Many of us first heard about the parsec from the cartoon “The Secret of the Third Planet,” in which brave astronauts easily overcome long distances in space.
And although this word is firmly etched in the memory, not everyone knows what it means. What is parsec? How far did the cartoon characters have to fly?
What does the word "parsec" mean?
Term "parsec" is an abbreviation of words "parallax" And "second" . Under the second in in this case understand not a unit of time, but a unit of measurement of plane 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, a distinction is made between daily, annual and secular parallax.
With daily parallax, the difference in direction to a celestial body from some given point on our planet and from the center of mass globe. The annual parallax indicates the same parameters, but taking into account , and the secular parallax allows us to determine the difference relative to the observer, taking into account the proper movements of the observed object in the galaxy.
What is parsec?
In simple terms, a parsec is a unit of measurement that determines the distance between celestial bodies beyond solar system.
The most common use of parsec is for measurements inside Milky Way. If it is necessary to establish distances on the scale of the Universe, multiple parsecs are used, that is, kiloparsecs (1000 parsecs), megaparsecs (million parsecs), gigaparsecs (billion parsecs).
This astronomical unit not only performs practical function, but also adds convenience to astronomers. It is much simpler to say that the distance from the Sun to the star is 1.5 parsecs, rather than 46.27 trillion kilometers.
Who invented 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 was able to perform reliable calculations of the annual parallax for the star 61 Cygni.
In his work, the scientist used one of oldest methods astronomy, according to which, to calculate the distance to a star, the difference in angles after two measurements was recorded. 
First, measurements were taken when the Earth was one side facing 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 British astronomer Herbert Hall Turner in 1913.
What is a parsec?
The annual parallax is used to calculate the parsec. To determine the distance to an object, astronomers build an imaginary right triangle, where the hypotenuse indicates the distance of the celestial body to the Sun, and the leg indicates the semi-axis earth's orbit. Size acute angle in this triangle is the annual parallax. Parsec in this case is the distance to a star whose parallax is 1 arcsecond.
In addition to parsecs, to measure the distance between space objects kilometers and light years are used. The relationship between all these units of measurement has long been calculated: 1 parsec is equal to 3.2616 light years or 30.8568 trillion kilometers. The symbol “pk” is used to designate a parsec in Russian, and “rs” in English.
Examples of distances in space
Since the advent of parsecs, astronomers have been able to calculate distances to many cosmic bodies and in the Universe. Thus, the distance from the Sun to the nearest star Proxima Centauri is 1.3 parsecs, to the center of the galaxy - approximately 8 kiloparsecs, to the Andromeda nebula - 0.77 megaparsecs. 
The total diameter of the Milky Way reaches about 30 kiloparsecs, and the distance from our planet to the observable edge of the Universe is approximately 4 gigaparsecs.
In the section on the question What is 1 parsec equal to? given by the author chevron the best answer is 3.2616 light years
Source: wikipedia.org
Answer from Lysander[newbie]
1 light year. I don't remember exactly, but one light second this is the distance from the Earth to the Moon, so you can look it up in a reference book and calculate how much it will be per year))
Answer from Plane[newbie]
The distance a beam of light travels at a speed of 300,000 km/sec in one year.
Answer from AB[guru]
Parsec (abbreviated pc) is a non-systemic unit of distance measurement common in astronomy. The name comes from parallax arcsecond and denotes the distance to an object whose annual trigonometric parallax is equal to one arcsecond. According to another equivalent definition, a parsec is the distance from which average radius Earth's orbit (equal to 1 AU), perpendicular to the line of sight, visible at an angle of one arcsecond (1″).
Answer from Help[guru]
A light year is the distance that light travels in a year.
Light travels from the Earth to the Moon in a little more than a second.
A parsec is the distance at which the Earth is visible at an angle of one second (1/3600 of a degree). I don’t remember exactly, it’s a little more than 3 light years.
Answer from Larisa Krushelnitskaya[guru]
A parsec is the distance from which the semimajor axis of the Earth's orbit appears at an angle of 1 arcsecond. That is
1 parsec = 1 astronomical unit / sin 1”
sin 1” = π/(180 60 60) = 1/206264.806
1 parsec = 206264.806 astronomical units =
= 206264.806 149 597 870.691 km = 3.08567758 10^13 km
Answer from Dmitry(C.)[guru]
1 parsec (parallax/second) is the distance at which an object has a parallax of 1 arcsecond. In one parsec 3.26 light years, or 206,265 astronomical units (distance from the Earth to the Sun), or 31 trillion kilometers (3.1 * 10 to the thirteenth power).
Image source: mattbodnar.com
Because of its uniqueness, every person who watched this cartoon remembered this word.
“It’s not far here, a hundred parsecs!” - thus Gromozeka, one of the heroes of “The Secret of the Third Planet,” reported the distance to the planet to which he recommended Prof. Seleznev and his team.
However, few people know what exactly parsec means, what distance we're talking about and how far the characters of the popular cartoon were forced to fly.
Meaning of the term "parsec"
This term was derived from the words "parallax" And "second", which here represents not a unit of time, but an arc second - an extra-system astronomical unit, which is identical to a plane second.
Parallax is a change in the location of a celestial body depending on where the observer is located.
Modern astronomy distinguishes the following types of parallax:
Daily– the difference in directions to a certain star in both the geocentric and topocentric directions. This angle directly depends on the height of the celestial body above the horizon.
At annual parallax changes in direction to a certain object directly depend on the rotation of the Earth around the Sun.
Concerning secular parallax, then it makes it possible to determine the difference in the direction of a celestial body depending on its movements in the Galaxy.
