Why does the moon have phases?

Answer 1

ANS:The phases of the Moon are the apparent shape of the Moon as seen from Earth. From the Earth only one side of the Moon can be seen. So although exactly half of the Moon is always illuminated (toward the Sun), what we see is almost always some of the lighted part and some of the unlighted part, as the Moon moves around the Earth in its orbit. At full moons, the face toward Earth is all lighted, and at new moons, the face toward Earth is all unlighted.

The following answer provides a more detailed description of how that illumination occurs.

ANS:One of the biggest misconceptions about the Moon is that its phases are caused by the Earth's shadow. The Earth's shadow causes eclipses, but it has nothing to do with phases. Rather, the phases of the Moon are produced by the alignment of the Moon and the Sun in the sky.

The orientation of Sun and Moon determines the phase of the Moon. At new moon, the angular distance of the Sun and Moon is small, less than a few degrees. At first quarter, when the moon is half full, the Moon lies 90 degrees east of the Sun. As an example, if you were to point to the setting sun with one arm and to the Moon with the other, the angle between your arms would be 90 degrees. At full, the moon is 180 degrees from the sun (and consequently rises shortly after sunset). At last quarter, it is 90 degrees west of the Sun.

The "new moon" is essentially invisible because it is between the Earth and Sun and therefore lost in the glare of the sun. Even on day 2 or 3, it is tough to spot the moon because it is just the tiniest sliver and still very close to the sun. The waxing crescent moon grows till it is about half full in the phase we call first quarter. From there it grows to full moon and then begins to shrink back to third quarter, waning crescent and finally back to new moon.

The first and last quarter moons mark the halfway points between the new moon and full moon. The first quarter moon is illuminated on the right hand side. The last quarter moon is illuminated on the left hand side. It seems conflicting to call a half full moon first and last quarter, but the quarter refers to the position of the moon in the sky, not it's phase. A quarter moon is one quarter of a full circle (90 degrees) away from the sun.

The lighted part of the Moon always points the way to the Sun. This means that a waxing crescent moon in the western sky at sunset has the lighted part on the right (west) which is the direction of the Sun. The sequence of the lunar phases always proceeds with the lighted part of the Moon growing from right to left until the moon reaches full. After full moon, the light recedes from right to left until new moon.

* ("left hand" and "right hand" above refer to the Northern Hemisphere, as seen from a north-south oriented observer)
The moon's phases, contrary to a widely-held belief, have nothing to do with earth's shadow. It is only during a lunar eclipse that earth casts a shadow on the moon. The rest of the time, the shadow you see on the moon is the moon's own shadow...the hemisphere of the moon which is turned away from the sun.

The moon is a sphere, like a Basketball. Only one half of the moon or ball can be sunlit at any moment. The other half is dark because the sun is below the horizon for that hemisphere of the moon or ball.

Now we know that only one half of the moon is lit at any moment. But why does it look like the moon changes shape? Well, you should also know that the line that divides the lit from unlit hemispheres (day from night) is called the "terminator". The moon is in motion around earth, so it is slowly but constantly changing its location in the sky relative to the sun. This motion causes us to view the terminator from different locations, causing the moon to look like a crescent, or a bloated egg (gibbous), etc.

Take a ball and color half of it black. Leave the other half alone. Now slowly spin the ball with your fingers and watch how the line between black and white causes the ball to appear to change shape. This is exactly why the moon appears to change.

Answer 2

As the moon orbits Earth, varying portions of the moon's illuminated surface are visible from Earth.

because the moon revolves around the earth and the sun makes shadows on it such as the crescent or half moon phases....

Answer 3

The phases depend on the Moon's direction, from our point of view, compared to that of the Sun. For example, when they are in opposite directions, we see the illuminated side of the Moon - the full Moon. When they are in the same direction (the Moon being in front of the Sun, since the Moon is closer to us), we see the dark side of the Sun - the new Moon. When they are at 90° angle, we see half of the Moon bright, the other half dark. Etc.