How long does a Full Moon last?

Question

I know that there was a full moon last night. And the night before. and it seems like it's been a full moon all week? but I am not sure

Answer 1

It is reckoned almost a day. But the 100% fullmoon is in its middle.

Answer 2

A true full moon is only at the specific time when the moon is fully in Earth's shadow.  Since the Earth and moon are always moving, however, the difference is too small for us to see and it appears to be full for several days.

A full moon is often thought of as an event of a full night's duration. This is somewhat misleading, as the Moon seen from Earth is continuously becoming larger or smaller (though much too slowly to notice with the naked eye). Its absolute maximum size occurs at the moment expansion has stopped, and when graphed, its tangent slope is zero. For any given location, about half of these absolute maximum full moons will be potentially visible, as the other half occur during the day, when the full moon is below the horizon. Many almanacs list full moons not just by date, but by their exact time as well (usually in GMT). Typical monthly calendars which include phases of the moon may be off by one day if intended for use in a different time zone.

Answer 3

Only for a moment. The moon is waxing, then full, then waning.

Answer 4

A full moon happens at a specific time. At any other time it isn't a full moon even if it may appear so to the observer.

The last full moon was on July 18th at 7:59 am UT. The moon you saw last night was not full but waning gibbous.

Answer 5

This is an interesting question because there does not seem to be any firm rule as to how long a Full Moon lasts. Perhaps the most common answer would be it lasts all night long (from moonrise to moonset) on the evening when the Moon becomes 100% illuminated, as seen from Earth. However, as you stated in your question, it can appear as if there is a Full Moon on several consecutive nights. In a moment, I will explain why that is, but first we should start off with what condition has to be met in order to have a Full Moon.

A Full Moon occurs when the Moon is 100% illuminated, as seen from Earth. This happens each month when the Sun and Moon are exactly opposite each other in the sky. In other words, when the Moon is 180 degrees away from the Sun. Having stated that, you could say that a Full Moon only lasts for a minute because the Moon is constantly revolving around the Earth and just BEFORE it reaches that special point in its orbit - 180 degrees away from the Sun - it is 179.99 degrees away from the Sun and at that point it is not, technically speaking, 100% illuminated. Instead, it is 99.9999999% illuminated. If you want to be a stickler for details, that is not 100% illuminated and therefore it is not a Full Moon. Also, just moments AFTER the Moon reaches that special point in its orbit - 180 degrees away from the Sun - it is 180.01 degrees away from the Sun and at that point, once again, it is not, technically speaking, 100% illuminated. Instead, it is 99.9999999% illuminated.

It is not possible to discern the difference between a Moon that is 100% illuminated and one that is 99.9999999% illuminated. So, we would say that it is a Full Moon in both situations. However, there does come a point when an observer will notice that the Moon is not quite 100% illuminated. This point is different for different observers and is probably largely dependent on how much experience you have observing the Moon and how carefully you make your observations. I would like to think that most people would notice that when the Moon is 95% illuminated it no longer appears perfectly round and so it is no longer a Full Moon.

So, with this in mind, you may be interested to know how long it takes for the Moon to go from being 95% illuminated (Waxing Gibbous) to 100% illuminated (Full Moon) to 95% illuminated (Waning Gibbous). I will state times and dates for an observer located in Toronto and will use the most recent Full Moon as an example.

95% illuminated (Waxing Gibbous) 15 Jul 2008 9:00:00 PM (EDT)

100% illuminated (Full Moon) 18 Jul 2008 4:00:00 AM (EDT)

95% illuminated (Waning Gibbous) 20 Jul 2008 10:00:00 AM (EDT)

So, as you can see, it took 109 hours for the Moon to go from 95% illuminated (Waxing Gibbous) to 100% illuminated (Full Moon) to 95% illuminated (Waning Gibbous). Therefore, it would appear that we had a Full Moon on the nights of July 15/16, July 16/17, July 17/18, July 18/19, and July 19/20. That's five nights in a row! Although, the actual Full Moon occurred on the night of July 17/18, and to be more specific, it occurred at exactly 3:59 AM (EDT) on July 18.

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