Moon Phases

Have you ever wondered what causes the moon phases? We all know that its appearance changes over time. But why? The good way to understand the phases of the moon is to examine an earth-moon-sun diagram:Sunlight is shown coming in from the right. The earth, of course, is at the center of the diagram. The moon is shown at 8 key stages during its revolution around the earth. The moon phase name is shown alongside the image. The dotted line from the earth to the moon represents your line of sightwhen looking at the moon. The large moon image shows what you would see at that point in the cycle. For the waning gibbous, third quarter, and waning crescent phases you have to mentally turn yourself upside down when imagining the line of sight. When you do this, you'll "see" that the illuminated portion is on your left, just as you see in the large image.

  • One important thing to notice is that exactly one half of the moon is always illuminated by the sun. Of course that is perfectly logical, but you need to visualize it in order to understand the phases. At certain times we see both the sunlit portion and the shadowed portion -- and that creates the various moon phase shapes we are all familiar with. Also note that the shadowed part of the moon is invisible to the naked eye; in the diagram above, it is only shown for clarification purposes. Finally, please realize this diagram is only meant to demonstrate how the phases work; the small inner moons in the diagram do not show the fact that the Same Side of the moon always faces Earth.


A solar eclipse is a natural event that takes place on Earth when the Moon moves in its orbit between Earth and the Sun (this is also known as an occultation). It happens at New Moon, when the Sun and Moon are in conjunction with each other. If the Moon was only slightly closer to Earth, and orbited in the same plane and its orbit was circular, we would see eclipses each month. The lunar orbit is elliptical and tilted with respect to Earth’s orbit, so we can only see up to 5 eclipses per year. Depending on the geometry of the Sun, Moon and Earth, the Sun can be totally blocked, or it can be partially blocked.

During an eclipse, the Moon’s shadow (which is divided into two parts: the dark umbra and the lighter penumbra) moves across Earth’s surface. Safety note: do NOT ever look at the Sun directly during an eclipse unless it is during a total solar eclipse. The bright light of the Sun can damage your eyes very quickly.


Plate tectonics is the theory that Earth's outer shell is divided into several plates that glide over the mantle, the rocky inner layer above the core. The plates act like a hard and rigid shell compared to Earth's mantle. This strong outer layer is called the lithosphere.