Days That Keep Getting Longer: teacher key

Day and Night, grade 8 edition, written for 8th grade. Earth's spin is slowing, and the Moon is drifting away. Learn how tides, lasers and ancient corals reveal how long a day used to be.

Read the book. Print this page for the key.

Questions and answers

1. About how much do tides add to the length of a day each century?
   • 1.7 milliseconds (answer)
   • 17 minutes
   • 1.7 hours
2. How do scientists measure how fast the Moon is moving away?
   • They watch it with telescopes at dawn
   • They bounce lasers off the Moon and time the round trip (answer)
   • They count growth bands in coral
3. What can growth bands in corals and layers in rocks show?
   • How long the day was long ago (answer)
   • How hot Mercury is
   • How much melatonin people make
4. Which planet spins fastest, giving a day of under 10 hours?
   • Mars
   • Venus
   • Jupiter (answer)
5. What does the word circadian describe on these pages?
   • Body clocks on a roughly 24-hour cycle (answer)
   • Rocks that hold fossils
   • The Moon's orbit
6. What happens to melatonin in the evening?
   • It falls
   • It rises (answer)
   • It stops being made for good

Words

• sidereal: measured by stars
• bulge: swollen part
• circadian: about a day
• melatonin: sleepy hormone
• hormone: body chemical