1 parsec is equal to light years. What is a parsec equal to? distance in astronomy

Many of us first heard about the parsec from the cartoon "The Mystery of the Third Planet", in which brave astronauts easily traveled long distances in space.

And although this word is firmly embedded in the memory, not everyone knows what it means. What is a 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 a second in this case understand not a unit of time, but 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 the globe. The annual parallax indicates the same parameters, but taking into account , and the secular parallax allows you to determine the difference relative to the observer, taking into account the proper motions of the observed object in the galaxy.

What is a parsec?

In simple terms, a parsec is a unit of measurement that determines the distance between celestial bodies beyond solar system.


The parsec is most commonly used for measurements inside Milky Way. If it is necessary to establish distances on the scale of the Universe, multiple parsecs are used, that is, kiloparsec (1000 parsec), megaparsec (million parsec), gigaparsec (billion parsec).

This astronomical unit not only performs practical function, but also adds convenience to astronomers. It is much easier to say that the distance from the Sun to the star is 1.5 parsecs, not 46.27 trillion kilometers.

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 equal to?

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 from the Sun, and the leg indicates the semiaxis earth orbit. The size acute angle in this triangle is the annual parallax. A 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 use kilometers and light years. The ratio of all these units of measurement to each other has long been calculated: 1 parsec is equal to 3.2616 light years or 30.8568 trillion kilometers. As a designation for a parsec in Russian, the symbol "pc" is accepted, in English - "rs".

Examples of distances in space

Since the advent of parsecs, astronomers have been able to calculate distances to many space bodies and in the universe. So, the distance from the Sun to the nearest star to us, Proxima Centauri, is 1.3 parsecs, to the center of the galaxy - about 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 about 4 gigaparsecs.

The distance between space objects is not comparable with the earth, and one could "drown in zeros" by measuring them in kilometers. Therefore, astronomers needed special units for measuring distances, and one of them is the parsec.

What does this word mean

The word "parsec" is made up of two words: parallax and.

A second in this context is not not time, but an angle. As you know, angles are measured in degrees, each of which is divided into 60 parts, called , and each is subdivided into 60 seconds.

Parallax is the displacement of an object relative to the background, determined by the position of the observer. Astronomers deal with three types of parallax - daily, annual and secular. With regard to the parsec, it is the annual one that is of interest.

By determining the annual parallax of a particular star, astronomers calculate what is the distance from the Earth to it. To do this, you need to build an imaginary right triangle. The hypotenuse in it will be the distance from this star to the Sun, and one of the legs will be the semi-major axis of the Earth's orbit. The size of the angle in this triangle corresponding to the star is the annual parallax.
The distance to a star at which the size of this angle is one second is called a parsec. The international of this unit is pc, and in Russian it is referred to as pc.

Why parsec

When talking about long distances in cosmic scale, are often measured in . This unit of measurement corresponds to the distance that a light beam travels in a year, and it is equal to 9,460,730,472,580.8 km. An impressive amount, but a parsec is even larger!

Parsec is 3.2616 light year, this is 30.8568 trillion km. It is this unit of measurement, and not the light year, that professional astronomers usually use. Distance in light years is more often indicated in popular science publications or science fiction novels and films.

But even such a unit of measurement was not enough for the needs of space exploration. I had to introduce units equal to a million parsecs - kiloparsec (kpc) and megaparsec (Mpc).

Thus, the distance that the heroes of the "Secrets of the Third Planet" were offered to overcome turns out to be very impressive. 100 pc is more than 326 light years! However, modern astronomy knows more significant distances. For example, the distance to the Virgo cluster, the closest cluster of galaxies to Earth, is 18 Mpc.

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 “Secrets of the Third Planet”, reported the distance to the planet, which he recommended to fly to prof. Seleznev and his team.

However, few people know what exactly parsec means, what distance we are talking about and how far the characters of the popular cartoon were forced to fly.