Parsec - meaning of the term
If we talk accessible language, then “parsec” is a unit of change in the distance between celestial bodies located outside the Solar System. Typically, parsec is used to calculate the distance within the Milky Way. These are basically multiple units: kiloparsecs, megaparsecs And gigapersecs. Submultiple units are usually not used because it is more convenient to use standard astronomical units instead.
Parsec greatly simplifies calculations for astronomers, because it is much easier to say that the distance from the Sun to a certain star is one and a half parsecs than it is more than 46 trillion km.
Who invented parsec?
in 1838, the German Friedrich Bessel was the first to achieve success in measuring the distances to objects in space. He was the first to produce accurate calculations Cygnus stars 61 annual parallax. To calculate the distance from this star, Bessel used the old method, calculating the difference in angles resulting from two measurements.
Determining the distance to stars using the parallax method. Image source: bigslide.ru
First, measurements were taken with the Earth facing the Sun on one side, and six months later repeated measurements were taken (with the Earth facing the Sun on the other side).
However, the term “parsec” itself appeared only in 1913 thanks to the English astronomer Herbert Turner.
How is parsec calculated and what is it equal to?
Schematic representation of a parsec (not to scale) Image source: wikipedia.org
One parsec is defined as the distance at which one astronomical unit (the average distance between the Earth and the Sun) represents the angle of one arc-second.
The annual parallax is used to calculate the parsec. When using an imaginary triangle with right angles, parsec is the distance to the star, provided that its parallax is 1 arcsecond.
A parsec is 3.26 light years or about 30 trillion km. It represents one of the first ways to determine distances to stars and is designated as "pc"
The essence of parsec is to use the principle of parallax to determine the distance to celestial bodies in space due to their tiny shift as the Earth moves around the Sun.
Some distances to space objects in parsecs:
The distance to the star closest to the Sun, Proxima Centauri, is 1.3 parsecs.
The distance from the Sun to the center of the Milky Way is about 8 kiloparsecs.
The distance from the Sun to the Andromeda nebula is 0.77 megaparsecs.
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How simpler words, the more there are. I warned you - now don't complain!
The Earth has an elliptical orbit. An ellipse, unlike a circle, does not have a “radius”, but has two “semi-axes” of different lengths - major and minor. Accordingly, there are two points in the earth's orbit that lie on the major axis and are the most distant from each other compared to any other pair of orbital points. Exactly in the middle of the segment between these points we draw a perpendicular to the plane in which the orbit lies (the ecliptic plane). An observer moving along a perpendicular will see the Earth's orbit under different angles. That is, if we draw rays from the observer’s location to the two previously mentioned points in the Earth’s orbit, the angle between the rays will depend on the distance to the ecliptic plane. Very close to the plane the rays form very obtuse angle(almost 180°). Very far - very sharp (almost 0°). And there is a distance at which this angle will be equal to exactly 2" (two arc seconds; one second is equal to 1°/3600). This is a parsec.
For a stationary alien sitting on the above-described perpendicular one parsec from the Earth and able to somehow see it (this is quite difficult, since the Earth is not bright enough for such a distant observer), the Earth will change its apparent location slightly due to its orbital movement. The displacement angle between the two extreme visible positions of the Earth will be exactly 2" (we specifically placed the alien at exactly this distance in order to obtain such a displacement angle). And relative to a certain "average" visible location, the Earth will move a maximum of 1" (half from 2"). An alien may say that the "annual trigonometric parallax" of the Earth is 1" (one arcsecond). And call the distance to Earth a “parsec” (PARALLAX - SECOND).
The parsec was needed, of course, not by aliens, enthusiastically observing the Earth from perpendicular to the ecliptic, but by terrestrial astronomers. The stars are so far away from us that they own movement does not lead to a change in position in the sky even within a year. But they seem to “rotate” in the sky in a circle due to the rotation of the Earth around its axis (one revolution per day). In addition to this, the stars ADDITIONALLY “move” across the sky due to the movement of the Earth in orbit, although this is hardly noticeable (for complete happiness, another influence will be added earth's atmosphere and hesitation earth's axis, but let’s say we took this into account and overcame it). If you try really hard, you can identify this subtle (against the background of daily “rotation” and other interference) movement and measure the annual trigonometric parallax of the star. And if the star were located near the above-described perpendicular to the ecliptic and had an annual parallax of 1", then it would be (damm!) exactly one parsec from us. After all, in the reference frame associated with the Earth, it is not the Earth moving in an elliptical orbit , and for some reason the rest of the world makes a similar movement, but in reverse side. Accordingly, for an earthly astronomer watching the above-described alien (or the star next to it), this alien (or the star next to it): 1) for some reason rotates around the Earth at wild speed (with full turn per 1 day) and 2) additionally moves along an elliptical orbit (with a full revolution of one year and semi-axes, like the earth’s), parallel to the ecliptic plane.
The distance to the remaining stars can also be easily calculated (only geometry with trigonometry and nothing more) in parsecs, if you can measure their annual parallax and (additionally) take into account their position in the sky. The parsec itself is equal (by definition and from trigonometry) to the cotangent of 1", multiplied by the semimajor axis of the earth's orbit (by the "astronomical unit"). Small angle cotangent equal to one, divided by the angle itself in radians. 180° is pi radians, 1° is pi/180 radians, 1"=1°/3600=pi/(180×3600). Cotangent 1" is 180×3600/pi≈206.000. Accordingly, a parsec is approximately equal to (slightly more) 206 thousand “astronomical units” (semi-major axes of the earth’s orbit). And since we know the parameters of the earth’s orbit (including its semimajor axis), we can express the parsec itself in other distance units (meters, light years, etc.) - this is approximately 3.2 light years. The stars closest to us have an annual trigonometric parallax of less than (but on the order of) 1" and, accordingly, are located at a distance of more than (but on the order of) one parsec.
