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 10¹¹ m

The 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 10⁸ x 3600 x 24 x 365 = 9.46 x 10¹⁵ 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 10¹¹ x tan(1 arcsecond)

1pc = 3.1 x 10¹⁶ m or 1 pc = 3.26 ly

You can calculate the angle in arcseconds and pc with the following simulation: