Welcome to Lunar Calendar 2012!
A lunar calendar is most commonly used to evaluate the different phases of the moon. The main widely used lunar calendar is the Islamic calendar or Hijri calendar, which consists of 12 lunar months. An interesting characteristic of this Islamic lunar calendar is that it is not linked to the seasons. This leads to a drift each solar year of 11 to 12 days. The phases of the moon are caused by the relative positions of the earth, sun, and moon. It takes the moon 27.3 days, or 27 days 7 hours 43 minutes, on average to go round the earth, completing one full lunar cycle. This measurement is taken relative to the stars and is called the orbital period. However, since the earth also orbits around the sun, a complete moon cycle (new moon to new moon) is actually a couple of days longer according to people on earth; 29.5305882 days ends up being the exact number. This number is called lunation and is relative to the sun.
During normal moon phases, excluding lunar eclipses when the moon passes through the earth’s shadow, the sun always illuminates the half of the moon facing the sun. But because the moon is orbiting the earth, we only usually only see a portion of this lit up moon, depending on its location relative to the earth and sun.As the moon reaches the opposite side of the earth from the sun, the moon appears full to us. As the moon moves between the earth and the sun, it appears completely dark, or as a new moon. It is the time in between that you get the varying degrees of fullness and newness. This period is the waxing (grows to full) and waning (decreases to new moon) periods of the moon.
Because the lunation period is shorter than an average calendar month, every 2.7 years you will actually get two full moons in one month. This second full moon of the month is called a “blue moon”. So the phrase “once in a blue moon” refers to this amount of time. Even though there is an exact date and time that the moon becomes full, it may appear to be full for a couple days on either side of this date just because the percentage of the moon that appears illuminated changes slower and slower as it approaches it’s full moon state.
The most common lunar phases that are documented on the lunar calendar 2012 are the new moon phase, the first quarter, full moon, and last quarter phases. So you can typically find a date and time for these different phases in various almanacs and calendars. The two crescent and to gibbous phases are intermediate phases, each lasting about a week between the primary phases.