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You might have questioned at least once how the Moon phases are caused. You all know that a changeover is taking place in its appearance. Why and how is this changeover taking place? The best way for understanding the Moon phases is to examine the following diagram of sun-moon-earth:
In the beginning, the illustration may look complicated but it would be easy once you start understanding each concept.
As its shown, the Sunlight is coming from the right. It isvery well recognized fact that the Earth is present at the center. In the diagram, you can find that the 8 stages of the Moon during its revolution are shown. Phase name of the Moon is shown in the image. The dotted lines between the Moon and earth are representing your sight line while looking the Moon. You will see the large moon image when you are looking at that point of the cycle. To see the waning gibbous, waning crescent and third quarter phases mentally you have to turn yourself upside down. This imagination would be made when you are imagining the sight line. While doing this, you will be feeling that illuminated portion is towards your left. This is depicted on the larger image.
Utter most important thing is that always one half of the Moon will be lightened up by the Sun. This is logically correct, but you must visualize these things in order to understand the concept of moon phases better. Sometimes there are chances that you can see both shadowed and sunlit portion, this will create moon shapes of different phases which you are all familiar. For the naked eyes, the shadowed portion will be invisible. For clarification purpose, this is represented in the above diagram.
By changing the relative positions or changing angles of the Earth, the Sun and the Moon, the lunar eclipse takes place.
If you are willing to examine the Moon phases in details, you can take the help of Lunar Calendar for iOS or Moon Phases Calendar for Android which are the best lunar calendars for the most popular mobile platforms.
The moon cycle can be understood better in an order of the full moon and new moon, first quarter, third quarter and few other phases in between in this explanation.
When the position of sun and earth is in between the Moon, then occurrence of the new moon can be seen. This is shown in the above large image. In appropriate alignments, these three objects are placed. The illuminated part of the Moon cannot be seen by us since it is present on the other side.
When a full moon takes place, sun, moon and the Earth are also positioned in approximate alignments, similar to the positions of the new moon, but you will see the Moon be present on the contradictory side of the Earth. During this phase, the Sunlit part will be facing towards us. Due to this, the shadowed portion will be hidden from the view entirely.
Usually the third quarter and the first quarter are called half-moon. This is going to happen when the Moon is exactly at 90 degree to the Sun and the Earth. Because of this reason, you can see the half illuminated portion during this phase. Remaining half portion will be under shadow which could not be seen.
By understanding these four, key phases; it would become rather easy for visualizing the illuminated portions during the transition.
Simple and unique way to understand and remember the in between phases is by defining and breaking out the 4 words. The four words are waning, waxing, gibbous and crescent. The crescent word refers to the Moon phase where it is illuminated less than half. In the gibbous phase, the Moon will be lightening up more than half. Waning means decreasing or shrinking in illumination. The word waxing means expansion or growing in the illumination. You can combine two words at a time to create the name of two different intermediate phase names.
Soon after the phase new moon, the portion of sunlit will start rising but it would be less than half, so the name can be given as a waxing crescent to this phase. When the first quarter is over, sunlit portion will start increasing again. But it would be more than half hence it is called waxing gibbous. The light will decrease continuously after the maximum illumination or full moon. This phase is termed as the waning gibbous. Waning crescent occurs after the third quarter, which diminishes till the light is gone completely.
Within a span of one month, the Moon is going to undergo all the above mentioned phases. As a matter of fact, this duration of one month is not true. The lunation or synodic period will take 29.5305882 days if it is properly calculated. It is the span required by the Moon to move back to the same position. If you are viewing the Moon cycle from outside the Earth, then you will notice that the time required for completion of one cycle is 27.3217 days. Two days less than the time viewed from the Earth. This period is also termed as the orbital period or sidereal period. Why is the sidereal period different from the synodic period? The reason is that since you are viewing the Moon from the traveling platform: the Earth is going to move a certain distance in this yearlong cycle during the cycle of the Moon. This alters the angle of moon view due to which the phase will be slightly altered. For the Earthbound observers, the Earths’ orbital direction is going to lengthen the period.
Even though the sidereal periods and synodic periods are accurate numbers, they cannot be precisely calculated by the method of simple division of days. The reason is that the Moon’s motion will be perturbed and affected by the various strengths and forces. To determine or find the exact position and moon phase at a given time, you must use different complex equations.
When you look into the diagram, you will find that during the phase of a new moon, sun is not blocked by the Moon, and at the phase of a full moon the Earth is not blocking the reach of sunlight to the Moon. From the Sun-earth orbital plane, the Earth and moon are 5 degrees off due to this; the Sunshine is not obstructed by the Earth.
At some special times, sun, moon and the Earth is going to line up in the year. When sun is blocked by the Moon, it is termed as a solar eclipse, and this will take place only during the phase of a new moon. When the Moon is casted shadow from the Earth, it causes a lunar eclipse, and this is going to occur only during the phase of the full moon. In every year, around 4 to 7 eclipses are going to happen. Most of them are partial or minor eclipses. Major eclipse is a solar eclipse which is found in very rare cases.
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