### Measuring the distance to stars for A Level physics - parsec

When it comes to distances involved in the objects in space, three major units are used. They are,

- astronomical unit
- light-year
- parsec

**Astronomical unit - AU**

This is the average distance between the Sun and the Earth.

1AU = 150 million km = 1.5 x 10

^{11}mThe astronomical unit, AU, is used to express the distance between the Earth and the other planets in our solar system.

**Light-year - ly**

The distance travelled by light in one year is called a light-year.

1 light-year = 3 x 10

^{8}x 3600 x 24 x 365 = 9.46 x 10^{15}m**parsec -pc**

The distance at which 1AU subtends an angle of 1 arcsecond is called a parsec.

1 arcsecond - (1/3600)°

tan (1 arcsecond) = 1AU/1pc

1 pc = 1AU x tan(1 arcsecond)

1pc = 1.5 x 10

^{11}x tan(1 arcsecond)1pc = 3.1 x 10

^{16}m or 1 pc = 3.26 lyYou can calculate the angle in arcseconds and pc with the following simulation:

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