Understanding The Moon Phases
Have you ever wondered what causes the moon phases? We all know that its appearance changes over time. But why? The good way to understand the phases of the moon is to examine an earth-moon-sun diagram:
The illustration may look a little complex at first, but it’s easy to explain.
Sunlight is shown coming in from the right. The earth, of course, is at the center of the diagram. The moon is shown at 8 key stages during its revolution around the earth. The moon phase name is shown alongside the image. The dotted line from the earth to the moon represents your line of sight when looking at the moon. The large moon image shows what you would see at that point in the cycle. For the waning gibbous, third quarter, and waning crescent phases you have to mentally turn yourself upside down when imagining the line of sight. When you do this, you’ll “see” that the illuminated portion is on your left, just as you see in the large image.
One important thing to notice is that exactly one half of the moon is always illuminated by the sun. Of course that is perfectly logical, but you need to visualize it in order to understand the phases. At certain times we see both the sunlit portion and the shadowed portion — and that creates the various moon phase shapes we are all familiar with. Also note that the shadowed part of the moon is invisible to the naked eye; in the diagram above, it is only shown for clarification purposes. Finally, please realize this diagram is only meant to demonstrate how the phases work; the small inner moons in the diagram do not show the fact that the same side of the moon always faces Earth.
So the basic explanation is that the lunar phases are created by changing angles (relative positions) of the earth, the moon and the sun, as the moon orbits the earth.
Moon Phases Simplified
It’s probably easiest to understand the moon cycle in this order: new moon and full moon, first quarter and third quarter, and the phases in between.
As shown in the above diagram, the new moon occurs when the moon is positioned between the earth and sun. The three objects are in approximate alignment (why “approximate” is explained below). The entire illuminated portion of the moon is on the back side of the moon, the half that we cannot see.
At a full moon, the earth, moon, and sun are in approximate alignment, just as the new moon, but the moon is on the opposite side of the earth, so the entire sunlit part of the moon is facing us. The shadowed portion is entirely hidden from view.
The first quarter and third quarter moons (both often called a “half moon“), happen when the moon is at a 90 degree angle with respect to the earth and sun. So we are seeing exactly half of the moon illuminated and half in shadow.
Once you understand those four key moon phases, the phases between should be fairly easy to visualize, as the illuminated portion gradually transitions between them.
An easy way to remember and understand those “between” lunar phase names is by breaking out and defining 4 words: crescent, gibbous, waxing, and waning. The word crescent refers to the phases where the moon is less than half illuminated. The word gibbous refers to phases where the moon is more than half illuminated. Waxing essentially means “growing” or expanding in illumination, and waning means “shrinking” or decreasing in illumination.
Thus you can simply combine the two words to create the phase name, as follows:
After the new moon, the sunlit portion is increasing, but less than half, so it is waxing crescent. After the first quarter, the sunlit portion is still increasing, but now it is more than half, so it is waxing gibbous. After the full moon (maximum illumination), the light continually decreases. So the waning gibbous phase occurs next. Following the third quarter is the waning crescent, which wanes until the light is completely gone — a new moon.
The Moon’s Orbit
You may have personally observed that the moon goes through a complete moon phases cycle in about one month. That’s true, but it’s not exactly one month. The synodic period or lunation is exactly 29.5305882 days. It’s the time required for the moon to move to the same position (same phase) as seen by an observer on earth. If you were to view the moon cycling the earth from outside our solar system (the viewpoint of the stars), the time required is 27.3217 days, roughly two days less. This figure is called the sidereal period or orbital period. Why is the synodic period different from the sidereal period? The short answer is because on earth, we are viewing the moon from a moving platform: during the moon cycle, the earth has moved approximately one month along its year-long orbit around the sun, altering our angle of view with respect to the moon, and thus altering the phase. The earth’s orbital direction is such that it lengthens the period for earthbound observers.
