What are parsecs, how they measure long distances in space

(ORDO NEWS) — A parsec is a unit of distance, approximately 20 trillion (20,000,000,000,000) miles, 31 trillion kilometers, or 206,264 distances from the Earth to the Sun.

Also, a parsec is equivalent to 3.26 light years (the distance you would cover in 3 years and 3 months if you could reach the speed of light).

What does the word “parsec” mean?

The term “parsec” is an abbreviation of the words “parallax” and “second”. In this case, a second is understood not as a unit of time, but as a unit of measurement of flat angles, that is, an angular (or arc) second.

Parallax is a meter by which the change in the position of a space object relative to the observer is determined. In astronomy, diurnal, annual and secular parallax are distinguished.

With daily parallax, the difference in the direction to the celestial body from some given point on our planet and from the center of mass of the globe is calculated. The annual parallax indicates the same parameters, but taking into account the rotation of the Earth around the Sun, and the secular parallax allows you to determine the difference relative to the observer, taking into account the proper movements of the observed object in the galaxy.

Invention of the parsec

For the first time, an astronomer from Germany, F. W. Bessel, began to study the distances between objects in space. He began his work in 1838.

As a result of long work, he was able to determine the annual parallax for the star 61 Cygnus.

During the calculations, the scientist used the well-known and old astronomical method, according to which the distance to the star could be indicated in the difference in angles.

To do this, you need to conduct 2 studies and measurements.

First, the indicators were measured when the Earth was turned to the Sun on one side.

The next measurements were carried out 6 months later, when the planet turned to the sun on the other side.

The term “parsec” was coined by an astronomer from the UK – G. H. Turner. He did this at the beginning of the last century.

Who Invented the Parsec?

The first successful measurements of distances to space objects were made by the German astronomer Friedrich Wilhelm Bessel in 1838. Then, for the first time in history, he managed to perform reliable calculations of the annual parallax for the star 61 Cygnus.

In his work, the scientist used one of the oldest methods of astronomy, according to which the difference in angles after two measurements was recorded to calculate the distance to the star.

First, measurements were taken when the Earth was on one side of the Sun, and then the same indicators were measured six months later, when it turned the other side to the Sun. The term “parsec” was coined by the British astronomer Herbert Hall Turner in 1913.

What is a parsec?

Unfortunately, many people have very poor knowledge in astronomy and physics. This does not make them any worse than the rest. Just how great it would be if every person on Earth could sing and draw and calculate logarithms. So, what are we talking about… A. Well, that’s it. Some citizens believe that a “light year”, like a parsec, is a unit of time. However, this is not at all the case.

One parsec is equal to a distance of about 3.26 light years. Or about 31 trillion kilometers. But if a light year is used to determine cosmic distances, that is, the distance that light travels in one year, then why did they come up with a parsec? The thing is that a terrible secret is involved here (no, of course not). In fact, this happened due to the use of one of the very first methods for determining the distance to stars.


Using the principle of parallax, it is possible to calculate the distances to some of the stars closest to us. You just need to measure a tiny shift in their position. To do this, their location in the sky is fixed, for example, in spring and autumn. That is, in those times of the year when the Earth is at opposite points of its orbit around the Sun. And then, based on the detected displacement, the distance to them is calculated. It’s actually quite primitive geometry. But this method works only for relatively close stars.

One parsec is the distance an object must be removed in order for its annual trigonometric parallax to be one arc second. Our nearest star, Proxima Centauri, is about 1.3 parsecs from the Sun. And our Milky Way galaxy is over 31,000 parsecs across.

And yes. The parsec is an off-system unit. Most often, astronomers measure cosmic distances in astronomical units …

Examples of cosmic distances

From the moment the parsecs appeared, scientists have been able to calculate the distances to most cosmic bodies that are not only in the Milky Way, but throughout the Universe.

As you know, the closest star to Earth is Proxima Centauri. It is located at a distance of 1.3 parsecs from the Sun. It is also removed from the galactic center by 8 kiloparsecs.

Between Proxima Centauri and the Andromeda Nebula, the distance is 0.77 megaparasec.

The total size of the Milky Way is about 30 kiloparsecs. Between the Earth and the edge of the universe, which is visible in the night sky, is about 4 gigaparsecs.

As you can see, the parsec is a very convenient unit of measure. It is useful to know what a parsec is in kilometers and light years.

This facilitates the process of calculating the distances between any celestial bodies that are in space.

Often it is used when calculating the distance between bodies in the Milky Way.

Given that technology is constantly evolving, soon scientists will be able to study and calculate the distances between different universes.

The disadvantage of the parsec is that only experienced astronomers can work with it. It is difficult for an ordinary person to make calculations.

Methods for studying distances in the universe


  • parallax method
  • Laser measurements
  • Radar measurements


  • Standard candles
  • Measuring rods
  • Hubble law
  • And about 30 more…


In astronomy, parallax is defined as the phenomenon of a visible change in the position of an object on the celestial sphere in relation to other objects as a result of a change in the observation location caused by the displacement of the observer.

Parallax is an apparent change in the position of an object as a result of the movement of the observer, that is, a change in its position.

The concept of parallax works in various systems:

  • diurnal parallax (geocentric equatorial parallax) associated with the Earth’s rotation
  • meridional geocentric parallax associated with a change in the latitude of the observer
  • annual parallax (heliocentric parallax) associated with the Earth’s orbit around the Sun

Parsec Definition

In a situation where the parallax angle is fixed, the distances to the star can be determined using the principles of trigonometry (since the distance from the Earth to the Sun is known). The distance from the Sun to the star with a parallax angle of 1 arcsecond was adopted as the unit of measurement, and Turner named it the parsec in 1913.

This name was created by combining the words parallax and second. The concept of parallax is associated with the rotation of the Earth (daily parallax), with a change in the observer’s latitude (geocentric meridian parallax), or with the Earth’s movement in orbit around the Sun (annual parallax). The definition states that 1 parsec (pc) is the distance from which half of the major axis of the earth’s orbit is visible as an arc 1 second long. This distance is the reciprocal of the body’s heliocentric parallax. Parsec is now abbreviated as pc, formerly as ps. The abbreviation ps is omitted today because it is the same as the picosecond abbreviation.

Light year (LY)

Light can only travel at a limited speed, which is 299,792,458 m/s. Thus, it covers a distance of about 300,000 km per second. Multiplying this distance by the number of days in a year (365 days), we get that in a year light travels a distance of 946,800,000,000 km. This value was defined as 1 light year.

Conversion table from Parsecs to Kilometers

Parsec Kilometer

3.09*10131.54 * 10143.09*10147.71*10141.54 * 10153.09*10157.71 * 10151.54 * 1016

Conversion table from Astronomical units to Parsecs

Astronomical unit Parsec

4.84824.24248.485121.212242.423484.8471 212.1172 424.234

Parsec to Astronomical Units Conversion Table

Parsec Astronomical unit

206 250.7561 031 253.7802 062 507.5595 156 268.8981.03*1072.06*1075.16*1071.03*108


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