The 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 off-system astronomical unit, which is identical to a second of a flat angle.

Parallax is a change in the location of a celestial body depending on where the observer is located.

Modern astronomy highlights the following types parallax:

Daily- the difference in directions to a certain luminary, both in the geocentric and topocentric direction. This angle directly depends on the height of the celestial body above the horizon.
At annual parallax direction change to certain object are directly related to the rotation of the earth around the sun.
Concerning secular parallax, then it makes it possible to determine the difference in the direction to the celestial body, depending on their movements in the Galaxy.

Parsec - the meaning of the term

If to speak in plain language, then "parsec" is a unit of change in the distance between celestial bodies located outside the solar system. The parsec is usually used to calculate the distance within the Milky Way. Basically, these are multiple units: kiloparsecs, megaparsecs and gigapersecs. submultiple units usually not used due to the fact that it is more convenient to use standard astronomical units instead.
A parsec greatly simplifies calculations for astronomers, because it is much easier to say that one and a half parsecs are from the Sun to a certain star than more than 46 trillion km.

Who invented the 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 make accurate calculations of the Cygnus star of 61 annual parallax. To calculate the distance from this star, Bessel used the old method, calculating the difference in angles obtained after taking two measurements.

Determining the distance to stars by the parallax method. Image source: bigslide.ru

First, measurements were made when the Earth was located relative to the Sun on one side, and six months later, repeated measurements were made (when the Earth turned to the Sun on the other side).

However, the term "parsec" itself appeared only in 1913 thanks to the English astronomer Herbert Turner.

How is a parsec calculated and what is it equal to?

Schematic drawing 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.

An annual parallax is used to calculate the parsec. When using an imaginary triangle with right angles, a parsec is the distance to a star, assuming its parallax is 1 arc second.
A parsec is 3.26 light years or about 30 trillion km. It is one of the first ways to determine the distances to stars and is denoted as "pc"

The essence of the 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 closest star to the Sun - Proxima Centauri - 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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For their calculations, astronomers use special units of measurement that are not always clear. ordinary people. It is understandable, because if cosmic distances were measured in kilometers, then the number of zeros would ripple in the eyes. Therefore, to measure cosmic distances it is customary to use much larger values: astronomical unit, light year and parsec.

Quite often used to indicate distances within our own solar system. If you can still express it in kilometers (384,000 km), then the closest way to Pluto is about 4,250 million km, and this will already be difficult to understand. For such distances, it is time to use the astronomical unit (AU), equal to the average distance from earth's surface to the sun. In other words, 1 a.u. corresponds to the length of the semi-major axis of the orbit of our Earth (150 million km.). Now if we write that shortest distance to Pluto is 28 AU, and the longest way can be 50 AU, it's much easier to imagine.

The next largest is the light year. Although the word "year" is present, one should not think that we are talking about the time. One light year is 63,240 AU. This is the path that a ray of light travels in 1 year. Astronomers have calculated that it takes more than 10 billion years for a beam of light to reach us from the farthest corners of the universe. To imagine this gigantic distance, let's write it down in kilometers: 950000000000000000000000. Ninety-five billion trillion habitual kilometers.

The fact that light does not propagate instantly, but at a certain speed, scientists began to guess since 1676. It was at this time that a Danish astronomer named Ole Römer noticed that the eclipses of one of Jupiter's moons were beginning to be delayed, and this happened precisely when the Earth was heading in its orbit towards opposite side Sun, the opposite of the one where Jupiter was. Some time passed, the Earth began to return back, and the eclipses again began to approach the previous schedule.

Thus, about 17 minutes of time difference was noted. From this observation, it was concluded that it took 17 minutes for light to travel a distance the length of the diameter of the Earth's orbit. Since it has been proven that the diameter of the orbit is approximately 186 million miles (now this constant is 939,120,000 km), it turned out that a beam of light moves at a speed of about 186,000 miles per second.

