"Phase" refers to the fact that the moon shows differing amounts of lighted hemispheres as viewed from the earth during its orbit around the earth.
"Cycle" refers to the repetition of these phases, as well as cycles of eclipses. We will investigate these here.
Understanding the observed phases of the moon, requires understanding how light and shadowing works in relation to the sun's light and the orbit of the moon and earth around the sun.
A superstition: It is sometimes believed that when the moon is bright and full, people act crazy. In fact, there is no statistical scientific evidence to support this. It is a myth.
Also: note that there are no monsters devouring the sun, as you might have been told if you were listening to a professor in 2000 BC.
One thing about studying moon phases: it begins to address specifically how what we see observe in astronomy has a very clear scientific explanation.
Given these facts, can you infer if the moon rotates around its own axis? (figure 3-2)
Other Key Points:
Moon does not produce its own light, and so the light that we see from it is that which is merely reflected from the sun.
As the moon moves around the sky sun illuminates different amounts of its surface. The phase of the moon is thus entirely determined by looking at the Earth's location relative to the sun. (figure 3-3). The best way to see the phases is to study the figure.
One can see the progression of phases: New, Waxing Crescent, First quarter, waxing gibbous, full, waning gibbous, 3rd quarter, waning crescent.
Note: the use of the word "quarter" here does not refer to how much of the moon is visible, but the phase in the cycle.
Cycle of lunar phases takes 29.5 days this is the SYNODIC PERIOD.
Why is this longer than the SIDERIAL PERIOD which was 27.3 days? very simple: this is because the moon returns to the same place on the sky once every siderial period, but the sun is also moving on the sky. When the moon returns to the same spot on the sky the sun has moved 27 degrees. Thus the moon now has to take some extra time to catch up. (figure 3-4). The moon takes about 2 days to catch up.
The conceptual way to understand moonrise and moonset from the book is to follow the directions in the book on how to make a moon-phase dial.
Imagine standing with the human figure on the globe pictured. Now the key point is that the horizon above which the moon is visible, is the plane perpendicular to your body. The time of the day is given by "sunrise, sunset, noon, midnight." Those times of day appear to us as the Earth rotates so that the US passes through them. While the Earth rotates, the position of the moon in its orbit doesnt change much, so we can talk about moonrise and moonset. As you imagine the Earth turning and the various times of day passing you can see how the moonrise and moonset differs for different moon phases.
If you live near the sea or you surf you are familiar with the tides and the periodic high and low tides.
Then pick a particular phase of the moon. The planets, as viewed in the sky, exhibit characteristic aspects and phases. "Aspects" refers to the location of the planet with respect tour overhead sky reference (objects on the celestial sphere); "Phase" again refers to the fact that the planets, through a telescope, exhibit phases (differing amounts of lighted hemispheres as viewed from the earth) just like the moon. The terminology associated with these aspects and phases is different, depending on whether we refer to an inferior planet or a superior planet.
Gibbous phases are phases between quarter and full phases. Greatest Elongation refers to the largest separation of the planet from the Sun in our sky, either to the East, or to the West. Thus, we see that the inferior planets exhibit a complete set of phases (just like the Moon) as viewed from the earth, and can never be further from the Sun than the angles defined by greatest elongation.
Aspects and phases of the inferior planets.
When a superior planet is at quadrature, it is on our celestial meridian at sunrise or sunset. Comparing with the preceding diagram for the inferior planets, we notice two basic differences: (1) The superior planets do not exhibit a full range of phases; they are always gibbous or full. (2) The superior planets can be located at any distance East or West of the Sun in our sky, unlike the inferior planets where there is a limiting angle away from the Sun (greatest elongation).
Shockwave movie illustrating aspects and phases of the superior planets.