The speed of light, approximately 299,792 kilometers per second, is a constant in the universe, making it a reliable unit for measuring vast astronomical distances. By defining a light-year as the distance light travels in one year, astronomers can effectively express the immense scales of the universe in a comprehensible manner. This consistent speed allows for standardized comparisons of distance across the cosmos, which is essential for studies in astrophysics and cosmology as covered in NCERT and UPSC reference materials.