Sam's Tide Post
At Sam's harbor, boats float high at breakfast and sit on the mud at lunch. By evening the water is back up again.
If the Moon is only on one side of Earth, why are there two high tides a day?
Page 1
Science · Grade 6 Science
Chapter 1: Earth, Sun, and Moon
Two a day, and one of them on the wrong side.
At Sam's harbor, boats float high at breakfast and sit on the mud at lunch. By evening the water is back up again.
If the Moon is only on one side of Earth, why are there two high tides a day?
Make your guess to turn the page.
Still to come: 10 pages · Words to Know · What We Learned · 4 questions · Explain It in Writing · Ask and Wonder
The idea this lesson is working on — Children who know the Moon causes tides usually picture one bulge, pulled toward the Moon. The book makes that Ben's hypothesis and lets the data refute it: the second high tide arrives with the Moon on the far side. The idea that replaces it is a difference in pull, which stretches Earth. Ask: which gets pulled hardest, the near water, the middle of Earth, or the far water?
The Sun matters too — When Sun and Moon line up, their effects add and tides are largest. When they are at right angles the tides are smallest. The Sun's tidal effect is about 46% of the Moon's, which is why neap tides are smaller but still happen.
What this model leaves out — Real tides are also shaped by the ocean basin the water sloshes around in. That is why the Bay of Fundy has a range of fifty feet and parts of the Gulf of Mexico get only one tide a day. Sam's numbers are a smooth, typical two-tide coast; the two-bulge model explains the cause, and the coastline explains the local details.