# Astronomical Distances

A parsec is the distance at which a star would have a parallax of one second of arc:

1 parsec = 1 pc = 3.26 light years = 3.09 x 1018 cm = 206265 A.U.

So what is parallax? Parallax, or trigonometric parallax, is a way in which astronomers can measure distances to relatively nearby objects. If they are measuring stars, they measure the relative change of the position of the nearby star with respect to distance background stars over the course of a year. In half a year, the earth is 300,000,000 km from where it started. Because we are viewing the stars from different places, the nearby stars appear to change position. This change in position is measured as a change in angle and is called the parallax angle. When the star's change in angle is one arc second, or .000278 degrees, the distance to the star is defined as a parsec. It is usually abbreviated as pc. In many instances you will see the abbreviations, kpc and Mpc, these stand for kiloparsec and megaparsec respectively. A kiloparsec is a thousand parsecs, or 103 parsecs. A megaparsec is a million parsecs, or 106 parsecs.

An astronomical unit is the average distance of the earth from the sun. Astronomical unit is abbreviated as AU.

1 A.U. = 1.5 x 1013 cm = 4.85 x 10-6 pc = semimajor axis of earth's orbit.

A Lightyear is the distance that light travels in a year. Since light's speed in a vacum is 3 x 108 m/s...

1 light year = 9.46 trillion kilometers = 9.46 x 1012 km = 63,292 A.U.

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