How far is far?
How fast is fast?
A light year is the
DISTANCE that light
will travel, through a
vacuum, in one year.

How many seconds are in a
year?
=

31,557,600 s/yr
The Speed of Light
c
= 299,792,458 meters
per second
or
186,000 miles per second
 To get an idea of how
fast this is. . . light can
travel about seven times
around Earth in one
second!


One light year is approximately
5.880 trillion miles or 9.5 trillion
km.
The symbol for light year is “ly”.

Light years are used to
measure the vast distances
in space.
 In the universe, the
kilometer measure is too
small to use.

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.


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.


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.