Lesson 6: The Light Year and Look-back Time
Overview
This lesson covers the meaning of light years and why astronomers use such a measurement of distance. Because light travels at a finite speed, we can also refer to “look-back” time and how telescopes are really time machines: the farther away we look, the farther back in time we are seeing. We are seeing the object as it was hundreds, thousands, millions, or billions of years ago. Look-back time is demonstrated with a fun activity passing tiny-to-large plastic snakes on foam board to represent pictures taken at 1-year intervals going out to space stations light years away.
Learning Outcomes
- Explain what is meant when a distance is described in “light years.”
- State why it is important for astronomers to have such a measurement.
- Define “look-back time.”
Materials
- Touch the Stars Braille book, pages 72 – 74.
- 3 – 4 rubber snakes of various sizes (small, medium, large, and extra-large) each attached to a board (to represent a photograph of the snake), plus 1 extra board with nothing attached.
Figure 3.6.1. Various size rubber snakes representing the species “sunbeam” that lives 4 – 5 years.
Some students may be hesitant to touch a snake, even a plastic one, but this demonstration is a great and effective visualization of “look-back” time: The farther away we are observing a celestial object, the farther back in time we are seeing. The plastic snakes (Figure 3.6.1) were purchased from a number of on-line sources, and from Archie McFee in Seattle, Washington. From the left: 1 year, 2 years, 3 years, 4 years old. (Final “image” says “R.I.P. SUNBEAM You were a good snake” with Braille labeling as well.)
Pre-assessment Questions & Discussion
Q. What do you recall about distances between the Sun and all of the planets from our solar system walk?
Q. What is a light year?
A. The distance light to travels in a year, about 6 trillion miles or 10 trillion kilometers.
Q. How fast does light travel?
A. Light travels at about 300,000 kilometers per second.
Q. How long does the light from other stars take to reach Earth?
A. The light from the closest star to our sun, Proxima Centauri, takes 4.243 years to reach us since it is 4.243 light years away. A beam of light reaching us today left this star 4 years, 2 months, 27 days, and 12 hours ago.
Text
Students read about Measuring the Universe: pp. 72 – 74 of Touch the Stars. This section covers distances, light years, and looking back in time very well.
Follow-up Questions on Reading
- Why is it important to use “light years” when talking about distances in the Universe?
- In what way can telescopes be considered time machines when they look farther and farther away at objects in the Universe?
- The air travel distance between New York and Los Angeles is about 3900 km. Let’s say that a jet has a cruising speed of 780 km/hr. Would it be clear to us if we said, “New York is 5 air-cruising hours from Los Angeles”? Why or why not?
- The closest star, Proxima Centauri, is a little over 4 light years away. If there were a planet orbiting this star, and if there were aliens there with technology, how long would it take us to get an answer back if we sent a radio message to them?
Reinforcing Hands-On Activity
Exploring:
In this activity, an arm’s length equals the distance light would travel in one year. Each participant (ideally at least 5, but the number can be adjusted) is seated one arm’s length away from another. The teacher is located an arm’s length from the first student, and his/her location is Earth (0 light years). The first student is at a space station 1 light year away, the next at a space station 2 light years away, and so on.
READ ALOUD: “We will passing around plastic snakes now to help us understand the idea of light travel time. This particular snake we will call a sunbeam snake, named after a real, non-poisonous snake native to Indonesia. The average life of a sunbeam snake is 4 years and over that time it grows to be about 1 meter long. The snake gets its name from the fact that its scales appear rainbow colored in strong sunlight. For this activity, let us pretend that I am taking photographs of a sunbeam snake here on Earth and sending it out to you in space.”
The first photograph is of the baby snake 1 year after its birth. I am sending it to the closest person to me at the speed of light. How long will it be before they receive the photo? Will my neighbor know about the baby snake before receiving the photograph? What happens to the snake during the time its photograph travels to the first space station?
Pass 1-year-old snake to first student
After another year has gone by, the person at the nearest space station has just received the the photograph of the one-year-old snake that I had sent them. Here on Earth the baby snake has grown and is celebrating its second birthday. To celebrate its birthday, I take an other photograph and send this one out to my friend at the nearest space station. The person at the nearest space station is simultaneously sending out the photograph of the baby snake that I had sent them the year before. How long will it take for these photographs to travel to their destinations? How much time has passed by since the snake hatched when the nearest space station receives the photograph of its first birthday? How about when the second space station receives the baby snake photograph? What does my snake look like now on Earth? Is it different from the photographs my space-station neighbors are looking at?
Pass 2-year-old snake to first student, while first student passes 1-year-old snake to second student.
Three years have now gone by and I took another photograph to celebrate the snakes third birthday. Remember these snakes live only 4 years on average. How much longer does my sunbeam snake have to live? What does the snake look like for the 3 different students at the space stations? Does the person at the farthest space station (4 light years away) know about the snake yet? I am going to send on the photograph of the 3 year old snake to my nearest neighbor. Will the snake still be alive when they receive it?
Pass medium (3-year old) snake to first student, while they pass the 2-year-old snake to the second student, and the second student passes the 1-year-old snake to the third student.
What does the last student think the snake looks like now? How about students one and two? How does the snake look like to me? Knowing that my sunbeam snake does not have long to live, I take one more photograph on its fourth birthday. With a touch of sadness, I send the photograph off to my neighbor on the nearest space station. Sunbeam has grown into a magnificent, friendly pet.
Pass the large (4-year old) snake to first student, who passes the 3-year-old snake to the next student, who passes the 2-year-old snake to the third student, who passes the 1-year-old snake to the fourth student. The fifth student doesn’t even know sunbeam exists yet.
Another year passes. Since the sunbeam snake lives only four years the snake is now dead. I take a memorial photograph of the snake’s tombstone and send it out to the first space station. How much time passes before each of the different stations find out the snake is dead?
Pass the tombstone photograph along, and discuss the time passing as each station receives the picture.
Summary and Post-Assessment Questions
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- Discuss the relationship between this activity and how the text describes a light year and look back time.
- How might an echo of sound be similar to sending a message using light to another planet?
Relevant Information and Links
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- Hubblecast Video Explains How Hubble “Looks Back in Time” — https://scitechdaily.com/hubblecast-video-explains-hubble-looks-back-time/