# Arithmetic/Geometry » Astronomical measurements

### Speed of light

The speed that light travels. The abbreviation for light speed is `c`.

299 792 458 m/s ≈ 3,0 × 10^{5} km/s

### Light year

The distance travelled by light in one year. The abbreviation for light year(s) is `ly`.

9 460 730 472 580 800 m ≈ 9,46 × 10^{12} km

### Astronomical unit

The average distance between the earth and sun is called the astronomical unit. The abbreviation for astronomical unit is `AU`.

149 597 870 700 m ≈ 1,5 × 10^{8} km

### Miles and kilometres

1 mile = 1.609344 km

1 km = 1.609344^{–1} miles = 0.621371192... miles

*Example 1*

The space probe Voyager 1 is the furthest man made object in space and was launched in 1977.

On August 2, 2011 it has travelled 16.367 light hours.

What is this distance in `AU'`s?

Answer:

16.367 light hours1^{ }`AU` = 16.367 × 60 × 60 × 3,0 × 10^{5}1,5 × 10^{8} ≈ 117.84 AU

*Example 2*

The speed of Voyager 1 at that moment was 61 400 km/h.

How long would it take to reach the closest star at that speed, if it were traveling to that star. The closest star is Proxima Centauri, at a distance of 4.24 light years.

Answer:

61 400 km/h = 61400 × 24 × 365 = 5.37864 × 10^{8} km/year

4.24 × 9.46 × 10^{12}5.37894 × 10^{8} ≈ 7.46 × 10^{4} ≈ 74 600 years