8.8D: Light Years
Earth and Space
Part I: The Speed of Light
Fundamental Question: What is the speed of light, and how far does it travel in one Earth year?
A beam of light from the Sun or from a flashlight travels at 300,000 km per second. This is known as
the speed of light. Nothing in the known universe travels any faster than light. Let’s do some
calculations to determine how far light can travel in a year.
Speed of Light = 300,000 km per second
1. In part I of your Student Journal, follow the instructions to complete the calculations. (These
numbers are too big for a calculator, so will you have to do them on a piece of scratch paper.)
2. Compare your answer to your teacher’s answers.
Part II: Calculating a Light Year
Fundamental Question: How do astronomers measure distance?
The huge distances between objects in space are really big numbers and difficult to work with! If you
were an astronomer, would you like to have to use these numbers every day? Probably not! That’s
why astronomers came up with a new unit for the distances in the universe. Astronomers don’t use
kilometers or miles for distances – they use the unit, light year (ly). A light year is the distance light
travels in an Earth year, so is a measure of distance. This can be confusing, since
a year is a measurement of time. Astronomers also use light-seconds and light-minutes.
1. Your teacher will review with you how to use the cancel zeros method, which you learned in a
previous lesson.
2. Complete the calculations in part II of your Student Journal.
3. Compare your answers with your teacher’s answers.
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8.8D: Light Years
Earth and Space
Part III: Modeling Light Years- The “Student-Minute”
Masking tape
Fundamental Question: How do astronomers measure distance?
A light year is the distance light can travel in one Earth year. It is a defined
unit based on the speed of light. Anyone can define a unit as long as it is
accepted by everyone.
1. In the area, mark the starting spot with a piece of masking tape and
label it “Start.”
2. Beginning at the "Start" marker, have one student volunteer walk
heel-to-toe in a straight line for exactly 30 seconds. Use the timer to
make sure your time is accurate
3. Mark the spot where the walker stops with a piece of masking tape.
4. Measure and record the distance traveled during the 30 seconds
(from the “Start” marker to the masking tape.) In Part III of the
Student Journal, record this distance in the data table for Trial #1.
5. Repeat steps 3-5 two more times for a total of three trials.
6. Calculate and record the average of the three distances.
END – Trial #
7. Multiply the average by 2 to find your “student-minute.”
Part IV: Time through Travel
Fundamental Question: How long has light from other stars been
traveling toward Earth?
Stars are millions and millions of kilometers away from Earth. For us to
see a star’s light, it has been traveling a long time. Proxima Centauri is
4.2 light years away from us. That means its light has been traveling for
4.2 years. If Proxima Centauri exploded 3 years ago, then we would not
see the explosion for another 1.2 years! So, in a sense, when we look at
the stars, we are looking at the past. We have accomplished time travel!
1.
2.
3.
4.
Place the masking tape for
each trial. Measure the
distance between the end
mark and the start mark
for each trial. Find the
average.
Complete the chart in Part IV of your Student Journal.
Answer questions 1-3.
Compare your calculations and answers with your teacher’s answers.
Wrap-up the activities by completing the Reflections and Conclusions section in your Student
Journal.
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