Which one of the following is a reason why astronomical distances are measured in light-years?
- Distances among stellar bodies do not change
- Gravity of stellar bodies does not change
- Light always travels in straight line
- Speed of light is always same
Explanation
Option (d) is correct
- For most space objects, we use light-years to describe their distance. A light-year is the distance light travels in one Earth year. One light-year is about 6 trillion miles (9 trillion km). The reason why astronomical distances are measured in light-years is because the speed of light is constant and very high. Light travels at a speed of approximately 299,792 kilometers per second (186,282 miles per second) in a vacuum. This speed is so immense that when we talk about the vast distances in space, using common units like kilometers or miles becomes impractical. Since the speed of light is same, if we know the time it takes for light to travel a certain distance, we can easily calculate that distance in terms of light-years.
