How Distances are Measured in Astronomy

Dealing with the numbers involved with the distances to the stars or even with those found in the solar system can be hard going. Astronomers make their lives easier by using a number of rulers (units of distance) for the distances and although they have some strange names they can be very useful for comparing the distances to stars, other galaxies and even the planets in our solar system.

AU (astronomical units)

One AU is the distance that the average distance that the Earth orbits the Sun at. The AUis most commonly used for the distances of objects with in our solar system. Pluto, the last planet in the solar system is found at an average distance of 39.47 au from the Sun. Sedna the new body nearly as large as a planet found beyond Pluto is never nearer to the Sun than 76AU and then goes to 880AU from the Sun in its giant elliptical orbit.

Light Years

One of the most common rulers is the light year. The light year is is the distance that light travel in one year (365 1/4 days). It is most commonly used for the distances to stars and other galaxies.

The nearest star is 4.2 light years away from our sun. We are 8.3 light minutes away form the Sun. the distance to the outer most planet Pluto is about 13 light hours.

Some other interesting distances in light years:

Object Distance in light years
Nearest Star (Proxima Centuri) 4.2
Sirius the dog star (the brightest star in the sky) 8.6
centre of the galaxy approximately 30 000
Andromeda (one of our nearest neighbouring galaxies) approximately 2 million
The stars of Orion. (Betelgeuse and Rigel) 1400 light years

Parsec (pc)

Astronomers started measuring distances from the amount that a star moves as the Earth goes from one side of the Sun to the other. Try moving your head and you will see that the postion of everything around you changes. One parsec is derived from the smallest angle measurement of 1/3,600th of a degree or an arc second that is the angle that a star at this distance would appear to move in 6 months as the Earth journeys around the Sun. A parsec is 3.2616 light years or 30,857,000,000,000 km.. Two parsecs is 6.5532 light years or twice the distance, it is not a measure of change in angles of the stars. Due to the massive distance in the universe astronomers often use multiples of parsec commonly found are kiloparsec (kpc) a 1000 parsecs or a megaparsec (Mpc) 1,000,000 parsecs.

Below is a conversion table for some useful astronomical distance units

  kilometres (km) Astronomical units (AU) Light Years (l.y.) Parsec (pc)
kilometres (km) 1 149.6 million 9,460,000,000,000 30,857,000,000,000
Astronomical units (AU) 0.0000000067 1 63,240 206,263
Light Years (l.y.) 0.00000000000011 0.000016 1 3.2616
Parsec (pc) 0.000000000000033 0.0000048 0.3066 1