** My apologies on the lateness - it's been a busy month. **
Full moon. Crescent moon. New moon. There's no doubt you've previously heard these or even talked about them. Perhaps you even know the less common or more specific phases of the moon. Yet, there still seem to be misconceptions about the moon's appearance. I've already covered why the full moon can seem so large and why the moon can appear orange when it's on the horizon. Here's some more information about the moon's appearance:
The Moon's Orbit & Apparent Size
Let's start with some important background knowledge. The moon orbits the Earth on a nearly, but not perfectly, circular path. As a consequence, there are two points where the moon will be at its closest (perigee) and farthest (apogee) distance from the Earth with every other point somewhere in between. The difference in distance between the two extremes is enough to be noticeable, at least with pictures. It might be a bit difficult to see the difference by just going outside and taking a look. Another neat thing about the moon is that it's tidally locked in its orbit. For every one orbit around the Earth, it will rotate exactly one time. The result is that someone on the Earth can only ever see one side of the moon. If you compare the two pictures above, you'll notice that the dark lunar mare near the bottom right corner of the crescent moon is the same as that on the right side of the gibbous moon. On the flip side, there is one side of the moon that we can never see from the Earth. This is where the "dark side" of the moon comes from. The dark side of the moon isn't dark in the true sense of the word. There are times when the sun shines on it. However, it's considered "dark" because people were never able to see it until lunar orbiter spacecraft were built and sent into space.
Moon Phases
Unlike the sun, the moon does not emit light on its own - we only see it because of the sunlight that it reflects. Like the Earth, the moon will only have sunlight illuminating half of it at a time. The half that is illuminated is not always that half of the moon that faces the Earth, which leads to the phases. I'll be a bit more specific with a few examples to help clear up any confusion. When the moon is full, the half that is illuminated is also the half that faces the Earth. The moon is on the opposite side of the Earth than the sun, so the sunlight illuminates the half that faces us. The opposite of this is the new moon. In this case, the moon and sun are on the same side of the Earth, so the half that is illuminated is the "dark" side of the moon.
This case also brings up another interesting point: the new moon should be located close to the sun in the sky. In other words, we see the new moon during the day rather than at night. It doesn't disappear from the sky or become dark. Instead, it is just visible at a different time of day.
Let's leave this little detour and get back to the phases. The last example that I'll give is the half moon. To see a half moon in the sky, the moon would be 90 degrees from the sun. If you have trouble visualizing that, picture it this way: if the Earth is a clock, the sun will appear near the 12 and the moon will appear near the 3. For this phase, half of the "dark" side is illuminated and half of the side that we see is illuminated. To get the rest of the phases, we can just rotate the moon around the Earth. The farther it gets from the sun, the closer it will get to a full moon. As it moves from full to new, a smaller and smaller section (visible to us) will be illuminated. If you need some extra visualization, you check out this chart or try the activity listed below.
Eclipses
I'm sure most people are well aware of solar eclipses, but did you know that the moon can be eclipsed, too? A lunar eclipse happens when all of the moon, Earth, and sun are aligned with the Earth in between the other two. As the moon passes behind the Earth, it moves into the Earth's shadow. Since the moon appears lit due to reflected sunlight, being in a shadow will make the moon appear darker. At the start of the lunar eclipse, a growing fraction of the moon will appear blacked out. As more and more of the moon moves into the Earth's shadow, it starts to turn a dark orange color. The next total lunar eclipse visible for the Americas will be on October 8, 2014, so get your binoculars ready.
Try It at Home
You can see for yourself how this process works with just a (solid) ball and a flashlight. Begin by placing the ball on a table with the flashlight pointed at it from a couple or so feet away. Walk around the table with the surface at eye level and watch the appearance of the ball. When you're opposite the flashlight (sun), the ball (moon) appears as a dark, "new moon" phase. Go around to the same side as the flashlight, and you'll see a full moon (or rather full ball?). As you walk around, you'll see the phases of the ball changing just like the moon. However, the moon orbits the Earth, not the other way around. For a more realistic demo, place a larger ball (or globe if you have one on hand) a few feet from the flashlight. You can simulate the moon and its phases by moving the small ball around the large one. Place a "person" on the large ball. How does the small ball's phase change from your person's point of view as it moves around in its orbit?
Keep in mind that the moon's orbit is slightly tilted. This means that the moon and Earth rarely appear directly between the sun and the other. By aligning all three, you are creating a solar (moon in middle) or lunar (Earth in middle) eclipse.
References & Further Reading
Lunar Eclipse Page - by NASA
Volcanism on the Moon - by Robert Wickman
The Lunar Orbiter Program - by Lunar and Planetary Institute (NASA)
