Why the Moon Has Phases
Aligned to MS-ESS1-1 — Next Generation Science Standards.
What this lesson teaches
The Moon does not make its own light; we see the half the Sun is lighting. As the Moon orbits Earth every month, we view that lit half from changing angles, so it seems to grow and shrink. When Earth, Sun, and Moon line up exactly, one can cast a shadow on another, causing an eclipse.
Worked example
About a week after a new moon you see a 'first quarter' moon: the right half glows and the left half is dark. The Sun is off to the right lighting that side, and from Earth you catch the lit half at a right angle, so exactly half looks bright.
Practice questions
- Use a lamp (Sun) and a ball on a stick (Moon) and slowly walk it around your head (Earth) to watch the lit part change shape.
- Draw the Moon's shape on the same night for one week and describe how it changes.
- Explain why a solar eclipse can only happen at new moon, when the Moon is between Earth and the Sun.
Watch the lesson
Every lesson comes with a video taught in English and Spanish — the same video the QR code in the printed workbook opens.
▶ Watch this lessonEn español
Por qué la Luna tiene fases
La Luna no produce su propia luz; vemos la mitad que el Sol ilumina. Como la Luna orbita la Tierra cada mes, observamos esa mitad iluminada desde ángulos que cambian, por eso parece crecer y encoger. Cuando la Tierra, el Sol y la Luna se alinean exactamente, uno puede proyectar su sombra sobre otro y ocurre un eclipse.
Ejemplo: Una semana después de la luna nueva ves un 'cuarto creciente': la mitad derecha brilla y la izquierda está oscura. El Sol está a la derecha iluminando ese lado, y desde la Tierra ves la mitad iluminada en ángulo recto, así que exactamente la mitad se ve brillante.
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