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The parsec (symbol pc) is a unit of length used in astronomy. It stands for "parallax of one arc second".

It is based on the method of trigonometric parallax, one of the most ancient and standard methods of determining stellar distances. The parallax of a star is half of the angular distance a star appears to move against the celestial sphere due to the orbit of the Earth around the Sun. (See the diagram at right.) Equivalently, it is the angle subtended at a star by the radius of the Earth's orbit. One parsec is defined to be the distance from the Earth to a star that has a parallax of 1 arcsecond. It is, therefore, approximately:

• 360×60×60/(2π) = 206,264.8062 AU
• 3.08567758×1016 m
• 19,173,511,600,000 miles
• 3.26156378 light years

See 1 E16 m for a list of comparable lengths and scientific notation for an explanation of the notation.

## History

The first direct measurements of an object at interstellar distances (of the star 61 Cygni, by Friedrich Wilhelm Bessel in 1838) were done using trigonometry using the width of the Earth's orbit as a baseline. The parsec follows naturally from this method, since the distance (in parsecs) is simply the reciprocal of the parallax angle (in arcseconds).

Though it had probably been used before, the term parsec was first mentioned in an astronomical publication in 1913, when Frank Watson Dyson expressed his concern for the need of a name for that unit of distance. He himself proposed the name astron, while Carl Charlier had suggested siriometer. However, Herbert Hall Turner's suggestion, parsec, was eventually adopted.

## Usage and Measurement

Since the parallax method is the fundamental calibration step for distance determination in astrophysics, its unit of choice, the parsec, is the most used unit of distance in scholarly astronomical publications. Newspapers and popular science magazines prefer a more intuitive unit, the light year.

There is no star whose parallax is more than 1 arcsecond. The closest star to the Earth (having the largest measured parallax) is Proxima Centauri with a parallax of 0.772 arcseconds; it is thus 1.295 parsecs, or 4.225 light-years, away from us. Because of the extremely small scale of parallactic shifts, ground-based parallax methods provide reliable measurements of stellar distances of no more than 325 light-years, or about 100 parsecs, corresponding to parallaxes of no less than 0.01 arcsecond, or 10 mas (milliarcsecond).

Between 1989 and 1993 the Hipparcos satellite, launched by the European Space Agency (ESA) in 1989, measured parallaxes for about 100,000 stars, with a precision of about 0.97 mas, and obtained accurate measurements for stellar distances of around 1000 pc.

NASA's FAME satellite was due to be launched in 2004, to measure parallaxes for about 40 million stars with sufficient precision to measure stellar distances of up to 2000 pc. However, the mission's funding was withdrawn by NASA in January 2002.

The ESA's GAIA satellite, due to be launched in mid-2012, will be of sufficiently high astrometric precision to measure stellar distances to within 10% accuracy as far as the Galactic Center about 8 kpc away in the Sagittarius constellation.

## Distances in parsecs

### Distances less than a parsec

Distances measured in fractions of a parsec usually involve objects within a star system.

• One astronomical unit (the distance from the Sun to the Earth) is 4.85 × 10−6 parsecs.
• The most distant space probe, Voyager 1, was 4.6 × 10−4 parsecs away from Earth in September 2004. It took Voyager 27 years to cover that distance.
• The Oort cloud is approximately 0.6 parsec in diameter.

### Parsecs and kiloparsecs

Distances measured in parsecs include distances between nearby stars, such as those in the same spiral arm or globular cluster.

One kiloparsec, abbreviated "kpc", is one thousand parsecs, or 3,261 light years. Kiloparsecs are typically used to measure distances between parts of a galaxy.

• The nearest known star (other than the Sun), Proxima Centauri is 1.29 parsecs away.
• The center of our galaxy, the Milky Way, is about 8 kiloparsecs away. The Galaxy is about 30 kiloparsecs across.
• The Triangulum Galaxy (M33), at a bit under 800 kiloparsecs away, is the most distant object visible to the naked eye.

### Megaparsecs and gigaparsecs

One megaparsec, abbreviated "Mpc", is one million parsecs, or 3,261,564 light years. Megaparsecs are typically used to measure distances between neighboring galaxies and galaxy clusters.

One gigaparsec, abbreviation "Gpc", is one billion parsecs — one of the largest distance measures used. One gigaparsec equals 3.261564 billion light years, or roughly 1/4 the distance to the horizon of the observable universe (dictated by the cosmic background radiation). Gigaparsecs are typically used to measure distances to supergalactic structures, such as clusters of quasars or the Great Wall.

## How to calculate the value of a parsec

In the diagram above (not to scale), S represents the Sun, and E the Earth at one point in its orbit. D is an object at a distance of one parsec from the Sun. By definition, the angle D is one arcsecond and the distance ES is one astronomical unit. By trigonometry, the distance SD is

$SD = \frac{ES}{\tan 1^{\prime\prime}} = 206 265 \mbox{ au}$

One astronomical unit is equal to approximately 1.49598×108 km, so

$1 \mbox{ pc} = 206 265 \times 1.49598 \times 10^{11} \mbox{ m} = 3.08568 \times 10^{16} \mbox{ m} \,$