The speed of light in a vacuum is exactly 299,792,458 m/s. This speed is sometim

Question

The speed of light in a vacuum is exactly 299,792,458 m/s. This speed is sometimes used to provide a convenient yardstick for large astronomical distances. (a) A light-second is the distance light travels in one second. If the Moon is 3.84×108 m from the Earth (center to center), how many light-seconds away is it? (b) A light-minute is the distance light travels in one minute. If the Earth is 1.50×1011 m from the Sun, how many light-minutes away is it? (Your answer also represents the number of minutes it takes for light to reach the Earth from the Sun.) (c) A light-year is the distance light travels in one year. The Sun is 28,000 light-years from the center of the Milky Way galaxy. What is this distance in meters?

Explanation / Answer

Speed of light = 299,792,458 m/s
Distance of moon = 3.84×108 m
Divide speed of light into distance of moon: 3.84×108 m / 299,792,458 m/s = 1.28 light-seconds

Distance of sun = 1.50×1011 m
# of seconds in minute: 60 s/min
Divide speed of light into distance of sun, then divide by 60 s/min: (1.50×1011 m / 299,792,458 m/s) / 60 s/min = 8.34 light-minutes

Distance of sun to center of Milky Way: 28,000 light-years
# of seconds in year: 31,536,000 s/y
Distance of sun in meters = (28,000 light-years)(31,536,000 s/y)(299,792,458 m/s) = 2.65x1020m

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