Distance Measure

In astronomy, the most commonly used measures of distance are the light year, parsec and astronomical unit.

Astronomical Unit (A.U.)
An astronomical unit (A.U.) is the average distance between the Earth and the Sun, which is about 93 million miles or 150 million kilometers. Astronomical units are usually used to measure distances within our solar system. For example, the planet Mercury is about 1/3 of an A.U. from the Sun, while the farthest planet, Pluto, is about 40 A.U. from the Sun (thats 40 times as far away from the Sun as the Earth is).

Light-Year (ly)
Most objects in space are so far away, that using a relatively small unit of distance, such as an astronomical unit, is not practical. Instead, astronomers measure distances to objects which are outside our solar system in light-years. A light-year (ly) is the distance that light can travel in one year in a vacuum (empty space). The speed of light is about 186,000 miles or 300,000 kilometers per second. So, in one year light travels a distance of about 5,880,000,000,000 miles or 9,460,000,000,000 kilometers or 63,240 A.U.. This distance is 1 light-year. For example, the nearest star to us is about 4.3 light-years away. Our galaxy, the Milky Way, is about 150,000 light-years across, and the nearest large galaxy, Andromeda, is 2.3 million light-years away.

Parsec (pc)
Astronomers use another unit of distance called a parsec (pc), which is equal to 3.26 light-years. The definition of the parsec is based on a triangle. A parsec is the length of the long leg of a right triangle, whose short leg is one astronomical unit when the angle between the Sun and the Earth, as seen from an object in space (a star for example), is one arcsecond The word parsec stands for "parallax of one arc second" One kiloparsec (kpc), is one thousand parsecs. One megaparsec (Mpc) is one million parsecs.

SUMMARY

1 Astronomical Unit (A.U.) is the average distance between the Sun and the Earth.
1 A.U. = 93 million miles = 150 million kilometers

1 light-year (ly) is the distance that light travels in one year in a vacuum.
1 ly = 5,880,000,000,000 miles = 9,460,000,000,000 kilometers = 63,240 A.U

1 parsec (pc) = 3.26 light-years