Although the synodic and sidereal periods are exact numbers, the moon phase can’t be precisely calculated by simple division of days because the moon’s motion (orbital speed and position) is affected and perturbed by various forces of different strengths. Hence, complex equations are used to determine the exact position and phase of the moon at any given point in time.
Also, looking at the diagram (and imagining it to scale), you may have wondered why, at a new moon, the moon doesn’t block the sun, and at a full moon, why the earth doesn’t block sunlight from reaching the moon. The reason is because the moon’s orbit about the earth is about 5 degrees off from the earth-sun orbital plane.
However, at special times during the year, the earth, moon, and sun do in fact “line up”. When the moon blocks the sun or a part of it, it’s called a solar eclipse, and it can only happen during the new moon phase. When the earth casts a shadow on the moon, it’s called a lunar eclipse, and can only happen during the full moon phase. Roughly 4 to 7 eclipses happen in any given year, but most of them minor or “partial” eclipses. Major lunar or solar eclipses are relatively uncommon.
In preparation for your upcoming visit to the Space Centre, you are to undertake a project on Mars. Click on the link below to find all that you need to complete this task.
Good luck astronauts!
(Documentary is on the 5 shared server)
Week 4 – Lesson 3 and 4 – Shapes and Sizes
- Students will use a basketball, tennis ball and marble to explore why the Sun and Moon appear to be the same size when viewed from the Earth.
- Students will be able to work as a part of a team.
- Students will use spherical objects to represent their understanding of the positions and distance between the Earth, Moon and Sun.
1. Share ideas or experiences of how people travel around Earth.
2. What objects do you think are the same shape as the Earth, for example, a basketball. Ask students the name of the shape and discuss how a sphere is different from a circle or disc. Discuss the shape of the Sun and Moon.
3. Discuss what the students’ know about the sizes of the Sun , Earth and Moon, particularly in comparison with each other.
4. Introduce three spherical objects such as a basketball, a marble and a peppercorn. Ask students to match each object to the Sun, Earth and Moon to indicate their size, and give reasons for their match.
5. Discuss the common observation that the Moon appears to be similar in size to the Sun.
6. Explain that students will be working in collaborative teams to find out more about sizes and positions of the Sun, earth and Moon.
7. Explain that for this activity the basketball will represent the Sun and the tennis ball to represent the moon. This activity is more practical to be completed outside.
8. One member will hold the tennis ball while another will hold the basketball. Ask the student to imagine that the observing student is standing on Earth looking at the Sun and the Moon.
9. The student with the basketball will move backwards until the student viewing the balls observes that the basketball appears to be the same size as the tennis ball. Students swap position so that each member has a turn at being an observer. In teams students discuss their observations and relate them to the sizes of the Sun and Moon. Students to take photos.
10. Compare the distances of the Sun and Moon from Earth. Show 1st and 2nd YouTube videos – How far away is the moon? (above).
11. Ask students to create and label a new diagram of the Sun, Earth and Moon.
12. Discuss the idea of scale and why it is used in science diagrams. For example, it would not be possible to represent the distances experienced in this activity without using scaled representations.
Perspective Photos (not related to Space)
Individually complete this task.
Share ideas with your cooperative group and discuss similarities and differences. People often have different experiences and ideas about topics. Sharing ideas is a useful way to learn.
Create a labelled diagram with the title ‘Spinning in space’ on your Inspiration app. Record your ideas about :
– The size and shape of the Sun, Earth and Moon
– The position of the Sun, Earth and Moon and
– how the Sun, Earth and Moon might move in space
Movements might be indicated by lines with arrows or turning arrows.
2. When you have finished, share it with a partner.
* How did the characters use information in their world to form their ideas?
* Why did the characters disagree?
* What did Eratosthenes suggest when they disagreed? Why?
* Why do you think the companion listened to Eratosthenes’ idea?
*What evidence do you now have that would further support Eratosthene’s ideas that the Earth is a sphere?
Visit the page below. Create a Keynote to demonstrate your understandings. Remember to add a bibliography to provide evidence of the depth of your research.