# A) How many AU are there in 1.00 light year? B) What is the speed of light in AU/h?

An astronomical unit (AU) is the average distance from the sun to earth, 1.50 x 10^8 km.

• A)

1 l-y = 63,239.8 au

(Taken directly from the Observer's Handbook of the Royal Astronomical Society of Canada)

B)

first, as an approximation:

Speed of light = 300,000 km/s

300,000 km/s * 3600 s/h = 1,080,000,000 km/h

1 au = 150,000,000 km

Speed of light = 1,080,000 000 km/h / 150,000,000 km/au

= 7.2 au/h (astronomical units per hour).

using more precise numbers:

Speed of light = 299,792,458 m/s (by definition).

= 1,079,252,848,800 m/h (metres per hour)

1 au = 149,597,871,500 m

Speed of light in au/h:

7.214...

• There are about 206,000 AU is a parsec and 3.26 light years in a parsec. So, dividing, you get about 63,000 AU is a light year.

Light goes 186000 miles per second and one AU is 93 million miles, so 1 AU is 500 light seconds. That means that light travels 7.2 AU in 1 hour.

• 63,239.73 AU ≈ 1 light year

The speed of light 186,000 mile per second