1. How Far is a Light Year? Claire Hodgin – Science Instructional Coach
University of Texas – University Charter Schools
Armosky and Hodgin  University of Texas at Austin, 2009

2. About the Relative Size of Things in the Universe
Engage
Powers of Ten
About the Relative Size of Things in the Universe
What was the smallest to the largest distance on the video?
1040
10,000,000,000,000,000,000,000,000,000,000,000,000,000 m

3. Explore
messagesfromearth.wordpress.com
If we could shrink the Sun to the size of this balloon, how big would the Earth be?

4. Our Solar System
kiroastro.com

5. What is one way to measure a planet or star?
By the Diameter of the sphere…

6. Diameter of a planet… Diameter of a sphere
The distance from one edge to the opposite edge

7. It would take 108 Earths laid side-by-side to go across the Sun’s diameter
messagesfromearth.wordpress.com
findfreeclipart.com

8. Finding the Earth/Sun Diameter Ratio
The Earth’s diameter is
12,756,000 meters or ~ 1.3 X 107 m
The Sun’s diameter is
1,400,000,000 meters or ~ 1.4 X 109 m
The ratio of the Earth : Sun diameter is
1.3 X 107 m : 1.4 X 109 m
Simplify the ratio...
1.3 X 107 m : 1 m
1.4 X 109 m : m
Earth to Sun Diameter ratio is 1 : 108

10. Actual Distance from the Sun to the Earth

11. If we use the same scale as we used for the SIZE of Sun and Earth…
How far apart would the Sun and Earth be in our model?
Sun Diameter
Sun to Earth Distance
Model
.45 m = X
Actual
1.4 x 109 m
1.5 x 1011 m
Cross multiply and x = 4.8 x 101 m (48 m) or ~ 50 m

12. Distance from the Sun to the Earth
in our model

13. Let’s take a field trip To build a model to scale
the Sun will be a 450 mm balloon
the Earth will be a 4 mm lentil
and they will be 50 meters apart

15. The Milky Way Galaxy is about 100,000 light years across
Explain
The Milky Way Galaxy is about 100,000 light years across
Powersof10.com

16. One light-year is the DISTANCE that light travels in one year
If we are going to find out
HOW FAR is a Light Year,
we need to know two things…
The speed of light, measured in meters per second and
The number of seconds in a year

17. What is the fastest speed in the universe?
The Speed of Light
What is the speed of light?
(abbreviated “c”)
300,000,000 meters per second or
3 X 108 m/s
(remember the line in Powers of Ten?)

18. How Many Seconds in a Year?
60 seconds per minute
X 60 minutes per hour
X 24 hours per day
X days per year
= the number of seconds per year

19. On the Graphing Calculator…
Turn on and press “Mode”
At the top row are the words:
Normal Sci Eng
And the “Normal” is highlighted in black
Arrow right to “Sci” until it is highlighted, and press “Enter”
Quit (2nd Mode), and now all of your calculations are in Scientific Notation (Powers of Ten)

20. Let’s try this on the graphing calculator
In science we call this “Dimensional Analysis,” in math “Unit Conversion”
60 s
60 min
24 hr
days
3.2 x 107 s
1 min
1 hr
1 day
1 yr =
Let’s try this on the graphing calculator

21. So How Far is a Light Year?
Multiply the Speed of light (m/s) by the number of seconds per year (s/yr)
= number of m/yr
3 x 108 m/s X 3.2 x 107 s/yr =
“3 2nd LOG 8 ) X 3.2 2nd LOG 7 ) Enter”
9.5 x 1015 m/yr
Or 9,500,000,000,000,000 meters
That’s Nine Quadrillion, Five Hundred Trillion Meters

22. How long does it take light to get from the Sun to the Earth?
8 minutes
It takes 16 minutes to go there and back

24. If we use the same scale as we used for the SIZE and DISTANCE of Sun and Earth…
How long would a light year be in the model?
Sun Diameter
Light Year
Model
.45 m = X
Actual
1.4 x 109 m
9.5 x 1015 m
Cross multiply and x = 3.1 x 106 m

25. That’s 3 times wider than Texas!
3.1 x 106 meters
That’s 3.1 x 103 kilometers!
That’s 3 times wider than Texas!
That’s too much tape!

26. Let’s change the scale of our model.
What is the new scale factor so that 1 light year fits inside a 50 meter paper strip?
Light Year Distance
Sun Diameter
Model
50 m = X
Actual
9.5 x 1015 m
1.4 x 109 m
Cross multiply and x = 7.4 x 10-6 m