21. distances in space are vast. why is it necessary to use units, such the light - year and astronomical…

Brief Explanations:

Distances in space are extremely large. Using regular units like meters or kilometers would result in very large and inconvenient - to - handle numbers. A light - year is the distance light travels in one year in a vacuum, about 9.461×10¹² kilometers. For example, the distance from the Earth to Proxima Centauri, the closest star to our solar system, is about 4.24 light - years. This makes it easier to express and understand cosmic distances compared to using kilometers. Astronomical units (AU) are also used for distances within the solar system, where 1 AU is the average distance from the Earth to the Sun (about 149.6 million kilometers). They simplify discussions about the scale and characteristics of the universe.

Answer:

It is necessary to use units like light - year and astronomical unit because space distances are vast. Using normal units would lead to extremely large numbers that are hard to work with. For example, the distance to Proxima Centauri is about 4.24 light - years.