How is parsec is defined?

Question

Parallactic second. It is a unit for measuring the distance of stars from earth, but how parsec is defined, please clarify.

Answer 1

A parsec is defined as the distance from earth to an astronomical object which appears having a parallax angle of one arc second.

http://en.wikipedia.org/wiki/Parsec

a parsec is approx. 3.262 lightyears

Answer 2

Think of a huge right triangle ABC, with our sun at the right angle, at B, and a star at the vertex of a right triangle at A. The earth is in its orbit around the sun at C; 186,000 miles away from the sun (B). If the angle CAB happens to be 1 second of one degree in measurement from an earthbound telescope, then we say that the star at A is defined as being 1 parsec away from us. That is it is one parallel-second away in distance. we can determine that distance by using trig formulas for right triangles.

Answer 3

The distance equal to a heliocentric parallax of one arc second. That is the length of the long sides of a triangle with a base equal to the Earth's orbital diameter (186 million miles) and the opposite angle equal to one arc second (1/3,600 of a degree).

Answer 4

in equations:-
1 Parsec = 3.08568025 × 1016 meters

In Words and Numerics:-

A parsec is an astronomical unit of measurement that is equivalent to 3.26 light years distance, or the distance photons will travel in vacuum over the period of 3.26 years. Light travels at a speed of 186,000 miles per second (300,000 kilometers per second), so a parsec represents a distance of 19,131,554,000,000 miles, or just over 19 trillion miles, (close to 31 trillion kilometers).

By comparison the average distance to the Sun from Earth is only 93 million miles (150,000,000 km). This distance is referred to as one astronomical unit (AU). One would have to make 103,000 round trips to the Sun to cover the distance indicated by a single parsec (206,265 AUs = 1 parsec). Our solar system, defined for example by Pluto's orbit, is only 1/800ths of a light year across. It would have to be 2,608 times larger to equal 1 parsec across.

A parsec is calculated using the parallax of 1 arc second, hence the shorter term parsec. To understand what this means it will be helpful to define the terms parallax and arc second.

Imagine a spherical plane or a simple circle bisected evenly by 180 lines that form 360 equal sections. The distance between two adjacent lines equals one degree of arc. All arcs added together equal 360 degrees or the entire circle. Now imagine that each degree of arc is bisected further into 60 more equal sections. Each of these sections equals 1 arc minute, so 60 arc minutes equals 1 degree of arc. Finally bisect each arc minute into 60 more equal sections representing arc seconds. An arc second is therefore an angular measurement that equals 1/60th of an arc minute, or 1/3600 of a single degree of arc.

Parallax refers to the apparent motion of a fixed object along an angular trajectory, due to a change in the observer's position. For example, if you use one eye to gaze at the monitor in front of you, then switch eyes, the monitor will seem to "jump" horizontally in reference to the background. Scientists make use of parallax to measure the distance to stars. To achieve the parallax effect an object is photographed against background stars from a fixed position on Earth. Six months later when the Earth has traveled halfway around its orbit at a relative distance of 186 million miles (2 AU) from the first position, a second photograph is taken. By measuring the distance the object "jumped" scientists can calculate the arc seconds of the parallax to reveal the distance. (As an aside, a third photograph is taken in one full year from the original position to calculate and subtract any effects from natural seasonal shift.) If a star generates "3.8" parallax arc second annually, scientists know the distance to that star is 3.8 parsecs.

The further the object, the less parallax it will have. The closer the object, the more parallax. Therefore distance is inversely proportional to parallax. If an object has a parallax of .5 (half) an arcsecond, it will be twice the distance of an object with 1 arcsecond of parallax, or about 2 parsecs away (6.5 light years). Conversely an object with 2 arcseconds of parallax will be twice as close as an object with 1 arcsecond of parallax, or about half a parsec away (1.6 light years).

Parsecs are more convenient to indicate astronomical distances than light years. One thousand parsecs is known as a kiloparsec, or kpc. A megaparsec is equal to 1 million parsecs, abbreviated as Mpc. A jaunt from Earth to the center of the Milky Way Galaxy would be a lengthy trip at just over 8.5 kpc.

Although the terms kpc and Mpc come in handy, to actually measure very distant stars of more than 100 parsecs or over 400 light years away, parallax is no longer viable. In that case scientists use other methods involving the calculation of brightness, sometimes referred to as spectroscopic parallax.

Answer 5

Arc Second Definition