Measuring Distance in Space
Astronomy
Name:
Date:
Background
In our solar system, we tend to describe distances in terms of
the Astronomical Unit (AU). The AU is defined as the average
distance between the Earth and the Sun. It is approximately 150
million km (93 million miles). Mercury can be said to be about 1/3
of an AU from the Sun and Pluto averages about 40 AU from the Sun.
The AU, however, is not big enough of a unit when we start talking
about distances to objects outside our solar system.
A light-year is another unit of distance. It is the distance that
light can travel in one year. Light moves at a speed of about 300,000 kilometers (km) each second. So in one
year, it can travel about 10 trillion km. That means a light year is about 10 trillion km long. A light year is a
particularly handy unit of measurement for astronomers, because it also tells us how long ago the light that
we see left the particular object. If we look at the star Alpha Centauri, we see it as it was a little over four
years ago, for it took the light that long to get here.
Why would you want such a big unit of distance? Well, on Earth,
a kilometer may be just fine. It is a few hundred kilometers from New
York City to Washington DC; it is a few thousand kilometers from
California to Maine. In the Universe, the kilometer is just too small to be
useful. For example, the distance to the next nearest big galaxy, the
Andromeda Galaxy, is 23 quintillion km (23,000,000,000,000,000,000
km). This is a number so large that it becomes hard to write and hard to
interpret. So astronomers use light years. It is easier to say that the
Andromeda Galaxy is 2.3 million light years away.
For distances to other parts of the Milky Way Galaxy (or even further), astronomers use units of the
light year, or the parsec. The parsec is equal to 3.3 light years. Parsecs are used less frequently than light
years.
Procedure
We want you to be able to convert between kilometers, AU, light years, and parsecs, in order to
ensure that you are never puzzled by distances in the Universe. While calculations with large numbers can
be intimidating, if you remember to start by cancelling zeroes, your task will be much easier.
For Example: If a star is 25,000,000,000,000 km from Earth, how many light years away is it?
Complete the following questions:
1. Why is using kilometers to measure distances in astronomy a problem?
2. When measuring distances WITHIN the solar system, it’s best to use ___________________________,
while __________________________________ or _________________________ should be used when measuring
distances OUTSIDE our solar system.
3. Briefly define the following:
Astronomical Unit:
Light Year:
Parsec:
4. How many kilometers are in each of the following:
1 AU:
1 ly:
1 parsec:
Complete the following table using the given unit to help you calculate the missing units. You may
use scientific notation if you wish. Hint: it’s easiest to convert to AU, ly, or parsecs from kilometers, so
you may want to start each problem with kilometers.
Object
Size/Distance to
Object
Kilometers
Earth’s Diameter
14,000 km
Sun’s Diameter
1,400,000 km
Astronomical
Units
Light Years
Diameter of the Milky
Way
50,000 pc
Diameter of a Large
Cluster of Galaxies
10,000,000 ly
Diameter of the Milky
Way
Distance to Sirius
10,000,000,000 AU
43,000,000,000,000
km
Distance to Sun from
Earth
0.000015 ly
Distance to the center
of the Milky Way
7576 pc
Distance to
Andromeda
Distance light travels
over the age of the
universe
Parsecs
1980 ly
910,000,000,000,000
AU