What causes moon phases?

moon phase
by howzey

Question by Erin: What causes moon phases?
Why does the moon always changes phases? And why do we only get full moon once a month?

Best answer:

Answer by Cupcakes_Are_Good
My mom
& my balled headed granny.

Know better? Leave your own answer in the comments!

5 Responses to “What causes moon phases?”

  • GeoffG says:

    The Moon is a sphere which travels once around the Earth every 29 days. As it does so, it is illuminated from varying angles by the Sun. At New Moon, the Moon is between the Earth and Sun, so that the side of the Moon facing towards us receives no direct sunlight, and is only lit by dim sunlight reflected from the Earth. As it moves around the Earth, the side we can see gradually becomes more illuminated by direct sunlight.

    After a week, the Moon is 90° away from the Sun in the sky and is half illuminated, what we call First Quarter because it is about a quarter of the way around the Earth.

    A week after this, the Moon is 180° away from the Sun, so that Sun, Earth and Moon form a line. The Moon is fully illuminated by the Sun, so this is called Full Moon. The Earth’s shadow points towards the Moon at this time, but usually the Moon passes above or below the shadow and no eclipse occurs.

    A week later the Moon has moved another quarter of the way around the Earth, to the Third Quarter position. The Sun’s light is now shining on the other half of the visible face of the Moon.

    Finally, a week later, the Moon is back to its New Moon starting position. Usually it passes above or below the Sun, but occasionally it passes right in front of the Sun, and we get an eclipse of the Sun.

    The Moon’s phases are NOT caused by the shadow of the Earth falling on the Moon. In fact the shadow of the Earth only falls on the Moon twice a year when there is a lunar eclipse.

  • John R says:

    The moon orbits the earth once every 28 days, that is why we only have a full moon once a month, the phases of the moon are caused be the different locations of the moon in relation to where it is in it’s orbit around the earth.
    take a basket ball and place it on a table, then put a light at the end of the table, then take a soft ball and hold it in your hand and start moving it around the basket ball, and watch the soft ball, you will see that the light hits the soft ball at and angle to where it looks like a new moon, and as you move the soft ball on around the light will light up a little more area as you move it around.

  • Olympian says:

    I wish I had a diagram of my own drawing…oh well. This link should help.


    The moon takes about 27-28 days to orbit the Earth once, and about 29 1/2 to go through a complete lunar cycle. This is so because of the Earth’s orbit around the Sun; it causes the angle of sunlight to change, which in turn causes the cycle to be slightly delayed. Again, I would be better off showing you diagrams in my own hand for this one.

    The phases change because of the moon’s orbit. At full moon, the moon is behind the Earth, and its entire face is lit by the Sun. Because it is behind the Earth, we can see the whole thing. You would think Earth’s shadow would block out the moon (which it does, sometimes, during a lunar eclipse) but the moon’s orbit is not equal with the Earth’s equator. This causes the moon to orbit at an angle, sometimes looking very near or far from the Earth, or above or below the Earth at times.

    New moon is when the moon passes in front of the Earth and we cannot see the moon. Again, there isn’t always an eclipse here, either, due to the moon’s orbital plane.

    As the moon orbits the Earth, we see the different phases. We only see the phases because of the angles at which the Sun hits the moon, and the angle at which the moon is lined up with Earth.

    On the diagram, you probably notice the words “waning” and “waxing”. Those just tell us exactly which phase the moon is in and how close or far it is from the full or new moon phase. “Waning” means shrinking, and “waxing” means growing. So a waxing crescent, for example, is growing slowly towards full moon. You also may notice “First Quarter” and “Last Quarter” phases, when the moon is half-moon shape. If you include full and new moon as the missing phases, first quarter is literally the first quarter in the cycle, full moon is second, last quarter is third, and new moon is the repeat of the cycle.

    I really hope this wasn’t too confusing. It’s a lot easier to explain verbally.

  • coldfieldgirl says:

    While the moon is doing her rounds around earth, the sun always shines on the half of the moon! But since we stand on earth, we look some day to only the half of this half lit side (first quarter and last quarter), or just fully on it (full moon), or completely not (new moon).

Leave a Reply