Assuming you're asking how often a "half moon" appears, a half moon (First or Third Quarter moon) will usually occur every 14-15 days (half a lunar month).
because the moon is one quarter size of the earth.
First quarter and last quarter appear as a half moon. about halfway down this page are some pictures: http://www.calculatorcat.com/moon_phases/moon_phases.phtml
At the time of the "First Quarter" and "Third Quarter" phases, the moon appears half-illuminated. These occasions are roughly 7 days and 21 days after New Moon respectively, and Full Moon occurs at the time exactly midway between them. Note: Both phases are easily visible in the daytime. First Quarter rises in the east around Noon, and Third Quarter sets in the west around Noon.
The four phases of the moon that appear to last 1 or 2 nights are the New Moon, First Quarter, Last Quarter, and Full Moon. The New Moon and Full Moon both occur for one night, while the First Quarter and Last Quarter phases can last for about one to two nights.
The phases called "first quarter" and "third quarter" have those labels because theyoccur one quarter and three quarters of the period of time from one new moon untilthe next new moon.At both of those times, we see half of the moon lit up.
At the third quarter phase, half of the moon is illuminated by the sun, but the side facing Earth appears as a crescent. So, from the moon, you would be able to see half of the moon illuminated, which would appear as a half-moon shape.
The four main phases of the moon are New Moon, First Quarter, Full Moon, and Last Quarter. These phases occur as the moon orbits Earth and changes position relative to the sun, causing different portions of the moon to appear illuminated from our perspective.
If the moon is full today, in a week it will be in the last quarter phase. The lunar cycle progresses from full moon to last quarter over approximately a week, as the moon moves into a waning phase. This means it will appear as a half-moon, with the left side illuminated in the Northern Hemisphere.
The four faces of the moon are called the Phases of the Moon. They are the New Moon, First Quarter, Full Moon, and Last Quarter.
Both the 1st Quarter moon and the 3rd (last) Quarter moon can be referred to as the "half moon" since we see half of the sunlit side of the moon.
A half moon appears when only half of the moon's illuminated side is visible from Earth. This occurs when the moon is at a 90-degree angle relative to the Earth and the sun, known as the first quarter or third quarter moon.
Full moon,New moon,First quarter,Third quarter or Last quarter.