The Moon’s rotation takes 27.3 days which is the same amount of time it takes to revolve once around Earth. Because these two motions take the same amount of time, the same side of the Moon always faces Earth. If you check the calendar, you will find that it actually takes the Moon 29 days to completely go through the eight phases. The difference between the 27.3 and the 29 is that as the Moon is orbiting the Earth, the Earth is still orbiting the Sun so the Moon has to catch up. If the Earth were not orbiting the Sun, then it would only take the 27.3 days.
Everyone knows the surface of the Moon is reflecting the light of the Sun. As we just learned, the Moon and the Earth are constantly moving around the Sun. All of this movement constantly changes how much of the Sun’s light gets reflected back to Earth. NEWSFLASH! One half of the Moon is always having daytime and one half is having night time, just like the Earth. What we call the Moon phases are the different ways the Moon appears from Earth. The phase of the Moon depends on the relative positions of the Moon, Earth, and the Sun.
Let’s learn about those phases. A New Moon occurs when the Moon is between Earth and the Sun. During a new moon, the lighted half of the Moon is facing the Sun and the dark side of the Moon faces Earth. We can’t see the Moon because it rises at the same time as the Sun. Have you ever noticed how some nights you see the Moon and others, you can’t or sometimes you can see the Moon during the daytime? The Moon NEVER rises and sets at the same time from day to day, ever. Think about it, if everytime you look at something, the light is reflecting off of it differently, isn’t it going to look a little different? Remember, New Moon, we can’t see it!
Since the Moon is totally dark to us, it is going to start becoming more and more visible. We call this waxing, it means that more of the lighted half of the Moon can be seen each night.
Our first phase is called the waxing crescent moon and is just a sliver of illuminated surface.
Our second phase is called the first quarter and half of the Moon’s surface that we can see is illuminated. We call it a quarter because we know that the Moon is round and we are only seeing one fourth of it.
The third phase is the waxing gibbous Moon. When more than one quarter of the Moon’s surface is visible, it is called waxing gibbous. More than a quarter but less than a full Moon.
Next comes the Full Moon and it occurs when all of the Moon’s surface that faces Earth reflects light. The side facing Earth is “Fully” illuminated. The Full Moon is the fourth phase.
After the Full Moon, less and less of the Moon’s surface is illuminated. The phases are said to be waning. Waning means that you can see less and less of the lighted half of the Moon each night. The phases are now the reverse of the waxing phases. The fifth phase is Waning Gibbous. Remember gibbous means less than full but more than a quarter. The sixth phase is called the Third Quarter and the seventh phase is the waning crescent. Finally, the Moon disappears and we are back to another New Moon. The New Moon would be the eighth phase.
Knowing the phases of the Moon can help you keep track of time. From the New Moon until the First Quarter takes seven days and from the First Quarter until the Full Moon takes another seven days. So if you are keeping track, that is two weeks from the New Moon until the Full Moon. Now add another seven days until the Third Quarter and then another seven days and we are back to another New Moon. That adds up to four weeks which is what we think of as a month.
I’m sure you have heard about either a solar eclipse or a lunar eclipse. Well, an eclipse takes place when Earth blocks light from reaching the Moon, or when the Moon blocks light from reaching a part of Earth. Eclipses are all about shadows. On some occasions, during a New Moon, the Moon passes between the Earth and the Sun and the Moon’s shadow falls on Earth. This causes a solar eclipse. During a full moon, sometimes the Earth comes between the Moon and the Sun and may cast a shadow on the Moon. Thiscauses a lunar eclipse. Lunar eclipses happen much more often than Solar eclipses. An eclipse can take place only when the Sun, the Moon, and Earth are lined up perfectly. During an eclipse the darkest portion of the Moon’s shadow on Earth is called the umbra and surrounding the umbra is a lighter shadow on Earth’s surface. This lighter shadow is called the penumbra.