### Celestial measures

#### Solar system

Distances in the solar system can be measure in Astronomical Units (AU).

1 AU = 149 597 870.700 kilometres.

An astronomical unit is the mean distance between the Earth and the Sun. In 2012, the International Astronomical Union defined the distance to be 149 597 870 700 metres.

#### Light-years

Outside the solar system, the light-year is used to measure distance.

A light-year is the distance that light travels in one year ($$365\dfrac{1}{4}$$ days). Scientists use it to measure the distance between galaxies and stars.

The light-year is 9 460 000 000 000 000 metres = $$9.46×10^{15}$$ m.

The following table gives the distances between various celestial bodies.

Distances between celestial bodies
Bodies Distance Time taken by current space ship
Earth – Sun 1.48 × 10$$^8$$ km $$\approx$$ 1 AU 7 months
Width of our solar system 79 AU 21 000 years
Earth to Mars by spaceship 1.25 $$\pi$$ AU $$\approx$$ 3.9 AU 7.5 months
Earth to closest star 4.2 light years 70 000 years
Diameter of the Milky Way 100 000 light years