# 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 × 105 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 × 1012 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 × 108 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 × 1051,5 × 108 ≈ 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 × 108 km/year
4.24 × 9.46 × 10125.37894 × 108 ≈ 7.46 × 104 ≈ 74 600 years