. Name the unit which is used to express distances between the various celestial bodies (like stars and planets).

The unit commonly used to express distances between various celestial bodies, such as stars and planets, is called the "light-year." A light-year is defined as the distance that light travels in one year in a vacuum, which is approximately 5.88 trillion miles or about 9.46 trillion kilometers. This unit is particularly useful in astronomy since the vast distances involved are more comprehensible when expressed in terms of the time it takes for light to travel these distances, rather than using conventional measurement units like kilometers or miles.

Another unit that is often used is the "parsec," which is equivalent to about 3.26 light-years and is primarily used for measuring distances to stars and galaxies. Both of these units help astronomers convey the immense scales of the universe more understandably.