It's subtle question. In the days before electric lighting, the Moon provided the only reliable source of illumination at night for our ancestors. Except for the few nights around each Full Moon, it would have been too dark to travel about at night, so the activities of hunters would have been linked to the phases of the Moon.
But in big cities, we hardly notice the phases of the Moon, because the lights are on 24 hours a day. Indeed, we have almost lost our connection with the night sky altogether.
The Moon also causes tides, so it has an obvious and direct effect on people who live by the coast and who make their living from the sea.
Some creatures synchronise their breeding with the cycles of the Moon. One example is sea turtles, which come ashore to lay their eggs at a particular phase of the Moon. They do this to ensure that when the eggs hatch, the tides will be highest at a specific time of night, and the baby turtles won't have so far to crawl across the beach to reach the safety of the water.
Scientists have tried to find connections between the phases of the Moon and statistics such as crime rates, admissions to hospital emergency room or the number of babies being born. So far, nobody has demonstrated a convincing link between the Moon and any of these indicators of human activity.
A Blue Moon, according to the current popular definition, is simply the second Full Moon in a calendar month.
This is no more than a chance coincidence between the phases of the Moon, which are a natural phenomenon, and the months of the calendar, which is man-made.
Consequently, there's nothing special about the second Full Moon in a calendar month: that's something which is bound to happen once in a while, because the average length of the lunar month is slightly shorter than the length of most calendar months.
If you're not convinced, consider the Islamic calendar, which is defined by the phases of the Moon. Each month begins with a sighting of the new crescent Moon. Thus the Full Moon occurs, by definition, in the middle of every month, so it's impossible for any month of the Islamic calendar to have two Full Moons.
Nonetheless, every time a Blue Moon comes around, there's a lot of excitement and interest. Newspapers and television stations will report it, and we notice a surge in the number of visitors to our web site.
The chart below shows how many hits our web site has received each day since we launched it in late 1998.
The blue bands indicate the months which have a Blue Moon. The darker blue bands are the Blue Moon months for the United States, where most of our visitors live. The orange bands denote Blue Moons according to the older Farmers' Almanac definition. The occasional spikes outside of Blue Moon months are caused by search engines scanning our entire web site.