On the day next to amavasya, the moon is seen as a thin crescent immediately after sunset, in the west. On the following days, the crescent grows in size. Also, the position of the moon changes when observed at sunset each day. The moon's position is observed to shift towards the east each day when observed during the sunset. After 7-8 days, the moon is seen in mid-sky and its size is increased to half i.e. the moon is seen as a semi-circle. On succeeding days, the moon is observed to increase in size and move towards the east. And finally the climax! Oh yeah! This is when the moon appears in all its splendour. The Full Moon! Technically speaking, this is called the Poornima. This comes around about 14-15 days after amavasya. On a Poornima day, the moon rises in the eastern sky at sunset time. On the subsequent days, it diminishes in size. Also it rises later and later after sunset. One may wonder, why this waxing and waning in the moon's size occurs. That's because, the part of the moon that we do get to see is the part that's both illuminated by the sun as well as in the line of our sight! Since the sun and the moon keep moving in circles, this illuminated part that's visible to us changes in size, increasing progressively from amavasya to Poornima. This period is called 'Sukla Paksha'. On Poornima day, the moon and the sun are in exactly opposite positions. Thus the entire face of the moon is visible to us and this is what we call the full moon. Coming back to the discussion on the waxing-and-waning stuff, the exact opposite happens from Poornima to amavasya, i.e., the moon's size decreases progressively from Poornima to amavasya. This period is called 'Krishna Paksha'.
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The amavasya occurs 14-15 days after the Poornima which itself occurs 14-15 days after the previous amavasya. This cyclic change in the moon's size -- amavasya-Poornima-amavasya leads us to categorize the moon as being in 5 different phases at different periods of the amavasya-Poornima-amavasya cycle.
1) Invisible,
2) Crescent,
3) Half,
4) Gibbous (more than half)
5) Full.
