## Parsecs

**Abbreviation/Symbol:**

pc

**Worldwide use:**

Parsecs are a unit of measurement commonly used in astronomy to describe vast distances in space. The term "parsec" is derived from the words "parallax" and "second," reflecting its origin in the concept of parallax, which is the apparent shift in the position of an object when viewed from different angles. One parsec is equivalent to approximately 3.26 light-years or 30.86 trillion kilometers.

The use of parsecs is widespread in the field of astronomy due to its convenience in describing astronomical distances. It provides a more practical and intuitive way to measure the vast distances between celestial objects, especially when dealing with objects outside our solar system. Parsecs allow astronomers to accurately determine the distances to stars, galaxies, and other celestial bodies, enabling them to study the structure and evolution of the universe.

The international scientific community widely adopts parsecs as a standard unit of measurement in astronomical research. It is used in various astronomical calculations, such as determining the luminosity and absolute magnitude of stars, estimating the size and mass of galaxies, and measuring the expansion rate of the universe. The use of parsecs facilitates communication and collaboration among astronomers worldwide, ensuring consistency and accuracy in astronomical observations and calculations.

**Definition:**

Astronomers used trigonometry to calculate the distance to stars long before the term parsec was coined, but the new unit made it easier to conceptualise unfathomable distances.

A parsec is the distance from the sun to an astronomical object which has a parallax angle of one arcsecond (1/3600 of a degree). The parallax angle is found by measuring the parallax motion (or apparent movement of a star relative to stable, more distant stars) when the star is observed from opposite sides of the Sun (an interval of six months on Earth). The parallax angle is obtained by halving the angular difference in measurements.

Once the parallax angle is established you can calculate the distance to a star using trigonometry, because we know Earth’s distance from the Sun. The distance from the Sun of a body with a parallax angle of 1 arcsecond was thus defined as a unit and, thanks to Turner, named the parsec.

With the parsec defined, deriving and describing huge distances became easy, since a distance in parsecs can be calculated as the reciprocal of the parallax angle in arcseconds (if the parallax angle is 1 arcsecond, the object is 1 pc from the sun. 0.5 arcseconds means the object is 2 pc distant).

**Origin:**

The origin of the unit of measurement known as the parsec was coined by British astronomer Herbert Hall Turner in 1913. A unit of distance useful in astronomy had been defined but was without a name, and the Astronomer Royal appealed for suggestions. Turner’s was accepted – parsec being derived from the definition of the unit as the distance from the sun to an astronomical object which has a parallax angle of one arcsecond.can be traced back to the early 20th century. The term "parsec" is a combination of the words "parallax" and "second," which reflects its connection to the concept of parallax in astronomy.

Parallax is the apparent shift in the position of an object when viewed from different angles. Astronomers use this phenomenon to measure the distance to nearby stars. The idea behind the parsec was first proposed by the British astronomer Herbert Hall Turner in 1913. He suggested that a unit of distance could be defined as the distance at which an object would have a parallax of one arcsecond when observed from opposite sides of the Earth's orbit around the Sun.

The concept of the parsec was further developed by the American astronomer Harlow Shapley and his colleagues in the 1920s. They refined the definition of the parsec to be equal to 3.26 light-years or approximately 31 trillion kilometers. This value was chosen to make calculations more convenient and to align with existing astronomical measurements.

**Common references:**

Proxima Centauri – the nearest star to the Earth other than the Sun, is 1.29 parsecs away.

The centre of the Milky Way is over 8kpc from the earth.

**Usage context:**

The usage of parsecs is primarily in the field of astronomy, where it provides a convenient way to express distances on an astronomical scale. Astronomers use parsecs to measure the distances between celestial objects such as stars, galaxies, and quasars. Since the distances in space are incredibly vast, using units like kilometers or miles would be impractical. Parsecs allow astronomers to express these distances in a more manageable and meaningful way.

In addition to its use in measuring distances, parsecs also play a crucial role in determining the luminosity and size of stars. By measuring the apparent brightness of a star and knowing its distance in parsecs, astronomers can calculate its absolute brightness or luminosity. This information is essential for understanding the properties and evolution of stars.

While parsecs are primarily used in the field of astronomy, they are not commonly encountered in everyday life. However, their usage is crucial for astronomers to explore and understand the vastness of the universe.