revolve in elliptical orbits with the sun located at one of the foci
d) all planets revolve in parabolic orbits with the sun at the center
View Answer

Answer: c
Explanation: According to Kepler’s first law, all planets revolve around the sun in elliptical orbits, with the sun located at one of the two foci of the ellipse (a ‘stretched’ circle) traced out by each planet.

5. Kepler’s second and third law implies that _____________
a) the more distant a planet is from the sun, the faster it revolves
b) the closer the planet is from the sun, the slower it revolves
c) the more distant a planet is from the sun, the slower it revolves
d) how quickly a planet revolves around the sun is constant, independent of its distance from the sun
View Answer

Answer: c
Explanation: Kepler’s second law states that the line joining a planet and the sun sweeps out equal areas in equal intervals of time. This law arises from conservation of angular momentum. Now let’s say a planet ‘P1’ lies closer in towards the sun and the planet-sun (P1-S) line sweeps out an area ‘A’ in time ‘t’. Another planet P2, which revolves farther out, is clearly associated with a longer planet-sun line, i.e., P2-S > P1-S. So in order for P2-S to sweep out the same area ‘A’ in time ‘t’ as was done by P1-S, the planet P2 must orbit slower than P1.
Kepler’s third law states that the square of the period of revolution of a planet around the sun is proportional to the cube of the semi-major axis of the ellipse traced out by the planet. If ‘T’ is the period of revolution and ‘a’ the semi-major axis, then T^2∝a^3. Clearly, as ‘a’ increases, ‘T’ also increases, meaning that the more distant a planet is from the sun, the longer it takes to complete one revolution, and vice-versa.

Sanfoundry Global Education & Learning Series – Astronautics.

To practice all areas of Astronautics, here is complete set of Multiple Choice Questions and Answers.


If you find a mistake in question / option / answer, kindly take a screenshot and email to [email protected]

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Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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