parsec

Non-SI unit

Name	Symbol	Quantity	SI units
parsec	pc	length	≈30.86 Pm
The parsec, symbol pc, is a non-SI unit of length. The parsec is not accepted for use with the SI. One parsec corresponds to the distance at which one astronomical unit subtends an angle of one arc second. One parsec is equal to approximately 30.856 775 815 × 10¹⁵ m, or approximately 30.9 petametres.
Definition			c Δν_Cs^-1
$1 \mspace{4mu} \text{pc} \mspace{6mu} = \dfrac{(149 \mspace{4mu} 597 \mspace{4mu} 870 \mspace{4mu} 700)(9 \mspace{4mu} 192 \mspace{4mu} 631 \mspace{4mu} 770)}{(\text{tan} \mspace{4mu} 1^{\prime \prime})(299 \mspace{4mu} 792 \mspace{4mu} 458)} \mspace{6mu} \dfrac{c}{\Delta \nu _{Cs}}\\ \\ \\ 1 \mspace{4mu} \text{pc} \mspace{6mu} \approx 9.461 \mspace{4mu} 711 \mspace{4mu} 597 \mspace{4mu} 618 \mspace{4mu} 666 \mspace{4mu} 090 \times10^{17} \mspace{6mu} c \mspace{4mu} {\Delta \nu _{Cs}}^{-1}$

The length in metres of one parsec is calculated using trigometry:

$1 \mspace{4mu} \text{pc} = \dfrac{1 \mspace{4mu} \text{au}} {\text{tan} \mspace{4mu} 1^{\prime \prime} }\\ \\\\ 1 \mspace{4mu} \text{pc} = \dfrac{149 \mspace{4mu} 597 \mspace{4mu} 870 \mspace{4mu} 700 \mspace{4mu} \text{m}} {\text{tan} \mspace{4mu} 1^{\prime \prime} }\\ \\\\ 1 \mspace{4mu} \text{pc} \approx 30.856 \mspace{4mu} 775 \mspace{4mu} 815 \times 10^{15} \mspace{4mu} \text{m}$

Astronomy

The parsec is used in the field of astronomy, primarily to express distances on interstellar and intergalactic scales.

Examples of distances of stars from Earth

Name	Distance in parsecs	Distance in SI units
Proxima Centauri	1.30 pc	40.1 Pm
Barnard’s Star	1.82 pc	56.4 Pm
Sirius	2.64 pc	81.2 Pm
Tau Ceti	3.65 pc	112.5 Pm
Altair	5.12 pc	158 Pm
Vega	7.68 pc	237 Pm
Arcturus	11.2 pc	347 Pm
Aldebaran	20.0 pc	615 Pm
Betelgeuse	168 pc	5187 Pm
Rigel	264 pc	8136 Pm
One parsec is equal to approximately 30.9 petametres

Examples of distances of galaxies from Earth

Name	Distance in megaparsecs	Distance in SI units
Andromeda Galaxy (M31)	0.77 Mpc	24.0 Zm
Triangulum Galaxy (M33)	0.81 Mpc	25.0 Zm
Messier 83 (NGC 5236)	4.50 Mpc	139 Zm
Messier 63 (M63)	8.99 Mpc	277 Zm
Sombrero Galaxy (M104)	9.55 Mpc	294 Zm
Cartwheel Galaxy (PGC 2248)	150 Mpc	4730 Zm
One megaparsec is equal to approximately 30.9 zettametres

The radius of the observable universe is approximately 440 000 zettametres, or 440 yottametres.