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Bouncing Lasers Off Mirrors on the Moon

6th grade nonfiction passage about measuring the distance to the moon: 224 words, six questions, an answer key and vocabulary.

6th grade nonfiction measuring the distance to the moon 224 words

Bouncing Lasers Off Mirrors on the Moon

Apollo astronauts left a few unusual things on the Moon, and some of them are still being used. One is a set of special mirrors. Each one is built from many small glass corners that send light straight back to where it came from. A flat mirror would bounce a beam off to the side, but these mirrors return the beam toward the source.

Several observatories on Earth fire short pulses of laser light at the mirrors. The beam spreads out as it travels, so by the time it reaches the Moon it covers a wide patch of the surface. Only a tiny number of light particles make it back to the telescope. Detectors count them carefully, and the scientists note the exact time of the round trip.

Light always moves at the same speed in space. That makes the math simple. Multiply the speed by the time the light took, then divide by two, because the beam went to the Moon and back. The answer is the distance to the Moon, and the measurement can be very precise.

After many years of readings, scientists found something surprising. The Moon is slowly drifting away from Earth, by a few centimeters each year. That is about as fast as a fingernail grows. The mirrors left long ago keep helping people learn how the Moon moves.

Questions

  1. What did Apollo astronauts leave on the Moon that scientists still use? in the text

  2. What do observatories on Earth fire at the mirrors? in the text

  3. Why do scientists divide by two in the math? think about it

  4. How do scientists work out the distance to the Moon? in the text

  5. How fast is the Moon drifting away from Earth? in the text

  6. Why would a plain flat mirror not work well for this job? think about it

Words to know

observatories
buildings with telescopes for studying space
pulses
quick, short bursts
detectors
instruments that sense or notice something
precise
very exact and careful
source
the place where something starts
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One passage, three levels

The same topic and facts written three ways, so a mixed class can read and talk about one text. Each group’s sheet shows only a shape; the key says which is which.

● Easier · 187 words · Flesch-Kincaid grade 2.3

Apollo astronauts left some things on the Moon. Scientists still use some of them. One is a set of special mirrors. Each mirror is made of many small glass corners. They send light straight back to where it came from. A flat mirror would bounce light off to the side. These mirrors send it back.

Some observatories on Earth shoot short pulses of laser light at the mirrors. The beam spreads as it goes. When it gets to the Moon, it covers a wide patch. Only a tiny bit of light comes back. Detectors count it. Scientists note the exact time of the round trip.

Light moves at the same speed in space. So the math is easy. Multiply the speed by the time. Then divide by two. The beam went to the Moon and back. The answer is how far away the Moon is.

After many years, scientists found something new. The Moon is slowly moving away from Earth. It moves a few centimeters each year. That is about as fast as a fingernail grows. The old mirrors still help us learn how the Moon moves.

▲ This passage · 224 words · Flesch-Kincaid grade 5.7

Apollo astronauts left a few unusual things on the Moon, and some of them are still being used. One is a set of special mirrors. Each one is built from many small glass corners that send light straight back to where it came from. A flat mirror would bounce a beam off to the side, but these mirrors return the beam toward the source.

Several observatories on Earth fire short pulses of laser light at the mirrors. The beam spreads out as it travels, so by the time it reaches the Moon it covers a wide patch of the surface. Only a tiny number of light particles make it back to the telescope. Detectors count them carefully, and the scientists note the exact time of the round trip.

Light always moves at the same speed in space. That makes the math simple. Multiply the speed by the time the light took, then divide by two, because the beam went to the Moon and back. The answer is the distance to the Moon, and the measurement can be very precise.

After many years of readings, scientists found something surprising. The Moon is slowly drifting away from Earth, by a few centimeters each year. That is about as fast as a fingernail grows. The mirrors left long ago keep helping people learn how the Moon moves.

■ Harder · 242 words · Flesch-Kincaid grade 9.1

The Apollo astronauts left behind several unusual objects on the Moon, and a few of them remain in scientific use even today. Among them is a set of specially built mirrors. Each one is assembled from many small glass corners that return light directly to the place it originated. An ordinary flat mirror would deflect a beam off to one side, whereas these reflectors send the beam straight back toward its source.

Several observatories around the world fire brief pulses of laser light at the mirrors. The beam spreads out as it travels, so by the time it reaches the Moon it blankets a wide patch of the surface. Only a tiny number of light particles manage to make the whole journey back to the telescope. Sensitive detectors count them carefully, and the scientists record the exact time that the round trip required.

Light always travels at the same speed through space, which keeps the arithmetic straightforward. Multiply that speed by the time the light took, then divide by two, because the beam traveled to the Moon and returned. The result is the distance to the Moon, and the measurement can be remarkably precise.

After many years of readings, researchers discovered something surprising: the Moon is gradually drifting away from Earth, by a few centimeters every year. That is roughly the rate at which a fingernail grows. The mirrors placed there so long ago continue to teach people how the Moon moves.

Download as one PDF

How to use it

  1. Press Read it big (or B) to put the text on the projector, or Full screen (F) to fill the whole screen.
  2. Ask the questions one at a time and use Show answer when the class has decided. A shows or hides every answer.
  3. Print student copy for the class, or Print with answers for yourself.

Ideas for class

  • Before reading, put observatories, pulses, detectors, precise and source on the board with their meanings from the Words to know section and have students predict what the text will be about.
  • Read it aloud once while students follow along, then have them reread it alone or with a partner. At a calm 100 words a minute it takes about 2 minutes to read aloud.
  • Use the Read it big button (or B) to put the text on the projector, and Show answer on one question at a time so the class commits to an answer first.
  • Question 3 asks the reader to infer. Ask students to point to the words in the passage that led them to their answer.
  • Ask students to write one question the passage did not answer, then look for the answer in a book or on a classroom screen.
  • Print the student copy for independent work and keep the answer-key copy for yourself.

Related

Questions

What reading level is "Bouncing Lasers Off Mirrors on the Moon"?

It was written for 6th grade. It has 224 words and works out to a Flesch-Kincaid grade level of about 5.7. That is a formula's estimate from sentence length and syllables, and it is not a Lexile or a guided reading level, so check it against your students.

What do the questions ask?

There are six. 4 ask for something the text says directly and 2 ask the reader to work something out from clues. Each has an answer in the key, and the answers are written as a teacher would accept them, not as the only possible wording.

How long does it take to read?

About 2 minutes read aloud at 100 words a minute. For a one-minute fluency check, paste it into the fluency timer and use the word count.

Can I print it?

Yes. Print student copy gives the passage, the questions with lines to write on, and the word meanings. Print with answers gives the same page with the answers filled in. Both are made in your browser.

Is this passage from a published book?

No. It was written for Peabody Teach, so there is no book to buy and no login to read it.

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