A Full Moon is an astronomical phenomenon which occurs about every 29.5 days (every 29.53059... days – that’s 29 days, 12 hours, and 44 minutes, and 2.8 seconds – to be more precise), or, that is to say, (usually) once every calendar month. (Occasionally there are two Full Moons in the same calendar month. When this does happen, the second one is often referred to as a Blue Moon.)
The Full Moon marks the stage of the lunar cycle when the moon is passing behind the earth, relative to the sun, during which time it appears as a fully lit circle from the vantage point of the earth. Actually, in modern astronomical terms, the Full Moon is defined as the precise moment when the center of the moon, while traveling in the portion of its orbit on the far side of the earth and farthest away from the sun, passes through an imaginary plane extending through the center of the earth and the center of the sun.
While the New Moon traditionally marks the beginning of each lunar cycle, the Full Moon is regarded as its culminating event. The Jewish observance of Passover and the Christian celebration of Easter are both calculated according to the Full Moon occurring closest to the Vernal Equinox in the spring time. Also the names that various cultures have assigned to specific lunar cycles throughout the year are associated most particularly with the Full Moon that occurs within that given cycle. (A list of those names for the seasonal Full Moons can be found HERE.)
Because the moon’s orbit around the earth is elliptical rather than circular, it is slightly closer to the earth at certain times than at others. The point in its orbit at which the moon is closest to the earth is known as perigee. When the Full Moon happens to occur when the moon is also at or near perigee, this makes the moon appear slightly larger in the sky than it does at other times. This event is called a Supermoon.
As is commonly known, the moon’s gravitational pull is also what causes tides in coastal areas upon the earth. The days surrounding the Full Moon (and the New Moon as well) see the greatest differences between high and low tides, a phenomenon known as Spring Tides (think “spring” in the sense of jumping or leaping, not in connection with the season.)
As most people are aware, a lunar eclipse is caused when the Full Moon passes through the earth’s shadow. This fact, combined with the above explanation, is liable to raise the following question: why isn’t there a lunar eclipse with every Full Moon? The answer lies in the fact that the imaginary planes representing the earth’s orbit around the sun (also known as the ecliptic) and the moon’s orbit around the earth are not in perfect alignment (although they are close). Only when a Full Moon happens to occur at the same time when the moon is also at or near one of two points where its orbital plane intersects the ecliptic will there be an eclipse. (These two points are known as the Ascending and Descending Nodes). While this doesn’t happen with every Full Moon, it does happen fairly often. But even so, not every eclipse event is a total one – some are only partial and some may be partial or total, depending on your precise vantage point on earth. And what is more, every eclipse is only visible for a portion of the inhabitants of the earth anyway. Still, because the earth casts a much larger shadow than the moon does, lunar eclipses – even total ones – are a much more common event as seen from any given locale on earth’s surface than are solar eclipses.
