Distances in Space
MATH IN SCIENCE: EARTH SCIENCE
Objective: You will learn about the units of length used to measure distances in
our solar system and beyond and the speed of light.
It's very hard for us to understand just how large the Universe is. Scientists tell us that the largest
number our minds can really comprehend, or grasp, is about one hundred thousand (100,000).
Because astronomers study objects over such extremely large distances, they commonly use units of
length that are much bigger than the ones we usually use. Three common units of distance used in
astronomy are the astronomical unit (AU), the light-year, and the parsec.
Astronomical Unit
The astronomical unit (au) is the average distance from the Earth to the sun, measured to be about
1.5 x 108 km or 150,000,000 km (9.3 x 107 mi or 93,000,000 mi). An AU is a convenient unit to use
when discussing distances within our solar system.
1. Saturn has an average distance of 9.5 AU’s from the sun. How many kilometers is this? How many
meters?
2. Pluto, the dwarf planet in the solar system, is about 6 x 109 km from the sun. How many
astronomical units (AU) is this?
Speed of Light
The speed of light remains constant in the Universe at approximately 300,000 km/sec or
186,000 mi/sec.
3. The average distance to the moon is 382,500 km. How many seconds does it take for
the sun’s light reflected by the moon to reach us?
Light-year
The light-year is defined as the distance that light travels in a year. It is equal to a distance of
approximately 9.5 x 1012 km or 9,500,000,000,000 km. For instance, Proxima Centauri, the closest
star to the Earth after the sun, is 4.3 light-years from us. An LY is a convenient unit to use when
discussing distances beyond our solar system.
4. How long does it take light from this star to reach us?
5. The star Betelgeuse, meaning “armpit of the giant,” is 310 light-years from Earth. How many hours
does light from this star take to reach Earth?
6. How many AU’s are in a light-year?
Parsec
A parsec is an abbreviation for parallax second. (We will study parallax Friday) A parsec
is 3.26 light years or well over nineteen trillion miles (19,000,000,000,000). Scientists
use parsecs when even light years are too small to conveniently express distances.
7. How many light-years are in a parsec? How many AUs?