Leo's Ball on a Stick
Every evening for a month, Maya sketches the Moon. It grows from a thin crescent to a full circle, then shrinks away again, in about 29 days.
What is actually changing about the Moon?
Page 1
Science · Grade 6 Science
Chapter 1: Earth, Sun, and Moon
Half the Moon is always lit.
Every evening for a month, Maya sketches the Moon. It grows from a thin crescent to a full circle, then shrinks away again, in about 29 days.
What is actually changing about the Moon?
Make your guess to turn the page.
Still to come: 9 pages · Words to Know · What We Learned · 4 questions · Explain It in Writing · Ask and Wonder
The idea this lesson is working on — Children, and a large share of adults, believe the phases are Earth's shadow. The book gives two views of one ball — Leo's from the middle and Ben's from above — so the child sees that the lit half never changes while the view does. Ask: from above, how much of the ball was lit at 45°? Then ask where Earth's shadow would have to be to make a crescent.
Phases are not the Earth's shadow — If the Earth's shadow caused phases there would be one every month, and the shadow would be curved the wrong way. The Earth's shadow on the Moon is a lunar eclipse, which is a different and rarer event. Leo's head at 180° is exactly that event, which is why the book counts it as an error in the phase test and names it.
About the numbers — The lit fraction seen is (1 − cos θ) ÷ 2, where θ is the angle from new moon: 0, 15, 50, 85 and 100 percent at 0°, 45°, 90°, 135° and 180°, the values in the table. The cycle of phases is 29.5 days; the Moon goes once round Earth against the stars in 27.3 days, and the difference comes from Earth moving along its orbit in the meantime.