# Eli5 how moon phases work

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Are the phases of the moon created by the earth’s shadow? And, if so, how can i see the moon in its phase during the daytime?

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The phase of the moon is it’s own shadow you’re seeing. If you shine a light on a ball and look at it from the side, you get the same effect.

The earth’s shadow on the moon is uncommon – a lunar eclipse.

No the moon’s phases are due to the fact that only half of the moon is lit up by the sun at any given moment. If the Earth is approximately between the sun and the moon the we are seeing the lit up side which is a full moon. Then as the moon orbits the earth we see less and less of the lit up side until the moon is approximately between the earth and the sun, at which point there is a new moon.

If all three are precisely in line then we get an eclipse, but this doesn’t happen every cycle because the orbits are not quite aligned.

>Are the phases of the moon created by the earth’s shadow?

No, *lunar eclipses* are created by the Earth’s shadow. Lunar phases are just the result of the Moon being a sphere lit from a single direction, and the direction of the light changing over time as the Moon circles the Earth. Notice that the illuminated side of the Moon is always oriented towards the Sun, and that it’s always a gibbous or crescent Moon during the day; when the Moon is on the sunward side of the Earth, we see more of its shadowed side, and when it’s on the spaceward side of Earth, we see more of its bright side.

The phase of the moon is it’s own shadow you’re seeing. If you shine a light on a ball and look at it from the side, you get the same effect.

The earth’s shadow on the moon is uncommon – a lunar eclipse.

Exactly half of the moon is lit by the sun at all times, just like the earth is. The “phases” of the moon are how much of the lit side we can see.

The moon orbits the earth once a month, so for part of its orbit it’s on the same side of the earth as the sun (earth’s daytime), and for part of the orbit it’s on the opposite side of the sun (earth’s nighttime). If it’s on the same side as the sun, then obviously the sunlit half of the moon is the half that’s facing away from us (new moon). If it’s on the opposite side as the sun, then the sunlit half of the moon is the half that’s facing toward us (full moon). And if it’s off to the side, then we see part of the sunlit side, so the first and third quarter, and waxing/waning crescents.

The moon’s orbit is tilted with respect to the earth’s orbit around the sun, so it’s very rare that the earth’s shadow is cast on the moon. These rare occasions are called lunar eclipses.

>Are the phases of the moon created by the earth’s shadow?

No, *lunar eclipses* are created by the Earth’s shadow. Lunar phases are just the result of the Moon being a sphere lit from a single direction, and the direction of the light changing over time as the Moon circles the Earth. Notice that the illuminated side of the Moon is always oriented towards the Sun, and that it’s always a gibbous or crescent Moon during the day; when the Moon is on the sunward side of the Earth, we see more of its shadowed side, and when it’s on the spaceward side of Earth, we see more of its bright side.

It’s helpful to imagine a simple model to understand moon phases: it just takes two objects (balls are best) and a lamp.

If you place ball E (for earth) adjacent to ball M (moon) and shine the lamp from a few feet away, you’ll see a different shadow (phase) of M than if you place M between E and the lamp, or if you place M to the other side of E, or if you place M behind E.

Here’s a nice illustration of an even simpler way:

>Are the phases of the moon created by the earth’s shadow?

No, *lunar eclipses* are created by the Earth’s shadow. Lunar phases are just the result of the Moon being a sphere lit from a single direction, and the direction of the light changing over time as the Moon circles the Earth. Notice that the illuminated side of the Moon is always oriented towards the Sun, and that it’s always a gibbous or crescent Moon during the day; when the Moon is on the sunward side of the Earth, we see more of its shadowed side, and when it’s on the spaceward side of Earth, we see more of its bright side.