Definition:Astronomical Distance Units

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Definition

This page gathers together various units of measurement used in astronomy.


Astronomical Unit

The astronomical unit is a derived unit of length.


It is defined as being $149 \, 597 \, 870 \, 700$ metres.


The astronomical unit is the standard unit of measurement used by astronomers when discussing distances within the solar system.

It is derived as the approximate mean distance from the center of Earth to the center of the Sun.

\(\ds \) \(\) \(\ds 1\) astronomical unit
\(\ds \) \(=\) \(\ds 149 \, 597 \, 870 \, 700\) metres
\(\ds \) \(\approx\) \(\ds 92 \, 955 \, 807 \cdotp 273\) miles
\(\ds \) \(\approx\) \(\ds 1 \cdotp 58125 \, 07409 \, 8 \times 10^{-5}\) light years
\(\ds \) \(\approx\) \(\ds 4 \cdotp 84813 \, 68111 \times 10^{-6}\) parsec


Light Year

The light year is a derived unit of length.

It is derived as the distance travelled by light in a vacuum over the course of $1$ Julian year ($365.25$ days).


It is defined as being exactly $9 \, 460 \, 730 \, 472 \, 580 \, 800$ metres.

\(\ds \) \(\) \(\ds 1\) light year
\(\ds \) \(=\) \(\ds 9 \, 460 \, 730 \, 472 \, 580 \, 800\) metres
\(\ds \) \(\approx\) \(\ds 5 \cdotp 8786 \times 10^{12}\) miles
\(\ds \) \(\approx\) \(\ds 632 \, 41 \cdotp 077\) astronomical units
\(\ds \) \(\approx\) \(\ds 0 \cdotp 3066\) parsec


Parsec

The parsec is a derived unit of length.

It is derived as the distance at which $1$ astronomical unit subtends an angle of $1$ second of angle.


It is therefore exactly $\dfrac {648 \, 000} \pi$ astronomical units.

\(\ds \) \(\) \(\ds 1\) parsec
\(\ds \) \(\approx\) \(\ds 30 \cdotp 85677 \, 58149 \, 13673 \times 10^{15}\) metres
\(\ds \) \(\approx\) \(\ds 1 \cdotp 9174 \times 10^{13}\) miles
\(\ds \) \(\approx\) \(\ds 2 \cdotp 06265 \times 10^5\) astronomical units
\(\ds \) \(\approx\) \(\ds 3 \cdotp 26\) light years