What causes the phases of the moon?

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 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 of 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 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.


The line boundary between dark and light on the Moon is called the terminator. It is rarely an even line because the surface of the Moon is not smooth. The terminator is closest to a straight line at first and last quarter. Librations affect exactly what portion of the face of the Moon is facing the Earth, during first and last quarter phase, the terminator's exact location may vary. This is illustrated when the terminator does not line up with the Moon's meridian at first and last quarter. In other words, you'd think that during first and last quarter when the Moon is half full, that the meridian (the imaginary line which runs from north to south on the Moon,) would be right on the terminator, but it isn't so. In fact the terminator can be almost 8 degrees off to either side of the meridian.