Learning about the phases of the Moon

1. Make sure you use the following settings to start exploring the phases of the Moon:
   1. Expand the Look At drop-down menu at the bottom left side of your screen. Click SolarSystem.
   2. From the list of objects displayed at the bottom of your screen, click the Earth icon.
   3. Click the icon. Check the Planetary Orbits box. You should now see lots of lines in different colours. These lines are the orbits of the planets and the Moon.
   4. Zoom out to see the entire path of the Moon's orbit.

   

2. When looking at the orbit of the Moon from above, what part of the Moon is lit up? Does this part change as the Moon completes one revolution? Do you see the same part lit up when you look at the sky from Earth? Follow these steps to answer the questions above:
   1. Click the View tab at the top of the page. Click four times to fast forward the time by × 10,000.
   2. Watch a complete revolution. Write a description of or draw the part of the Moon that is lit up and note whether it changes.
   3. At the end of the revolution, pause the time and look at which part of the Moon is lit up.
   4. Click the icon. Uncheck the Planetary Orbits box.
   5. Change your view by zooming into Earth. Line up the Moon above Earth as shown in the figure below. It may take some practice to properly line up the view. Write a description of or draw the part of the Moon that is lit up after everything is properly lined up.
      • Is the same part lit up as the part you saw when looking from above?
      • Can you explain the differences? Switch between the two views a few times to help you take it all in and understand what is going on.
      • The view from near Earth with the Moon positioned above is like looking at the sky. From this view, you can see the phases of the Moon.
   6. Click the icon. Check the Planetary Orbits box.
   7. Zoom out to the wide view again and travel ahead in time by approximately 3.5 days.
   8. Pause the time.
   9. Click the icon. Uncheck the Planetary Orbits box.
   10. Write a description of or draw the part of the Moon that you see when looking from above and positioned near Earth.
       • Do you see any changes? Can you explain the changes?
   11. Repeat steps vi to x until the Moon has completed another full revolution and you were able to see several phases of the Moon.
   12. Centre the Moon on your screen. Line up Earth directly above the Moon, as if you were on the Moon looking up at Earth in the sky.
   13. Watch Earth.
   14. Repeats steps xii and xiii, travelling forward in time by 3.5 days each time.
       • How did Earth change each time?
       • Did the changes look similar to the changes you saw in the phases of the Moon?
       • Does the phase of Earth that you're looking at from the Moon look the same as the phase of the Moon that you had been looking at from Earth? Can you explain why?
   15. Can you calculate how much time it takes the Moon to cycle through all of its phases?
       • Try to figure out how to get the answer before moving on to the next section.

   

3. To calculate the length of a full cycle of the phases of the Moon, you will have to determine how much time passes between two of the same phases. While there are many ways to do this, we will explain one way below.
   1. Change your point of view so you can see the orbit of the Moon from above.
   2. Fast forward until the Moon is in a particular phase of your choice. Press pause.
      • Double-check that you have the phase you wanted by changing your view so that the Moon is lined up above Earth.
   3. Write down a description of the phase of the Moon and the date and time of the simulation.
   4. Switch back to a view from above where you can see the Moon's full orbit.
   5. Fast forward until the Moon has returned to the same phase you choose at the beginning. Check that the Moon is in the right phase by repositioning your view close to Earth.
      • Note: If you fast forward too quickly, you will have trouble stopping the orbit at the right place.
      • Is it difficult to identify the first phase?
      • Are certain phases easier to identify than others?
      • If you aren't seeing the right phase, go back to the view from above the orbit and go forward or backward in time until you see the right phase.
   6. Write down the date and time.
   7. Calculate the length of a full Moon phase cycle using the dates and times you wrote down.
   8. The result of your calculation should be approximately 29.5 days long.
      • Why isn't the length 27.3 days, which is how long it takes the Moon to complete a full revolution around Earth?