For some time now I have noticed that different sources quote different dates for the full moon, sometimes they agree, but many times there is a one day difference between the dates listed. Pondering the discrepancy and how the Earth and Moon revolve around the Sun it came to me that the varying dates quoted were dependent on the location on Earth the information originated from, and also that it was not 100% accurate. So how is the date of the full moon calculated or what governs its date? The full moon occurs when the moon is directly in line with the Earth and Sun, with the Earth being between the Sun and Moon. The orbit of the Moon is slightly off-level (by 5 degrees) which is why most of the time we see the full moon, and on more rare occasions we see an eclipse, either partial or full. The question of “what day/date is the full moon?” is where the errors begin. The Earth is always in motion around the Sun, and the Moon is always in motion around the Earth. With the Earth and Moon in constant motion there is only a specific moment when the Sun, Earth and Moon exactly align – so the question of when is the full moon should really be “what is the moment of the full moon, and from where I am located what is the corresponding date and time?”. Given that the moment of the full moon lasts just that long, a single moment, the actual full moon could occur while we are on the opposite side of the planet enjoying a warm winters day. For other people it could be morning, for some it could be evening or midnight. The actual moment of the full moon should be quoted as a date and time (generally based on GMT). The next question is - when do I see the full moon? Before going there let it be said that due to the 5 degree tilt in the lunar orbit, the full moon as we call it is actually only 99.9% fully illuminated by the sun. It is so bright the small amount not illuminated is not discernible. The date for the full moon quoted at different locations around the world will be given as the night closest to that ‘moment’ of the full moon, some may include that moment, others will be the night before or after. The moon will still appear to be full, but will either be waning (decreasing) from the 99.9% or waxing (increasing) towards the 99.9% fullness we observe. The next full moon occurs on the 14th of July 2022 at 4.37am Brisbane time (GMT+10) which is fortunately during the night so it will be possible to observe the Moon at the moment of actual fullness. When the moon rises in the evening of the 13th July it will appear full, as best our eyes can observe, but will still be waxing (increasing) in fullness until 4.37am on the 14th of July. From that moment it will then start waning (decreasing) in fullness as the cycle continues. To summarize, the actual full moon occurs at a given moment when the Sun, Earth and Moon are in a straight line. As the Earth rotates we see the moon fairly close to the moment of fullness but it may be the evening before or after. With the Moon 99% illuminated by the Sun it will appear full although the moment of fullness may have passed.
This web site (www.timeanddate.com/moon/phases/australia/brisbane) gives accurate dates and times for Moon phases and can be customized for your location.
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AuthorI love to understand things, how they work, why they happen - I'm always learning and keen to investigate... Archives
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