The Speed of Light and its Relevance to a Light Year as a Means of Measuring Distances in the Milky Way Galaxy
OBJECTIVE: After completing this lesson, students will have an understanding of the distances covered at speed of light, and a light year, and know how to measure distances in the Milky Way Galaxy.
I. Give students a simulation or drawing of our Milky Way Galaxy, pointing out the position of our Solar System on the Orion arm of the pinwheel. (Most good astronomy books have an illustration of the Solar System located in the Milky Way Galaxy.) Ask them to take a few notes, first that we are located 30,000 light years from the center of the galaxy and that the galaxy is approximately 100,000 light years in diameter.
Ask the students if anyone has any idea how to measure the distance of a light year. Ask if anyone knows how long it would take us to get to the center or outer edge of the Milky Way Galaxy, traveling from our Solar System. Ask them to solve the mathematical word problem:
If light travels at 186,300 miles per second, how far does
it travel in one hour, one day, one year?
Once they have solved this problem, give them this problem:
How far, in miles, is it to the center of the Milky Way Galaxy from our Solar System and how far it is in miles to the outer edge of our Milky Way Galaxy?
Traveling at the speed of light, how long would it take us to get to the center and to the outer edge?
Add to their notes that the Universe has been measured at 30 billion light years in diameter, and that there are virtually thousands of galaxies out there, some larger and many smaller than ours. Add that the nearest large galaxy, Andromeda, is approximately 2 million light years away. Among the small galaxies closest to the Milky Way Galaxy are the Large and Small Magellanic Clouds, at a distance of approximately 130,000 light years.
Give them another word problem:
If the star Vega, a bright star in the constellation Lyra, is 26 light-years away, how many miles is that?
If you were to visit a planet circling Vega, and you could travel at the speed of light, how many years would it take you to make a return trip?
If your best friend were 20 when you left for Vega, how old would he or she be when you returned?
II. Word problems to measure the students'understanding of distances covered at the speed of light and the concept of a light year. (They should be allowed to use their notes.)
1. The sun is 93,000,000 miles away. How much time does it take for its light to reach the earth?
2. We observed a supernova in the Large Magellanic Cloud in l987-88. When did the supernova actually occur?
3. How would you set up the calculation for finding the distance from the Milky Way Galaxy to the Andromeda Galaxy? (You don't have to find the answer.)