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Space Facts






Parsec & Light Year

How far is it to the stars?

Both "parsec" and "light year" are measures of distance — very large distances.

1 Light Year =   5.8786318 trillion miles
(9.4607 x 1012 kilometers)
1 Parsec =   19.173745 trillion miles
(30.857 x 1012 kilometers)

[that's American "trillion," not British; i.e. 1012 (=1,000,000,000,000)]

Both distances are based at least partly on the size of Earth's orbit about Sol (our sun).

Light year, for one, is based on the amount of time it takes Earth to make one trip in its orbit about our sun. In that amount of time, light, traveling at more than 186,000 miles per second, will have traveled exactly one light year.

For that matter, one "light second" would be roughly 186,000 miles, and one "light day" would be more than 16 billion miles — or almost four times the average distance of Pluto from our sun. Indeed, one "light year" is a very large distance. The two main stars of our closest neighbor, Alpha Centauri, are about 4.34 light years from Earth and our sun.

Likewise, parsec is tied to Earth's orbit, but as a surveying baseline. Astronomers use two opposite points in Earth's orbit in order to calculate distances to nearby stars.

With a simple experiment you can easily see the effect. Hold your pointer finger about a foot in front of your face and look across the room through first one eye then the other. If your finger remains stationary, it still appears to jump against the more distant background. This apparent shift is called "parallax."

For even the closest stars, the apparent shift is very slight. With angles in the sky measured in degrees, minutes and seconds, Alpha Centauri is said to shift by (or have a parallax of) less than a second of arc. And that's where the term "parsec" comes from: an object with a six month shift in parallax of one second of arc is said to be at a distance of one parsec. Alpha Centauri, our next-door neighbor, is roughly 1.33 parsecs from Earth.

Conversion:

To convert from parsecs to light years, multiply the value in parsecs by 3.2616.

The display above simulates the apparent shift of a nearby star when seen from two widely spaced viewpoints. Even though the distant, bluish white star has not moved, it appears to move against a field of more distant stars. The amount of apparent motion, or "parallax," can be used to measure the distance to the star, if the length of the "baseline" is known. This needed length is the distance between the two viewing points. The baseline astronomers use is 186 million miles long — the diameter of Earth's orbit, or two views from Earth, six months apart.

Examples:

How far are the stars? As stated above, the closest neighboring star is about 4.34 light years distance. Some of the brightest stars in our sky (like Rigel, Betelgeuse and Deneb) are several hundred light years away. The Andromeda galaxy — visible as a hazy patch in the northern skies — is 2.54 million light years distance. And the farthest galaxies are several billion light years away.

For another unit used in astronomy for distance measure, compare Astronomical Unit.

References:
Astronomy Data Book, by J.H. Robinson & J. Muirden — John Wiley & Sons, New York
A Field Guide to the Stars and Planets, by D.H. Menzel — 1964, Houghton Mifflin Company, Boston