Expert: Jayendra Upadhye - 3/9/2005

Question
Hello,
Just a simple one here- During the Full Moon Phase, I'm curious on what the average number of hours the moon is 100% full before the slightest degree of waning begins. Thanks for your time.

Answer
Hi Stephen,
Your question has a simple and a difficult answer.
The simple answer as i discovered on many sites on the web is:- "But an instant..just as the noon is but an instant of the whole day".

The difficult part is more satisfactory and clear.
The moon subtends a clear (but small) angle when viewed from the earth.

That means the lines joining the viewer and the opposite east west ends of the lunar disc are NOT quite, but ALMOST parallel.

During the start of the full-moon, the following happen.
(1) - The moon appears full, meaning, from no point in the viewer's field of view, the day-night divide can be seen on the lunar disc.
(2) - The moon continues to move in its orbit.
(3) - At some point when the moon has traversed the distance equal to it's radius or width, along it's orbit,
The limb or day-night line begins to intrude into view.
That signifies end of the full moon phase.

To get the exact duration, you need to know the moon's orbital speed, and time taken to travel distance equal to its own radius, along the orbit.
In reality, each phase will "endure" that much time, as the moon is NOT a point object but a bodu of finite known width!
And to complicate matters, it is not too far from us for us to treat as a point!

Had it been very far in its orbit, one could technically say each phase is an instant.

Since it is not so, the new moon and full moon phases, (where the demarcating boundary ..the day night line is absent), tend to appear to last longer.

Once the day night line appears, its smooth transition can be deemed instantaneous for the intervening phases. Just for conformities sake, one considers all the hundreds of instantaneous traverses of that line as "one particular phase" lasting one specific fraction of a lunar month.
(1/28th? since the lunar month is 28 days).

Jayen