1. One light year is equal to the distance traveled by light in one year (through empty space).
One light year = 1 ly
= 6 trillion miles
= 9.5 trillion kilometers

2. How do we know how far one light year is?
Wait a minute!
How do we know how far one light year is?

3. QUESTION: How far will you travel in 3 hours
if your speed is 50 miles per hour?
ANS: ___________________
The secret to this calculation is
___________ = _____________ x _____________

4. Calculating a light year
Using the formula d = vt, with
v = c = 3.00 x 105 km/s
t = one year = 3.15 x 107 s
1 ly = 3.0 x 105 x 3.15 x 107 or

5. Some interesting distances: To star nearest the sun…
Some interesting distances: To star nearest the sun….4 ly To edge of our galaxy.…. 50,000 ly To nearest galaxy …. 2,000,000 ly