Light travels at a constant speed of about 300,000 km/s. this speed is referred to as the speed of light. a light year is the distance that light travels in one year. one light year is equal to 9.46 trillion kilometers. the closest star to earth, other than the sun, is proxima centauri. it takes light from this star about 4.2 years to reach earth. how far from earth is the star proxima centauri? a. 4.2 light years b. 9.46 trillion km c. 300,000 light years d. 300,000 km.