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What Determines the Moon Phases?

heidi Kent, Artem Cheprasov
  • Author
    heidi Kent

    Heidi has taught middle school science, health, and English for more than 22 years. She has a Master's Degree in General Science from North Dakota State University. She is a member of the MSTA, has chaired Professional Development and Continuing Education at the Ashby Public School.

  • Instructor
    Artem Cheprasov

    Artem has a doctor of veterinary medicine degree.

Study moon phases by correlating the moon's position relative to the earth and sun. Know what causes the moon to shine, what determines the phases of the moon, and identify the moon phases. Understand how and when eclipses happen. Updated: 11/11/2021

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How Is the Moon Dependent on the Sun?

The two brightest objects shining in the sky are the sun and the moon. However, they shine for entirely different reasons. The sun, a star at the center of the solar system, shines because of nuclear fusion within its massive core. Vast amounts of energy are released in this process which the earth experiences as light. The moon, on the other hand, is dependent on the sun. It shines only because its surface reflects sunlight. Without the sun, the moon and the earth would be very cold, dark rocks floating in space.

The full moon is visible because it is reflecting light from the sun onto earth.

Full Moon at Sunset

Just as the earth revolves around the sun, the moon revolves around the earth. Any object that revolves around another is called a satellite. The moon is therefore considered a natural satellite of the earth. The path an object takes as it revolves is called its orbit. All orbiting or moving objects in space tend to remain in motion; this is called inertia. When the inertia of an object is in balance with the gravitational force of another object, it will orbit it. The changing location of the moon as it orbits causes tides, moon phases, and eclipses.

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What Causes the Moon to Shine?

Is the moon a reflection of the sun? The moon shines because the side facing the sun reflects the sunlight light into space. One side of the moon is always facing the sun and is lit up. However, the entire lit-up face of the moon is not always visible. Depending on where the moon is in its orbit, there are times when the moon's lit side faces away from earth. The visibility amount of the moon constantly changes as the moon makes its monthly orbit around the earth.

The visibility amount of the lit surface of the moon changes each month predictably. However, humans are always seeing the same side of the moon. The back or far side of the moon has never been visible; this is because the moon is in synchronous rotation meaning it takes the moon the same amount of time to rotate once on its axis as it does to make one revolution around the earth. The result is the same side of the moon facing the earth. The side of the moon that is never visible is called the 'dark side' of the moon.

Every so often, the earth or moon blocks sunlight from reaching one or the other. When this happens, it is called an eclipse. Eclipses occur in either the new moon or full moon phases and are described more thoroughly later. In new and full moon phases, the moon, earth, and sun line up in a straight line called syzygy.

Moon Phases with the Earth and Sun

The changing appearance of the moon as it orbits the earth is called phases. The photograph shows a sequence of moon phases. The moon phase shown in number 1 is the new moon phase. The moon is not visible from earth in a new moon phase because its lit side faces away from earth. Moon number 5 on the photo shows the moon with the entire lit side facing the earth; this is the full moon phase.

These photographs show changes in the visible lit surface of the moon growing from a new moon at 1 to a full moon at 5.

Photo sequence of Moon Phases

The moon's location on its path around the earth is what determines the phases of the moon. Scientists have given names to the changing appearance or phases of the moon. A waxing moon occurs before a full moon when the lit surface of the moon is increasing in size. A waning moon occurs after a full moon when the lit surface of the moon is decreasing in size. More precise names are labeled and described on both the table and the diagram.

The table lists the moon phases in order starting with a new moon. Note that numbers 1-5 on the table also correspond to numbers 1-5 on the moon phase photograph.

Moon Phase Amount of Lit Surface Visible
1. new moon none
2. waxing crescent 1/4
3. first quarter 1/2
4. waxing gibbous 3/4
5. full moon all
6. waning gibbous 3/4
7. third quarter 1/2
8. waning crescent 1/4
1. new moon none

The same phases listed on the table can also be seen on the diagram. The diagram shows the actual position of the moon relative to the earth and the sun. To analyze the diagram, here are important points of reference to keep in mind:

  • The earth is the circle in the center.
  • Sunlight is coming from the right side of the diagram.
  • Around the earth, there are two sets of moons: the inner set shows the actual lit surface of the moon, and the outer set of moons which are labeled, shows how much of the lit surface is visible from earth.
  • The diagram shows all moon phases at one time and on one diagram.
  • The moon orbits in a counter-clockwise direction, starting with the new moon phase.

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Frequently Asked Questions

Why does the moon only shine at night?

The moon can shine during the day also, but it is not as obvious. The moon reflects light from the sun, and sometimes it is visible even during the day.

What are the roles of Earth and the Sun in moon phases?

The earth and sun have roles in moon phases. The sun is constantly lighting up half of the moon. However, the amount of visible moonlight depends on the moon's location in its orbit around earth.

What moon phase is between Earth and sun?

When the moon is located between the earth and the sun, it is called a new moon phase. During this phase, the moon is not visible because the lit-up side faces away from the earth.

Does the sun go through phases?

The sun does not go through phases as the moon does. However, it has times and periods where it puts out more energy or has more storms on its surface.

What is it called when the Sun moon and Earth are in a line?

When the sun, moon, and earth are in a straight line, it is called syzygy. Syzygy is the arrangement that will occur in both full and new moons.

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