As it ever so slowly creeps across the night sky, the full moon casts a distinctly magical light on our beautiful planet. The moon may look very full, though, on nights when the calendar may list the full moon on the day before or the day after. How long then is the full moon and when would it actually be full?
On its way around the earth during its cycle of about 29 days we see eight visible phases of the moon.
This process is quite spectacular considering it involves a number of variables to produce such a wonderful and awe-inspiring visible cycle. A new moon, which kicks off a new 29 day rotation, is when the sun and moon are conjunct. The sun at that time is lighting up the side of the moon away from us. When the moon is halfway around its rotation, it is opposite the sun. This allows us to see the entire side illuminated, creating a full moon. It then continues around, reflecting less light until it reflects none again at the new phase.
All of these "phases" are interconnected and part of one big cycle of constant motion. This constant motion obviously means the moon is never in one place. The question on full moon 'duration' can be answered a couple ways then...an astronomical way and an astrological way.
Astronomical Scientific Answer: To go the scientific route, we must incorporate the above literally. This means that the full moon would strictly mean 100% maximum illumination. Given that this light again, is being observed on spherical object that is constantly moving, it occurs at an infinitely specific 'time'. Therefore the moon would only be at 100% full for an instant before it is 99.999...% full again. This rounded time of 100% illumination is the time used for the full moon placement on calendars. Most calendars do not specify the time but only put a full moon icon on the day where this occurs.
Astrological Symbolic Answer: Symbolically, the phase of fullness must be correlated with all the other phases. In astrology, it represents the stage of clarity and experience with ideas began in the new phase. The full moon at its fullest (the rounded 'time' from above) happens at a specific degree of the zodiac in a specific astrological sign. This information can be used in interpreting the meaning of the full moon.
The duration of the phase, however is typically interpreted as an equal eighth of the cycle, so about 3.5 days. Some start the phase at the pinnacle of fullness, however, some also say (including myself...more on this sometime) the maximum fullness occurs in the middle of this period as a celebration of the energy represented. So the full phase is about 1.75 days before and after the full moon's brightest point. (The symbolic way of looking at the moon phases can be beneficial for working with ideas and projects in our daily lives. Eventually I will provide my own spin on the information. For now if you want to learn more about the astrological symbolism of the moon phases, visit zodiac arts.)
The reason it looks full for so long though is that we can look at this cycle (as well as many others) in the form of a wave. The top and bottom of the wave here represent the new moon and full moon. The points of the line are much flatter at those times where there would be slowly little change. The part in between the waves, instead of a relative flatness, runs almost vertical at its halfway point, indicating change is happening quicker and more significantly. A great comparison to this topic would be the sun's angle and the changing of the seasons.
If we compare the three moon phases next to each other that represent these parts of the sine wave, it helps to illustrate how this factor is why the moon may appear completely full for a couple days.
This set of three would be centered between the wave peaks when the line is almost vertical, the first quarter moon being the exact center of the diagram above. Notice the dramatic difference between the first and third.
This set of three is centered at the wave's furthest point with the full moon. Notice the 1st and 3rd phase are the same amount of illumination. At the peak of this cycle there would be little change on either side of the total full phase, making the moon look totally full for a couple days. The same factor can be observed at the new phase when the opposite is true and the moon is dark.
So like most things, it depends on how you want to look at it. Ultimately the full moon's presence is much more than a label or a time. Maybe the full moon doesn't exist in the realm of time, but in the realm of symbolism and wisdom, a part of a majestically beautiful and symbolic cycle that represents so much more than any one part of it alone.
The orbits of the moon/earth and earth/sun are slightly offset. When the two orbits "line up" during a full moon or a new moon, that is when we have eclipses. When this happens at the new moon, the moon goes directly in front of the sun causing a solar eclipse somewhere on the planet. For the full moon, we see earth's shadow on the moon, causing a lunar eclipse.
We only see one side of the moon. This is because the moon is rotating itself as it is rotating around us. BUT the moon's rotation synchronizes near exactly with its rotation around us, causing the same side to continuously face us.
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