Jupiter2U School
Chapter 04

4. The Moon's Phases

Primary

Some nights the Moon is a full glowing circle; other nights it's a thin sliver. This chapter explains why the Moon seems to change shape.

At a glance
Core ideaPhases come from how much of the Moon's sunlit half faces us.
Key termLunar cycle — one full round of phases takes about 29.5 days.
You can…Name the phases in order: new, crescent, quarter, gibbous, full.
Watch outPhases are not Earth's shadow — that's a rare lunar eclipse.
01 · Theory

The Moon orbits the Earth

The Moon travels all the way around the Earth roughly once every 29.5 days — about a month (in fact, the word "month" comes from "Moon"). As it circles us, the Sun always lights up exactly half of the Moon — the half facing the Sun — just like it always lights up half of the Earth.

What changes is how much of that lit half we can see from Earth, depending on where the Moon is in its orbit. The main phases, in order, are: new Moon (dark), crescent, first quarter (half), gibbous, full Moon (whole circle), then back through gibbous, quarter, and crescent to new again.

02 · Explanation

It is not the Earth's shadow!

A very common mix-up: people think the Moon's phases happen because the Earth's shadow covers part of it. That's not true — that's a completely different, much rarer event called a lunar eclipse (Chapter 7). Phases happen every single month, caused simply by the changing angle between the Sun, the Moon, and you.

Key ideaHalf of the Moon is always lit by the Sun. Phases are only about how much of that lit half faces Earth — nothing is ever actually "missing" from the Moon.
New Crescent 1st quarter Gibbous Full Gibbous Last quarter Crescent ← waxing (growing) waning (shrinking) →
One lunar cycle: the sunlit fraction we see grows from new to full, then shrinks back — about 29.5 days end to end.
03 · Practical

Hands-on: a month of Moon-watching

Activity

Keep a simple Moon journal for two weeks.

  1. Each clear evening, look at the Moon (or check a Moon-phase app with a grown-up) and draw its shape.
  2. Label each drawing: new, crescent, quarter, gibbous, or full.
  3. Line your drawings up in order after two weeks.
  4. Notice the shape grows fuller night by night (waxing), then would shrink again (waning) over the following two weeks.

Answer: the Moon takes about 29.5 days total to go through every phase and return to where it started.

04 · Q&A

Test your understanding

Why does the Moon seem to change shape?

Because as the Moon orbits Earth, we see different amounts of its sunlit half — never because a piece of it disappears. Half of the Moon is always lit; only our viewing angle changes.

About how many days does a full cycle of phases take?

About 29.5 days — close to a month, which is exactly where the word "month" comes from.

What is a "new Moon"?

The phase when the Moon sits roughly between the Sun and Earth, so its sunlit half faces away from us entirely. The Moon is still there, just appearing dark to our eyes.

Is a Moon phase the same thing as a lunar eclipse?

No, and this is the biggest mix-up people make. Phases happen every month from the changing Sun-Moon-Earth angle. A lunar eclipse is rare and happens only when Earth's shadow actually falls on the Moon — you'll study that in Chapter 7.

Concept mind map

How the ideas connect

Every key idea in this chapter, branching from the core concept — use it to see the whole picture at a glance.

Moon orbits EarthSun lights halfNew MoonCrescentHalf moonFull MoonAbout one monthThe Moon's Phases
Infographic

The process, step by step

Step 1New MoonLit side faces away from us, so the Moon looks dark.
Step 2Waxing crescent to first quarterA growing sliver, then a half, becomes visible.
Step 3Full MoonWe see the whole lit face; Earth is between Sun and Moon side.
Step 4Waning to newThe lit part shrinks back to a new Moon over about 29.5 days.
Solved examples

Worked problems, step by step

Follow each solution line by line, then try to reproduce it on paper before moving on.

Example 1Why does the Moon change shape during the month?

  1. The Sun always lights half the Moon
  2. As the Moon orbits Earth we see different amounts of the lit half

Example 2Roughly how long is one full cycle of Moon phases?

  1. One orbit of phases is about a month
  2. Precisely about 29.5 days
Practice problem set

Now you try

Work each one out first, then tap to reveal the worked answer.

1What causes the phases of the Moon?
The Moon orbiting Earth, so we see changing amounts of its sunlit half.
2Are phases caused by Earth's shadow?
No. Phases come from our viewing angle, not Earth's shadow.
3What phase is it when the Moon looks completely round?
Full Moon.
4What phase is it when we cannot see the Moon at all?
New Moon.
5How long is one cycle of phases?
About 29.5 days, roughly a month.
6Does the Sun ever light the whole Moon at once?
No, the Sun always lights only one half of the Moon.