Science · Grade 6 Science
Chapter 1: Earth, Sun, and Moon
Why not every month?
At every new moon, the Moon passes between Earth and the Sun. Yet a total solar eclipse is seen from any one town only about once in 375 years.
The Moon lines up with the Sun every month. Why does the eclipse not happen every month?
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 — A child who has just learned that a new moon sits between Earth and the Sun reasons, correctly, that there should be an eclipse every month. Honour that: the flat hoop gives exactly that result. The missing premise is the tilt. Ask: with the hoop flat, how many eclipses? With it tilted, where is the ball at most new moons?
Two kinds, and which is rarer to see — A lunar eclipse is visible from the whole night side of Earth. A solar eclipse casts a narrow shadow track, so any one place sees one rarely — which is why totality is such an event. The Moon is about 400 times smaller than the Sun and about 400 times closer, which is why it only just covers the Sun.
About the model — In a desk model the ball and globe are enormous compared with the distance between them, so their shadows are wide and the model gives more eclipses than the sky would; the book names this as a limit. The real Moon is about 60 Earth-radii away, so its 5.1° tilt carries it several Earth-diameters off the Sun–Earth line at a typical new moon. Eclipse seasons drift slowly around the calendar year, so March and September are this model's months, not fixed dates.