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Distances in Astronomy

The Metric System
Astronomers measure factors such as size and distance using the metric system, a decimal system of weights and measures in which the kilogram (2.04 pounds), the meter (39.37 inches), and the liter (61.025 cubic inches) are the basic units of mass, length, and capacity, respectively. Names for the most common other units are formed by the addition of the following prefixes to these terms: kilo- (thousand), mega- (million), giga- (billion), centi- (1/100), milli- (1/1000), and micro (1/1,000,000). So, for example, a kilometer is a thousand meters, while a millimeter is one-thousandth of a meter. The metric system is used in every country except the United States, but we will soon adopt the system. It is extremely easy to convert in the metric system. One common tool to help you convert between metric prefixes is the Metric Staircase. 

Angular Measurement
Astronomers also utilize angles in measuring celestial objects, especially in quantifying their position and movement in the sky. An angle is formed by two lines that have the same endpoint. An angle is measured in degrees, from 0 to 180. The number of degrees indicates the size of the angle. A right angle has a measure of 90 degrees, while a complete circle would measure 360 degrees (see Figure 1 ). Angular measure is often given in terms of arc, which can be measured in minutes (60 minutes to a degree) or seconds (60 seconds to a minute). 

AU (Astronomical Unit)

One AU is the distance that the average distance that the Earth orbits the Sun at. The AU is most commonly used for the distances of objects within our solar system. Pluto 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 76 AU and then goes to 880 AU from the Sun in its giant elliptical orbit.

The AU changes throughout the year as the distance that Earth is from the sun changes throughout the year. This distance ranges from 92 million miles during summer in the northern hemisphere to 95 million miles during winter in the Northern Hemisphere. The average distance is 93.5 million miles, or 150 million kilometers.

Light Years

One of the most common rulers is the light year. The light year is the distance that light travels in one year (365 ¼ days). It is most commonly used for the distances to stars and other galaxies.

The nearest star, in the Centauri system, is 4.3 light years away from the Sun. We are 8 light minutes away from the Sun. The distance to Pluto is about 13 light hours.

Some other interesting distances in light years:


Distance in light years

Nearest star (Proxima Centuri)


Sirius the dog star (the brightest star in the sky)


Center of the galaxy

~ 30 000

Andromeda (one of our nearest neighboring galaxies)

~ 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 position of everything around you changes, a property called parallax. 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 are 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




Astronomical units (AU)

Light Years (ly)



kilometers (km)


149.6 million



Astronomical units (AU)





Light Years (ly)





Parsec (pc)





self-check questions

1.       List the basic units of measurement

a.       Distance –

b.      Mass –

c.       Volume/Capacity-

2.       Copy the metric staircase into your notebook. Using the staircase, convert the following quantities.

a.       1000 kg = ______________g

b.      5.5 cm = _______________ hm

c.       33.9 mL = ______________ L

3.       Why is an “arc” divided up into minutes and seconds?

4.       The moon’s angular diameter is ___________ degree or ______________ of arc .

5.       Define an AU. Why does the value of an AU change over the course of a year?


6.       Define a light year.


7.       How many light minutes is earth from the sun?

8.       How close is the nearest star (other than the sun)?

9.       Define a parsec.

10.   Why are megaparsecs often used as a distance measurement?