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**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

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**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

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

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Our Solar System kiroastro.com

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**What is one way to measure a planet or star?**

By the Diameter of the sphere…

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**Diameter of a planet… Diameter of a sphere**

The distance from one edge to the opposite edge

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**It would take 108 Earths laid side-by-side to go across the Sun’s diameter**

messagesfromearth.wordpress.com findfreeclipart.com

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**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

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**Actual Distance from the Sun to the Earth**

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**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

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**Distance from the Sun to the Earth**

in our model

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**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

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**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

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**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

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**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?)

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**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

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**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)

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**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

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**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

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**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

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**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

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**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!

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**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

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