Phases of the Moon

The Moon appears to change in the sky. One moment it’s a big white circle, and next week it’s shaped like a sideways bike helmet. There’s even a day where it disappears altogether. So what gives?

The Sun illuminates half of the Moon all the time. Imagine shining a flashlight on a beach ball. The half that faces the light is lit up. There’s no light on the far side, right? For the Moon, which half is lit up depends on the rotation of the Moon. And which part of the illuminated side we can see depends on where we are when looking at the Moon. Sound complicated? This lab will straighten everything out so it makes sense.

 

Materials

  • ball
  • flashlight

This first video will show you how the moon changes it’s appearance over the course of its cycle:

This video will show you how to demonstrate why the moon changes it’s appearance over the course of its cycle:

The sun illuminates half of the Moon all the time. Imagine shining a flashlight on a beach ball. The half that faces the light is lit up. There’s no light on the far side, right? So for the Moon, which half is lit up depends on the rotation of the Moon. And which part of the illuminated side we can see depends on where we are when looking at the Moon. Sound complicated? This lab will straighten everything out so it makes sense.

One question you’ll hear is: Why don’t we have eclipses every month when there’s a new Moon? The next lesson is all about eclipses, but you can quickly answer their questions by reminding them that the Moon’s orbit around the Earth is not in the same plane as the Earth’s orbit around the Sun (called the ecliptic). It’s actually off by about 5o. In fact, only twice per month does the Moon pass through the ecliptic.

The lunar cycle is approximately 28 days. To be exact, it takes on average 29.53 days (29 days, 12 hours, 44 minutes) between two full moons.  The average calendar month is 1/12 of a year, which is 30.44 days. Since the Moon’s phases repeat every 29.53 days, they don’t quite match up. That’s why on Moon phase calendars, you’ll see a skipped day to account for the mismatch.

A second full Moon in the same month is called a blue Moon. It’s also a blue Moon if it’s the third full Moon out of four in a three-month season, which happens once every two or three years.

The Moon isn’t the only object that has phases. Mercury and Venus undergo phases because they are closer to the Sun than the Earth. If we lived on Mars, then the Earth would also have phases.

 

Exercises

  1. Does the sun always light up half the Moon?
  2. How many phases does the Moon have?
  3. What is it called when the Moon appears to grow?
  4. What is it called when you see more light than dark on the Moon?
  5. How long does it take for a complete lunar cycle?