What causes lunar phases?
The phase of the moon (new, half, full, etc) is a function of how much of its lit surface we can see from the earth. Half the moon is always sunlit, half is dark, but as the moon orbits the earth, more or less of the lit face is pointed towards us here on earth.
The moon's synodic period around the earth is about 29.5 days, which coincides with the length of a month. Once in each of its synodic periods, its lit face is directly oriented towards earth -- a full moon.
Because the synodic period is less than some calendar months, on relatively rare occasions we can have two full moons within one month. The second of those full moons is referred to as a "blue moon" -- thus the phrase, once in a blue moon, which means "rarely."
Contrary to popular belief, the phases of the moon have nothing to do with shadows caused by the earth. Whenever the earth's shadow passes over the moon, it is full moon, and the passing of the shadow is a full or partial lunar eclipse. As the moon orbits around the earth, we get to see the moon in different degrees of illumination by the sun. These different degrees of illumination are the moon's phases. We…
How long is the moon phases and how long does the lunar cycle last and what causes the moon phase to change phase to phase?
Yes, it does. An observer on the earth side of the moon would notice the earth going through phases similar to the moon's, and the phases would take a lunar cycle to complete. However, when it is full moon to us, it would be 'new earth' to the lunar observer. When it is new moon for us, lunar observers would be treated to a full earth.
Lunar phases are the result of looking at the illuminated half of the Moon from different viewing geometries; they are not caused by shadows of the Earth on the Moon that occur during a lunar eclipse. The Moon exhibits different phases as the relative geometry of the Sun, Earth and Moon change, appearing as a full moon when the Sun and Moon are on opposite sides of the Earth, and as a new moon (also…