For millennia, humans have kept track of time by observing the changing face of the moon. In fact, you may have noticed that the word moon shares its first few letters with the word month and that s no coincidence. The phases of the moon new moon, first quarter, full moon and last quarter repeat themselves about once every month. But why does the moon have phases at all? To answer this question, it s necessary to understand two important facts. First of all, the moon revolves around the Earth once every 29. 5 days. And secondly, as the moon carries out its voyage around the planet, it s lit from varying angles by the sun. One half of the moon is always illuminated by the sun. But here on Earth, we can t always see the half of the moon that s lit up. What we call the phases of the moon represent the different fractions of the moon s lighted half that we can see as the moon circles the Earth. [
When the moon and the sun are on opposite sides of the Earth, we perceive the moon as full. However, when the sun and the moon are on the same side of the Earth, we say the moon is new. During a new moon, the side of the moon that we can see from Earth is not at all. Between the new moon and, the moon is a crescent (less than half illuminated). It then waxes grows bigger into a half-moon (half-illuminated). The first half moon after the new moon is called the first quarter because at that point, the moon is one-quarter of the way through its monthly cycle of phases. After the first quarter comes the gibbous moon (more than half illuminated) and finally a full moon.
This cycle of phases then repeats itself in reverse. After a full moon, the moon wanes becomes smaller into a gibbous moon, a half-moon (also called last quarter), a crescent and finally a new moon. Just before and just after the new moon, when a slim crescent of the moon is lit, you can also see the rest of the moon lit dimly. This owes to sunlight that bounces off the Earth and illuminates the otherwise dark portion of the moon that s facing us, an effect known as earthshine. The major phases of the moon new moon, first quarter, full moon, last quarter and next new moon occur, on average, about 7. 4 days apart. If you need some help tracking these phases yourself (or if you want to see where the moon was on an important day in history), NASA provides an of the dates and times of all phases of the moon for the six thousand year period between 2000 BCE to 4000 CE. NASA s, a coalition of amateur astronomy clubs from around the U. S. , also provides information that may be helpful to those who want to know more about the phases of the moon and the solar system in general. , provided by the Astronomical Society of the Pacific, demonstrates why the moon has phases. Follow Elizabeth Palermo on Twitter @ techEpalermo, Facebook or Google+. Follow LiveScience @livescience. We re also on Facebook Google+. More information about why the moon has phases: 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.