why do we have different phases of the moon

When you look up into the night sky, or sometimes the clear, blue sky of the middle of the day, you may see the moon. Sometimes it looks large and completely round. Other times it may seem as if a small slice has been removed or appear to be a slim crescent. What gives? How can the moon change? The answer lies not in the changing of the moon at all! Rather, it is our view of the moon that changes as the earth, moon, and sun shift positions each day and night. This lesson explains more fully the phases of the moon and its lunar cycle. How Do We See the Moon? Before we get into formal definitions, let's try to visualize what is happening in the relationship between the moon, the sun, and the earth over the course of about a month. The sun is stationary; that's the easy part. The earth moves in two ways - it spins on its own axis and it moves in a circular orbit
around the sun. Simultaneously, the moon moves in similar ways; the moon spins on its own axis while orbiting the earth. Also key to our lesson is the understanding that the moon is not a light source. What we see as the illumination of the moon is actually the light of the sun reflecting off of the moon. As the earth and the moon revolve and orbit, the amount of sunlight that hits the face of the moon changes; these shifts in sunlight hitting the moon are what account for the shape of the moon changing. You can simulate a simpler version of what's going on right at home. First, find a dark room and introduce to it a singular source of light, pointing at you. A flashlight sitting on a dresser would work well. Now, stand directly in front of this light, holding parallel to the light a ball.


The flashlight is the sun, your head is the earth, and the ball is the moon. Now, turn your body, slowly. As you turn, you'll notice that the amount of light that hits the face of the ball (the moon) changes. When you are facing your flashlight, the side of the ball facing you will be dark; none of the light can hit the side you see. As you turn, more light will hit the surface of the ball that faces you until you are turned with your back to the flashlight and the whole of the ball is illuminated. The same thing happens between the sun, the earth, and the moon every month(-ish! )! Although the shape of the moon as we see it will change (infinitesimally) every day, there are officially four phases of the moon. A phase of the moon is defined by the shape the moon appears to be when viewed from Earth. The four phases of the moon are: New moon, when the moon is not visible from Earth First quarter moon, when - after a new moon - one half of the moon is visible Full moon, when all of the moon is visible from Earth Third quarter moon, when - after a full moon - the other Now, of course, the moon does not shift exactly from no moon to half, half to full, full to half moon. Rather, the changes are subtle with each passing night. In between each new moon and first quarter moon, there will be a few nights where the moon's light is shaped like a crescent. Similarly, after a third quarter moon, there will be another crescent, as the shape of the moon shrinks back towards another new moon. Observing the moon from Earth, it is easy to see that it goes through a cycle of light and dark appearances.


Different stages of this cycle are known as phases, and there are technical names for them. Explaining the moon phases requires an examination of the moon's orbital position in relation to the Earth and the sun. Most people believe that the moon takes one month to orbit around the Earth. This is mostly (but not exactly) correct. The moon's orbit is explained scientifically by two different periodicities. The synodic period, also called lunation, is the time between when the exact same moon phase is observed by someone on Earth. This period lasts exactly 29. 5305882 days. The sidereal period, also called the orbital period, is the actual time it takes the moon to orbit the Earth. This period lasts exactly 27. 3217 days. The difference in period lengths is accounted for by the movement of the Earth. Someone observing moon phases from Earth is observing from a platform that is also in motion. During the moon's revolution, the Earth has moved approximately 1/12 of it's own yearly revolution around the sun. Moon phases describe how much and what parts of the moon are observed as light and shadow. As the moon proceeds through it's orbit, the change of phases is easily observed. During a full moon phase, the entire moon is seen as light. During a new moon, the entire moon is seen as shadow. At first quarter and third quarter moon phases, one half of the moon is seen as light, and one half as shadow. The times in between are known as crescent and gibbous, as the moon's lighted or shadowed area takes on a crescent shape.


Just like Earth, half of the moon is lit by the sun, and half is in shadow at any given time. As the moon travels around the Earth, we see the moon from different angles, and thus can see different percentages of light and shadow. When the moon is full, the moon is on the opposite side of the Earth from the sun. As a result, we can see the whole of the lit side of the moon. At new moon, exactly the opposite alignment exists, with the moon being between the Earth and the sun. At that point, we can only observe the shadowed side of the moon. At first and third quarter moons, the moon is at a 90 degree angle from the Earth and sun. We can see half of the lit side and half of the shadowed side. The crescent and gibbous periods are observed as the moon transitions between these points in its orbit. Waxing vs. Waning; Crescent vs. Gibbous There are four terms used to describe the "in between" moon phases: waxing, waning, crescent and gibbous. Waxing is when the moon's lit area appears to be increasing, whereas waning is when the lit area appears to be decreasing. Crescent is when the moon appears less than half illuminated, and gibbous describes when the moon appears more than half illuminated. An eclipse occurs at full moon phase when the Earth casts a shadow on the moon, temporarily making it go fully or partially dark. Partial eclipses occur several times per year, whereas a total eclipse occurs very infrequently. Eclipses are relatively short events, and you can observe over the course of a few hours the moon going from full to dark and back to full again.

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