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What Is a Parsec?

One of the units of measurement used in astronomy is the parsec. It is equivalent to 3.26 light years in distance. This is interchangeable with the distance a photon will travel in a vacuum for a period of 3.26 years (both definitions basically describe the same thing and the same event). With light traveling at 186,000 miles per second, a parsec basically represents the distance of a little over 19 trillion miles.

To put things in perspective, the distance of the Sun to the Earth is only 93 million miles, which is also called an astronomical unit. To cover one parsec, one will have travel 103,000 round trips to the sun. The solar system, when measured from the center to the edge of Pluto’s orbit is only 1/800th of a light year across.

The calculation of a parsec uses the parallax of 1 arc second. This is the reason why the short name for this unit of measurement is a parsec.

To further explain, the parallax is the perceived motion of a fixed object along an angular trajectory because of the change in the position of the observer. On the other hand, an arc second is the angular measurement that is equivalent to 1/60th of an arc minute or 1/3600 of a single degree of arc.

In order to observe the parallax effect, an object is photographed in a fixed position on Earth with the stars as a background. When the Earth has traveled halfway through its orbit (after six months), another photograph is taken. Scientists measure the distance an object “moved”, the arc second of the parallax can be measured in order to reveal distance.

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