by Sarah Todd

Created on: August 11, 2008

Despite little or no actual proof that the full moon causes the manifestation of the werewolf this ancient belief persists in some places to this day. Testament, perhaps to the power of the Moon's influence upon culture, and the reason many more modern myths about the effect of the Full Moon still thrive.

There is also no evidence that the Full Moon is responsible for increased cases of mental illness or the number of patients admitted to emergency rooms. Birth and suicide rates are not significantly higher during Full Moon, and it has no proven effect upon fertility. Neither does the Full Moon increase the probability of a man-made or natural disaster taking place.

The lack of proof has done little to diminish the respect and reverence many have for Earth's satellite. The myths and rumours are as reliable and varied as the Moon's monthly phases.

But the Moon isn't as consistent as we believe. A "Black Moon" is a rarely used astronomical term that describes several of the Moon's "inconsistencies", one of which is the absence of a Full Moon in a calendar month.

The lunar month is measured between New Moons, and is approximately 29 days 12 hours and 44 minutes long. However, this time period can vary by as much as seven hours, because the Moon's orbit around the Earth follows an oval rather than circular path. The Earth's own orbit around the Sun is also an oval ecliptic.

The oval path means the speed of both the Sun and the Moon along their respective orbital paths is not consistent. When the Moon's speed is faster than average and the Sun's speed is slower the Moon will catch up with the Sun sooner, resulting in a shorter lunar month. Conversely, when the Moon moves slower than average and the Sun is faster the Moon catches up with the Sun later, resulting in a longer lunar month.

When Belgian astronomer Jean Meeus calculated the exact dates and times of every New Moon during the two centuries from 1900 to 2100, he was able to identify the length of both the shortest and the longest lunar months. The longest lunar month was, at 29 days 19 hours and 55 minutes, four hours shorter than 30 days, proving all four 30 day months must have at least one Full Moon. The shortest lunar month measures 29 days 6 hours and 35 minutes, meaning the calendar's shortest month is the only month which could miss a Full Moon. The shortest lunar month is six hours longer than a leap year, so February is the only month likely to miss a Full Moon.

The last year February had no Full Moon was in 1999. In fact, the Black Moon was only experienced four times during the last century. February 2018 is the next year the month will have no Full Moon on its calendar, repeated in 2037, 2067 and 2094. Whenever February misses a Full Moon the months January and March are credited with two Full Moons. The astronomical term for an irregularly timed Full Moon two Full Moons in one calendar month - is "Blue Moon".

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