This document defines a light year as the distance that light travels in one year, which is approximately 5.88 trillion miles or 9.5 trillion km. It explains that light years are used to measure vast distances in space since the kilometer is too small, and that when viewing objects in the night sky we are seeing them as they appeared in the past due to the time it takes light to reach us. For example, the light we see from Sirius left it eight years ago.

3. How far is far?
How fast is fast?

4. A light year is the
DISTANCE that light
will travel, through a
vacuum, in one year.

5.  How many seconds are in a
year?
 31,557,600 s/yr

6. The Speed of Light

7. c = 299,792,458 meters per
second
or
186,000 miles per second

8.  To get an idea of how fast
this is. . . light can travel
about seven times around
Earth in one second!

9.  One light year is approximately 5.880
trillion miles or 9.5 trillion km.
 The symbol for light year is “ly”.

10.  Light years are used to
measure the vast distances
in space.
 In the universe, the kilometer
measure is too small to use.

11.  Every time you look at
objects in the night sky or
the Sun, the light from
that object is old. You are
looking at history.

12.  It takes eight light minutes for light
leaving the Sun to reach the Earth.
 To put this in perspective, if you
could drive from the surface of the
Sun to the surface of the Earth, it
would take you 180 years driving
nonstop at 60 mph.

13.  Sirius, in the constellation Canis
Major, is the sky’s brightest star. It is
easy to find on winter and spring evenings.
 When you look at Sirius, you are
looking back in time to see how
Sirius looked eight years ago. The
light left the star eight years ago.