Parsec Definition: Lesson for Kids

Instructor: Teresa Toller

Teresa has taught middle school math and science for 30 years. She has a masters degree in technology in education.

When studying astronomy, scientists needed to find a unit for the long distances in outer space. The units we use on Earth just aren't adequate enough! In this lesson, we will discover how the measurement parsec was developed to accommodate that need.

Distances in Outer Space

When looking at the sky at night, have you ever wondered how far away a star is or what unit of distance would you use to measure that distance? Using units that are used on Earth, like kilometers or miles, you would end up with VERY large numbers!

What instrument would you use to measure that distance? You certainly can't measure distances in outer space with a ruler nor can you attach a long tape measure to a space shuttle!

Astronomers developed a way to calculate distances in outer space by using reference points. Since we live on Earth, the Earth is used as one reference point. The other stars that you can see in the background around the one star you are wondering about are the other reference points.

Let's explore how astronomers, the scientists who study the planets and stars, measure these distances: the parsec.

Development of the Parsec

The parsec was developed by a British astronomer, Herbert Hall Turner, in 1913 in order to make it easier for astronomers to calculate distances in outer space. Today, the primary use for the parsec is to measure distances between the earth and stars. This method can also be used to calculate the distances between galaxies.

Naming the Measurement

The word 'parsec' is a combination of the words parallax and arcsecond. In order to truly understand what a parsec is, you first have to understand those two root words.


The 'par' in 'parsec' is an abbreviation of the word parallax. Parallax is the apparent change in location of any nearby star when the star is viewed against the background of more distant stars. To find parallax, you observe and record the star's position when the Earth is at one point of its orbit around the Sun. Then about six months later, when the Earth is at its opposite point in orbit around the Sun, you observe and record the same star's position when viewed against the same background stars. The difference between those two measurements creates an angle. Half of that angle is the star's parallax. Parallax is measured in arcseconds.

Example of Parallax
Parallax image


The 'sec' in the word 'parsec' is an abbreviation of the word arcsecond. An arcsecond is a very small unit used to measure angles. In your mathematics class, you've probably used a protractor to measure the size of an angle on a piece of paper. Protractors usually measure an angle in degrees. The arcsecond is 1/3600 of a degree, so a 90 degree angle (a right angle) is equal to 324,000 arcseconds! Since the angles of parallax are very thin right angles, arcseconds are the appropriate unit of measurement.

Math Protractor
Protractor image

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