The clocks in the sky: stretch · Grade 7

The passage and its 8 questions

Directions: Read the passage carefully. Then answer the questions in full sentences.

• When a phone shows a blue dot on a map, it can feel like magic. The real explanation is stranger and more precise than magic: your position depends on clocks circling Earth, and on the fact that time itself runs at a slightly different speed up there.
• The Global Positioning System uses about thirty satellites that orbit roughly 20,000 kilometers above the ground. Each one carries extremely accurate atomic clocks and constantly broadcasts a simple message: which satellite it is, where it is and exactly what time it was when the signal left. Your phone does not send anything back. It only listens.
• Radio signals travel at the speed of light, so a receiver can calculate how far away each satellite is from the tiny delay between when a signal was sent and when it arrived.
• With distances from at least four satellites, it can pinpoint its location. Three distances narrow the position down, and the fourth corrects for the receiver's own clock, which is far less precise than the satellites' clocks.
• Timing has to be almost perfect. If a clock were off by just one millionth of a second, the calculated position could be wrong by about 300 meters, roughly the length of three soccer fields.
• Here is the surprising part. Albert Einstein's theories predict that clocks run faster where gravity is weaker and slower when they move very fast. The satellites experience both effects, and together they make the satellite clocks gain about 38 millionths of a second each day compared with clocks on the ground. That sounds insignificant, but the error would accumulate. Without a correction, GPS positions would drift by more than ten kilometers in a single day.
• So engineers deliberately set the satellite clocks to tick very slightly slower before launch, and the system makes further adjustments along the way. Every time you find a bus stop on a map, you are relying on one of the strangest ideas in physics, proof that a big theory can have a very small, very practical job.
• 1. How many satellites must a receiver hear from to find its position, and what does the last one do?
• 2. The author writes, "It only listens." What can you infer about how many people can use GPS at the same time?
• 3. What does 'accumulate' mean in paragraph 5? Which words around it help you work it out?
• 4. Copy the sentence that explains why timing must be almost perfect.
• 5. Why does the author compare a 300-meter error to soccer fields?
• 6. What is the author's purpose in the final paragraph?
• 7. Find a word in the passage that was new or tricky for you. What do you think it means? Which words around it helped you?
• 8. Write a one-paragraph summary that a younger student could understand.