Review: Moon Phases


Background:

The four primary moon phases are:

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

The lunar cycle (synodic month) is just around 29.53 days, making it slightly shorter than a calendar month.аSo, it is possible to see two of the same phases in the sameаcalendarаmonth. 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

The Moon is also inclined with respect to the ecliptic by 5 degrees. As it orbits the Earth, it precesses about the ecliptic because the Sun’s gravity continually pulls it. When the Moon’s orbit is equal to the ecliptic, it is called a node. Depending if the Moon is moving from above or below the ecliptic, it is called an descending or ascending node. Look at the image to the right to help you visualize this. It takes 27.212 days for the Moon to return to the same node, and it is called a nodical or draconic month. Because the nodical month is shorter than the sydonic month, eclipses are rare events. Check out this simulation from UNLV about the nodical and sydonic months.

If you watch the path of the Moon with respect to the stars, you will find that it only takes 27.3 days for the Moon to return to the same patch of stars. This is the sidereal month, and the difference between aаsynodicаmonth and a sidereal month is due to the Earth’s motion around the Sun. Here’s a link to a nice animation explaining this.

Exercise

Moon Phases

1. Label the phases of the Moon with the correct name and what order it appears in.

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 toаfamiliarizeаyourself 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. If today is a full moon, how long until it is a full moon again (in days)?

3. If the moon is 5 degrees above the ecliptic, how long until it returns to this position (in days)?


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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