Part 5: Moon Phases

Image Credit:аBy Andonee - Own work, CC BY-SA 4.0, link


The four primary moon phases are:

  1. New Moon
  2. First Quarter
  3. Full Moon
  4. Third Quarter (Last Quarter)

The moon cycles through these four phases roughly every 7 days.аThe lunar cycle is just around 29 days, making it slightly shorter than a calendar month.аSo, it is possible to see two of the same phases in the same celandar month. This is where the phraseа‘once in a blue moon’ comes from. Theа‘blue moon’ is when there are two full moons in a calendar month.

There are also four secondary phases that occupy the space between the primary moon phases:

  1. Waxing Crescent
  2. Waxing Gibbous
  3. Waning Gibbous
  4. Waning Crescent

These names may feel arbitrary, so you should spend some time looking at the diagram at the top of the page to find patterns that help you remember their ordering. For example, the crescents occur only around the New Moon.

Also, knowing the direction of the sun and therefore the side of the moon is illuminated is very important.

If you have troubles convincing yourself of how the moon should look in a certain phase, check out this simulation from UNLV:аUNLV Moon Bisection Simulation


Moon Phases

1. Label the moon phases in the image in your packet and mark what order they occur in, starting with the New Moon asа‘1’. The sunlight is coming from the left margin, so you may wish to mark it.

2. Then, using the phases you marked in the first image, label them in the diagram with the moon, Earth, and Sun.

Telling Time with the Moon Phases

It is possible to tell the time based on the moon phases because the phases of the moon are a result of the orientation of the Sun, Moon, and Earth. Look over this lunar phase simulation from UNLV to familarize yourself with the geometry of the moon in various phases. Then, fill out the table in your packet and check your answer with the demo.

1. You see a waning gibbous moon rising, what time of day is it?

2. You see a third quarter moon rising, what time of day is it?

3. If it is 3 pm, what moon phase is directly above you (transiting the meridian)?

4. If it is 3 am, what moon phase would you see setting?

5. If today is a full moon, what will the phase be in two weeks?

If you want to quiz yourself on the phases, this is a good simulation from UNLV that will help you.а

If you want to learn about how eclipses work, this simulation from UNLV will help you.

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