Earth Is an Uneven Clock: teacher key

Day and Night, grade 12 edition, written for 12th grade and up. Humanity builds its clocks on a spinning planet that speeds up and slows down. Follow the measurements, the leap seconds and the open questions about day and night on other worlds.

Read the book. Print this page for the key.

Questions and answers

1. Why was the second defined from the sidereal year in 1952 instead of from the day?
   • Newcomb and others showed Earth's spin varies irregularly (answer)
   • Atomic clocks did not exist
   • Years are longer than days
2. What was Halley's Moon puzzle later shown to be?
   • A mistake in ancient eclipse records
   • Earth's spin slowing down through tidal braking (answer)
   • The Moon speeding up for good
3. What did the hamster transplant show?
   • Hamsters cannot keep a rhythm
   • The SCN is the master clock (answer)
   • Light has no effect on animals
4. What do fossil shells 70 million years old suggest about the day?
   • It was about 23.5 hours long (answer)
   • It was about 30 hours long
   • It was exactly 24 hours long
5. Which cause is part of the equation of time?
   • Earth's oval orbit (answer)
   • The ocean tides
   • The Moon's phases
6. What does eccentricity describe on these pages?
   • How oval an orbit is (answer)
   • How bright a night sky is
   • How long a day lasts

Words

• sidereal: measured by stars
• eccentricity: how oval
• meridian: line of longitude
• circadian: about a day
• melanopsin: light-sensing pigment
• carcinogen: cancer-causing thing