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The moon orbits the earth in about 24 hours and 50 minutes. This is called a lunar day. The resultant orbital lag between the sun and the moon causes the phases of the moon...
The luminosity of the moon is caused by the light from the sun.It is the varying relative positions of the sun and the moon that produce the phases of the moon.These lunar phases,earlier discussed as tithis, are extremely important in vedic astrology.
A lunar month is made up of 30 tithis. Each tithi is determined when the moon moves in advance of twelve degrees ahead of the Sun.
Tithi (Lunar Phase)
The first element of the Hindu pancanga is the tithi or lunar phase. This is perhaps the single most important element of the pancanga. It is the building block for the lunar month. Simply stated a tithi is a measurement of 12 degrees of longitudinal separation between the sun and the moon. Another way to put it is to say that a tithi is the daily phase of the moon. For example, at new moon (amavasya) the sun and the moon are separated by zero degrees. We can say they overlap. As they begin to separate the first tithi begins when the sun and the moon have separated by 12 degrees. The moon is now a tiny almost imperceptable sliver. The second tithi begins when they are separated by 24 degrees. The sliver is slightly larger. The third tithi begins when they have separated by 36 degrees. The digit of the moon is new clearly visible. And so it goes until the sun and moon have separated by 180 degrees. This tithi is called full moon, purnima. These first 15 tithis or phases of the moon make up the waxing phases of the moon which in Sanskrit this is called the sukla-paksa. This is the bright side of the lunar month. After purnima, full moon, the tithi begins again counting from one as the longitudanal separation between the sun and the moon decreases back to zero. This is called the waning phase of the moon or in Sanskrit, the krsna-paksa or dark side of the lunar month. At certain times of the month when the sun and moon can both be seen in the sky at the same time you can estimate the tithi by using the hand method to measure the longitudinal separation between the sun and the moon.
The tithis are sequentially numbered from both the points of the new moon as well as the full moon. See the accompaning illustration. In this way, the sukla-paksa, begining with the new moon (amavasya), is followed by the first tithi, then the second tithi, the third tithi and so on up to the 14th tithi. There is no 15th tithi. Instead, this tithi is called full moon (purnima). After the full moon, the waning phase (krsna-paksa) again begins with the first tithi, the second tithi, the third tithi and so on up to the fourteenth tithi followed by the full moon. Afterwards the cycle repeats itself. In this way thirty tithis make up a lunar month, which is known as a masa. Some parts of India begin the month from the full moon whereas other parts begin the month from the new moon. Today, the lunar calendar is still in use throughout the world for Hindu religious purposes.
Ancient Hindus devised a scheme to track the lunar days. They refer to a lunar day as a "tithi," with 15 tithis assigned to the bright half of the lunar month ("shukla paksha"), when the Moon is waxing, and 15 tithis assigned to the dark half of the lunar month ("krishna paksha"), when the Moon is waning. The tithi is a measure of the phase of the Moon, which is also a measure of the difference in longitude between the Sun and the Moon.