Sunday, December 6, 2009

Basic Astronomy: Phases of The Moon

Moon phases, or lunar phases, refer to the different appearances that the Moon takes on over the course of a lunar month. At the beginning of a lunar month, the Moon is dark. And then, over the course of the month, more and more of the Moon is illuminated until we see a full moon. Then the amount of illuminated moon decreases to a new moon again. Then the cycle starts all over again. These are the phases of the Moon.

When thinking about what causes the phases of the Moon, you've got to realize that the Moon is always half illuminated by the Sun. This is the same for all the objects in the Solar System. We see the different moon phases from here on Earth because our perspective of the Moon changes as it orbits around the Earth. When we can see the Moon fully illuminated, then the Sun and Moon are on opposite sides of the Earth; this is a full moon. The situation is reversed when the Moon and the Sun are on the same side of the Earth. This is when we see a new moon. The other lunar moon phases occur when the Moon makes various angles compared to the Earth.

Eight Phases of the Moon
Although the lunar phases actually transition smoothly from one phase to another, we have developed different terms for the 8 moon phases that look distinct. The Moon's appearance moves through each of these moon phases as the amount of sunlight falling on it changes from our perspective. this is a cycle that always moves in the same direction. The Moon will always go from new moon to first quarter then full moon, then last quarter and back to new moon again.

Here are the eight phases of the moon:

  1. New Moon – When the illuminated side of the Moon is away from the Earth. The Moon and the Sun are lined up on the same side of the Earth, so we can only see the shadowed side. This is also the time that you can experience solar eclipses, when the Moon passes directly in front of the Sun and casts a shadow onto the surface of the Earth. During a new moon, we can also see the reflected light from the Earth, since no sunlight is falling on the Moon – this is known as earthshine.
  2. Crescent – The crescent moon is the first sliver of the Moon that we can see. From the northern hemisphere, the crescent moon has the illuminated edge of the Moon on the right. This situation is reversed for the southern hemisphere.
  3. First Quarter – Although it's called a quarter moon, we actually see this phase when the Moon is half illuminated. This means that the Sun and the Moon make a 90-degree angle compared to the Earth.
  4. Waxing Gibbous – This phase of the Moon occurs when the Moon is more illuminated that half, but it's not yet a full Moon.
  5. Full Moon – This is the phase when the Moon is brightest in the sky. From our perspective here on Earth, the Moon is fully illuminated by the light of the Sun. This is also the time of the lunar month when you can see lunar eclipses – these occur when the Moon passes through the shadow of the Earth.
  6. Waning Gibbous – In this lunar phase, the Moon is less than fully illuminated, but more than half.
  7. Last Quarter – At this point of the lunar cycle, the Moon has reached half illumination. Now it's the left-hand side of the Moon that's illuminated, and the right-hand side in darkness (from a northern hemisphere perspective).
  8. Crescent – This is the final sliver of illuminated moon we can see before the Moon goes into darkness again.
And so, the Moon passes through each of these phases each lunar month. It takes a total of 29.53 days to go from new moon to new moon.

Source: universe today

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