The Moon and Its Phases
The Moon's phases come from our view of its sunlit half.
About this topic
The Moon orbits Earth every 27.3 days and shines by reflected sunlight. Half is always lit; phases occur because we see different amounts of the lit half as the Moon orbits: new moon, waxing crescent, first quarter, waxing gibbous, full moon, then waning back, with a cycle of about 29.5 days.
It is tidally locked, so the same side always faces us. The Moon causes tides, and its phases have shaped calendars. Earth's shadow does not cause phases; that's an eclipse.
To picture the phases, imagine standing on Earth while a ball (the Moon) circles you and a lamp (the Sun) shines from one side. Half the ball is always lit; what changes is the angle from which you view it. At new moon the lit half faces away, at full moon it faces us, and at the quarters we see exactly half of the lit side. Waxing means growing; in the Northern Hemisphere the right side appears lit when waxing.
The 29.5-day cycle is longer than the 27.3-day orbit because Earth also moves around the Sun, so the Moon must travel a little further to line up again. This synodic month underlies lunar calendars used in Islamic, Hebrew and many traditional calendars, and tides follow the Moon's position. A very common misconception is that phases are caused by Earth's shadow; the shadow is only involved in lunar eclipses. Another is that there is a permanent dark side; the far side gets as much sunlight as the near side, we just never see it from Earth.
Key ideas
- New moon to full moon in about 29.5 days
- Moon is tidally locked
- Same side faces Earth
Quick check
Use cases: where The Moon and Its Phases helps
- Homework: review the key idea behind The Moon and Its Phases before you start a problem set.
- Exam revision: use the quick check to test whether you can recall The Moon and Its Phases without notes.
- Visual learning: play with the interactive lab to see how The Moon and Its Phases changes when you adjust its inputs.
- Teaching a friend: explain The Moon and Its Phases in your own words, then compare your version with the key ideas.
Earth science helps with weather forecasts, natural hazards, energy and climate planning.
Frequently asked questions
What is The Moon and Its Phases?
The Moon's phases come from our view of its sunlit half.
What will I learn in The Moon and Its Phases?
New moon to full moon in about 29.5 days. Moon is tidally locked. Same side faces Earth.
Is The Moon and Its Phases part of Earth & Space?
Yes. It is one of 60 Earth & Space topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the The Moon and Its Phases lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study The Moon and Its Phases?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.