Many people often skittishly use the expression “once in a blue moon” in order to show that something happens really seldom. But do they actually understand what does this expression mean and where does it come from?
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The “blue moon” occurrence is actually an event which happens only once two or three years when it is possible to witness two full moons in just one month. Usually, a full moon occurs only once per month. The term “blue moon” was misinterpreted and derives from the 1946 issue of a monthly American magazine Sky & Telescope.
The Synodic cycle can be described as the time interval between two consecutive and identical phases of the Moon (e.g., full moons). It can also be defined as the time interval which is required for one period to repeat itself in relation to a solar body. On the contrary, the Sidereal cycle can be defined as the time it takes for one period to repeat itself in relation to stars. So it is the time interval between two consecutive returns of the Moon, in its monthly visible movement, to the same place of the celestial sphere (relative to the stars).
The Moon’s phase change is caused by the changes of the angle between the Sun, the Moon and the Earth. While the Moon is orbiting around the Earth, its angle is constantly decreasing or increasing. This changes in the relative position of the Earth, Moon and Sun cause a terminator (the boundary between the illuminated and non-illuminated part of the disk of the Moon) to shift, which, in turn, causes changes of the shape of the visible part of the Moon.
The reason behind the fact that we are able to see only one side of the Moon is that it is in synchronous rotation around the Earth. That means that the Moon rotates exactly one time per revolution around our planet.
In fact, the Moon does spin on its axis, indeed. It takes about 27.3 days for the Moon to make a full turn once on its axis. But at the same time it takes exactly 27.3 days for the Moon to complete its spin around the Earth. Thus because of this synchronization only one side of the Moon is faced towards our planet.
Are crime rates and full moon interconnected? Well, there have always been debates of one sort or another over this question but the answer is yet to be found. Many police officers and doctors suggest that night shifts during full moon prove to be more violent and dangerous than other nights. Serial killers such as Andrei Chikatilo suggested that full moon provoked them to murder. Some researchers and scientists tend to believe that this is caused by purely psychological reasons. Thus, if there is any sort of connection between these events, its nature is still too vague.
A lunar eclipse is an event which occurs when the Moon passes behind our planet’s shadow. On the contrary, when the Moon’s shadow falls on the Earth a solar eclipse is witnessed. It would be possible for us to witness both lunar and solar eclipses every full and new moon (thus, every month) if only the Moon orbited in the same plane as Earth.
But the Moon’s orbit is inclined to ecliptic by roughly 5 degrees. Every month the Moon intersects Earth’s orbital plane at points which are called nodes. In case full moon or new moon is close to one of these nodes, a lunar eclipse or solar eclipse respectively is definite to happen.
One of the most serious problems lunar calendar makers must face is the fact that synodic month duration is changing with the flow of time. Thus, they must decide how it will change over a certain period of time. The problem gets even worse, as the length of the day is also slightly changing (about 2 milliseconds per century).