Sun, Moon, and Stars: Astronomy for Beginners · Part II, Ch. V · Part 2: Part II: A closer look at the Solar System
17. Yet more about the Moon: tides and the Moon's face
Goal: Your child can explain how the Moon (with help from the Sun) makes two tides a day, why spring tides come at new and full moon, and can name some of the Moon's 'seas' and craters.
You'll need
- The book, open to Part II, Chapter V
- Paper, a pencil and a coin or bottle cap to trace circles
- The Moon diary
- Binoculars (for the Moon at night only), or a good photo of the full Moon
What's in the book
The Moon can be thought of as a sister-planet: the Sun pulls her more than twice as hard as the Earth does. The chapter explains her wavy path round the Sun, the tides (two a day, with spring tides and neap tides), why we never see her far side, and her craters, mountains and dark 'seas'. A note from a naval officer explains flood, ebb and slack water.
The lesson, step by step
Hook question
SayIf you build a sandcastle near the water at the beach, why might it be gone a few hours later?
Read together
Read the chapter aloud. On the tides part, read slowly:
If there were only one wave, there would be only one tide in each twenty-four hours.
SayThe book says the Moon is the chief cause of the tides, and the Sun helps.
Draw the two bulges
Draw the Earth as a circle. Draw a stretched oval of 'ocean' around it, bulging toward a small Moon on one side and also bulging out on the far side.
Turn the paper slowly under your finger, which stays on one spot as 'you'. Say: As the Earth turns, you pass through one bulge (high tide), a low part, the other bulge (high tide again) and another low part. Two high tides a day.
Spring and neap tides
Draw the Sun on the same side as the Moon (new moon), then on the opposite side (full moon). The book says the pulls work together then: spring tides, the biggest. At the quarters they pull at right angles: neap tides, the smallest.
Say'Spring' here has nothing to do with the season. It means the water springs up high.
The Moon's face
Look at the full Moon (or a photo). Find the dark 'seas' that make the face or the 'man in the moon'. If you can, find the bright crater Tycho near the bottom, with rays like a splash. The book says Tycho is about fifty-four miles across.
What we know now
High tides come about 12 hours and 25 minutes apart, not exactly 12, so they come about 50 minutes later each day, just as the Moon rises about 50 minutes later each day. Real tides are also shaped by coastlines and the shape of the ocean floor.
The book guessed some craters might be made by rocks from space. That guess was right: nearly all the Moon's craters were made that way.
The dark 'seas' are wide plains of dark, hardened lava that flooded huge basins long ago.
The book says water could only be there as ice. Spacecraft have found ice in dark craters near the Moon's poles. And the far side, photographed since 1959, has very few dark seas.
Tell it back
Ask your child to explain, with the drawing, why there are two high tides each day.
Check questions
Which pulls the Moon harder, the Sun or the Earth?
When do spring tides happen? Neap tides?
Are the Moon's 'seas' made of water?
Afterwards
If you live near a coast, look up a tide table and visit the shore at high and low tide on the same day. Inland, check the Moon diary: mark the days of new and full moon as 'spring tide' days.
Answers to the book's problems
The Sun; its pull on the Moon is more than twice the Earth's. Spring tides at new and full moon; neap tides at the first and last quarters. No, the book explains they were named 'seas' before it was found that there is no water there.
Worked out for this site from the scan. If a number in your copy differs, trust the book.