Astronomical Distances

Distance  in space is so large that astronomers had to come up with a unit that best describes these distances in terms we can relate. Within our solar system, we use Astronomical Units to measure distances between the planets and the Sun. We relate these distance to the distance of the Earth from the sun. The Earth is about 149 599 000 km away from the Sun. Based on this scale of astronomical units, the Earth is said to be 1 astronomical unit (A.U.) away from the Sun. Mars, on the3 other hand, is about 228 000 000 km away from the Sun. In terms of astronomical units, it is (228 000 000 divided by 149 599 000 km) 1.5 A.U.'s away from the Sun.

Light years are used to measure even greater distances in space. It is the distance light travels in one year. It works out to be about 63 240 A.U. In space, light can travel about 300 000 km in one second. In one year, it can travel 9 460 000 000 000 km. Since this number is quite large, we use scientific notation to write this number out  (9.46 x10 12 km).

The closest star to our solar system is Alpha Centauri. It is about 4.3 light years away. This tells us that it took 4.3 years for the light from that star to reach us. It also tells us that the light we receive is already 4.3 years old. In essence, we are looking at Alpha Centauri as it was 4 years ago. We are actually able to look back in time.

In relation to our Sun, the light that we receive takes 8 minutes to get here. When we look at our sun, we are seeing it as it was 8 minutes ago.