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Which one of the following is a reason why astronomical distances are measured in light-years?
(a) Distances among stellar bodies do not change.
(b) Gravity of stellar bodies does not change.
(c) Light always travels in straight line.
(d) Speed of light is always same.



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Astronomical distances are measured in light-years because the speed of light is constant. Light travels at a speed of approximately 299,792 kilometers per second (or about 186,282 miles per second) in a vacuum. Since the speed of light is incredibly fast and remains constant, astronomers use the term "light-year" to measure vast distances in space. A light-year is defined as the distance that light travels in one year. It provides a convenient unit of measurement for astronomical distances, considering the immense size and scale of the universe. Therefore, the speed of light being constant is the reason why astronomical distances are measured in light-years.


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