The Phases of the Moon

Directions: Point at a day of the month and the Moon will go to its orbital position for the beginning of the corresponding day. Point away from any days and the Moon's motion will resume.

The Left Panel shows the motion of Earth and Moon from an "overhead" viewpoint. The Sun is far to the right. This view does not represent the true relative sizes or distances of Earth and Moon. The Moon should be about one quarter the size of Earth and about 1 meter away from Earth on the screen.

The Upper Right Panel shows the phase of the Moon as we see it from Earth.

The Lower Right Panel tracks the motion and phase of the Moon based on a simplified 28-day calendar. The Moon actually takes 27.3 days to complete one orbit around Earth. The Moon takes 29.5 days to complete one cycle of phases.




Questions to Consider:

1. On what day (or days) does the following occur? (Click for Answer)

  • New Moon
  • Crescent Moon (two answers)
  • First Quarter Moon
  • Gibbous Moon (two answers)
  • Full Moon
  • Third Quarter

The astronomical month is considered to start with a New Moon. In this demo, the New Moon occurs at the beginning of Day 1. A New Moon appears completely dark because the part of the Moon that is visible is the Moon's night side.

During the next week (Days 1-7), less than 50% of the Moon's visible face is illuminated and we see a Crescent Moon.

After one full week (Day 8), exactly 50% of the Moon's visible face is illuminated. We see a First Quarter Moon. That is, one quarter of the month has elapsed.

During the next week (Days 8-14), more than 50% of the Moon's visible face is illuminated and we see a Gibbous Moon.

Halfway through the month (Day 15), the Moon's visible face is fully illuminated. A Full Moon is completely bright because the part of the Moon that is visible is the Moon's day side.

During the next week (Days 15-21), more than 50% of the Moon's visible face is illuminated and we see a Gibbous Moon again.

After three full weeks (Day 22), exactly 50% of the Moon's visible face is illuminated again. We see a Third Quarter Moon. That is three quarters of the month has elapsed.

After one month, the Moon returns to the same location in the sky as the Sun and we see a New Moon again. The demo represents a month as 28 days for simplicity. But the real cycle of phases takes 29.5 days to complete.

2. How much of the Moon do we see at any time? (Click for Answer)

Flip through the days in the month and notice how the dark and bright features on the Moon's surface don't move in the view from Earth. From Earth, an observer can see only one hemisphere (50%) of the Moon's surface. This hemisphere is known as the Moon's "near side".

The other hemisphere is never visible from Earth. It is called the Moon's "far side". This part of the Moon was not photographed until 1959 (the Soviet Luna 3 space probe) and not seen by human eyes until 1968 (Apollo 8).

3. How much of the Moon is illuminated by the Sun at any time? (Click for Answer)

Sunlight normally falls on one hemisphere (50%) of the Moon's surface. The only exception is when the Moon passes through Earth's shadow during a lunar eclipse. This can only occur when the Sun, Earth, and Moon are lined up during a Full Moon (at Day 15 in the demo).

4. Does the Moon rotate? Watch and think about this cafefully! (Click for Answer)

Yes, the Moon rotates. Watch the appearance of the Moon in the left panel of the demo. The Moon's bright and dark features rotate counterclockwise over 28 days. (Earth's does this once a day.)

Imagine standing on the Moon's near side for one day and watching Earth hang in the sky. Earth would appear as a spinning globe, with Australia, Asia, Africa, Europe, and the Americas visible in turn.

In contrast, the Moon rotates once a month and orbits Earth once a month. This curious situation is called "synchronous rotation". The Moon achieved this link between rotation and orbit due to the interaction of the tides of Earth and Moon. For this reason, this situation is also known as "tidal locking". The large moons of the other planets move in the same way.

5. What causes the phases of the Moon? Is it Earth's shadow or some other effect? (Click for Answer)

The Moon's visible face appears to be more or less illuminated over the course of the month. This is due to our changing view of the Moon's day and night side

The Moon can only pass through Earth's shadow during a single day of the month: Full Moon. At other times of the month, the Moon is far from Earth's shadow. So Earth's shadow plays no part in the cycle of the phases of the Moon.



Created by Kevin Healy, 2007