Already in our time, thanks to Professor Albert Michelson, who set out to determine as accurately as possible what a light year is, using a different method, the final result was obtained: 186,284 miles in 1 second (about 300 km / s). Now, if we calculate the number of seconds in a year and multiply by this number, we get that a light year is 5,880,000,000,000 miles long, which corresponds to 9,460,730,472,580.8 km.

For practical purposes, astronomers often use the unit of distance known as the parsec. It is equal to the displacement of the star against the background of other celestial bodies by 1 "" when the observer is displaced by 1 radius

How simpler words, the more there are. I warned you - don't complain now!

The Earth has an elliptical orbit. An ellipse, unlike a circle, does not have a "radius", but has two "half-axes" of different lengths - a large and a small one. Accordingly, there are two points in the earth's orbit that lie on the major axis and are as far apart as possible compared to any other pair of points in the orbit. Exactly in the middle of the segment between these points, we draw a perpendicular to the plane in which the orbit lies (the plane of the ecliptic). An observer moving along the perpendicular will see the Earth's orbit from a different angle. That is, if we draw rays from the location of the observer 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 a very obtuse angle (almost 180°). Very far - very sharp (almost 0°). And there is such a distance at which this angle will be equal to exactly 2 "(two arc seconds; one second is equal to 1 ° / 3600). This is the parsec.

For a stationary alien sitting on the above perpendicular one parsec from the Earth and able to see it somehow (this is rather 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 equal to exactly 2" (we deliberately placed the alien at such a distance in order to obtain such a displacement angle). And relative to some "average" visible location, the Earth will move a maximum of 1" (half from 2"). An alien can say that the "annual trigonometric parallax" of the Earth is 1" (one arc second). And call the distance to the Earth "parsec" (PARallax - SEC).

It took a parsec, of course, not for aliens enthusiastically observing the Earth from a perpendicular to the ecliptic, but for earthly astronomers. The stars are so far away from us that they own movement does not lead to a change in the position in the sky even for 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, the stars ADDITIONALLY "move" across the sky due to the Earth's orbital motion, although this is hardly noticeable (for complete happiness add more influence earth's atmosphere and hesitation earth's axis, but let's say we took this into account and overcame it). If you try very hard, you can identify and measure this subtle (against the background of the daily "rotation" and other interference) movement and measure the annual trigonometric parallax of the star. And if the star were near the above-described perpendicular to the ecliptic and would have an annual parallax of 1 ", then it is (ta-damm!) Exactly one parsec from us. Indeed, in the reference frame associated with the Earth, it is not the Earth that moves in an elliptical orbit , and the rest of the world for some reason makes a similar movement, but in reverse side. Accordingly, for an earthly astronomer following the above-described alien (or a star next to him), this is an alien (or a star next to him): 1) for some reason rotates around the Earth at a wild speed (with full circle in 1 day) and 2) additionally moves in an elliptical orbit (with a full revolution of one year and semi-axes, like that of the earth), parallel to the plane of the ecliptic.

The distance to the rest of the stars can also be easily calculated (only geometry with trigonometry and nothing else) in parsecs, if you can measure their annual parallax and (additionally) take into account the position in the sky. The parsec itself is equal (by definition and from trigonometry) to the cotangent of 1 "multiplied by the semi-major axis of the earth's orbit (by the "astronomical unit"). Cotangent of a small angle 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 of 1" is 180×3600/pi≈206.000. Accordingly, a parsec is approximately equal to (a little more than) 206 thousand " astronomical units"(major semiaxes of the earth's orbit). And since we know the parameters of the earth's orbit (including its major semiaxis), we can already express the parsec itself in other units of distance (meters, light years, etc.) - this is approximately 3.2 light The stars closest to us have an annual trigonometric parallax less than (but of the order of) 1" and, accordingly, are at a distance of more than (but of the order of) one parsec.