Posted by Anonymous on 2019/02/28 under Life How long does each phase of the moon last? A full lunar cycle lasts almost a month (about 29.5 days), and during that time the moon goes from new to crescent to first quarter to gibbous to full to gibbous to third quarter to crescent and back to new. However, a "full moon," a "new moon," and a "half moon" (first and third quarter) are instants, not periods of time. Because the moon is continuously orbiting the Earth, it will only be absolutely "full" (directly opposite the sun) for an instant before it moves along in its orbit. So the only phases that last any length of time are crescent and gibbous. The moon is a crescent for about 7.38 days before instantaneously passing first quarter and becoming gibbous. It remains gibbous for another 7.38 days, then becomes full for a split second before becoming gibbous again. After another 7.38-day period, it instantaneously passes third quarter and becomes a crescent. And after another 7.38 days, it completes the cycle by passing the moment of the new moon before becoming crescent again. The answer to this question is a bit more complicated than it seems at first sight. Note that the question is: Short answer: Longer answer: Technically there is only one short instant when the moon is exactly full, new, or half full (first or last quarter). The rest of the time in between then are the additional four phases. Now supposing you are looking with your eyes, without binoculars or telescope, than the moon may look full for one or two nights in a row, maximum. On the second night you’ll probably already see something ‘missing’ if you have good eyes. The same goes for new, and half full. So let’s say it takes 24 hours to human eyes until each of the four main phases is ‘gone’ again. That would mean the reast of the month is filled by the additional four phases (25.5 divided by four). Defining it this way, each of the four main phases lasts 3.4% of the cycle, and each of the four additional phases is 21.6% of the length of the cycle. New moon: 24 hours* (from 3.4% to 0% and back to 3.4% lit again) Note 1: Note 2: There are no discrete lunar “phases” that last any length of time at all. We talk about the “Full Moon” for instance, but the moon is only maximally illuminated for a single instant in time. However, it’s very close to fully illuminated for a period of time – even a day earlier or later will appear “Full” unless you pay very close attention. The same holds true for “New”, “First Quarter” and “Last Quarter” phases. These really only mark points in the lunar cycle, and actually last no time at all, although they won’t look much different to the eye for +/-24 hours or so. Informally, people may talk about crescent (less than 50% illuminated) or gibbous (more than 50% illuminated) “phases”, but these are continually and very slowly changing as well. So, if you group crescent days or all gibbous days together, and you consider that a “phase”, this “phase” will last multiple days (these periods mark the time between New, First Quarter, Full, and Last Quarter phases). Since the full cycle is about 29 1/2 days long, we can say it is approximately 4 weeks long (plus 1.5 days). So it takes a week to go from New Moon to First Quarter. Another week to get to Full Moon. And one week to get to Third Quarter and finally one more week to return to New Moon. New Moon and Full Moon are two weeks apart roughly with only about 3/4 day extra to add. Same for First and Third Quarter. The phases of the Moon are a result of it reflecting light from the Sun whilst it orbits the Earth. This takes about 29.5 days and is approximately one month. This is where the word “month” comes from – the time for the Moon to complete a sequences of phases. We should really call it a “moonth”! The Sun gives out light which is reflected by the planets, their moons, the asteroids and comets, and that is how we can see them. Imagine the the Sun, the Earth and the Moon are all in a line in that order. People on the side of the Earth that is lit by the Sun are in daylight; those on the other side of the Earth are experiencing night-time. With the Moon in the position described, people not only can see the Moon in their night sky, but it is reflecting the Sun’s light directly back to the Earth. As with the Earth, half of the Moon is lit up by the Sun, and in this configuration, that half is facing the Earth. As a result, people see a “Full Moon”. When the Moon is between the sun and the Earth, the side of the Moon that is facing the Earth is not lit by the Sun, and this is known as a “New Moon”. As the Moon moves around from this position, we can see a small sliver of it lit by the Sun – as a crescent – and the amount increases as the Moon continues in its orbit. This is a “waxing crescent”, and continues until the Moon is at 90º to the line of the Sun and the Earth. At this point, half of the side of the Moon that is facing the Earth can be seen, at “First quarter”. As it continues round to Full, it becomes a “waxing gibbous” Moon, before reaching Full. After Full, less is lit up, during its “waning gibbous” phase, until it reaches “Last Quarter”, and then on through “waning crescent” until it is back at New Moon, between the Sun and the Earth. It is at the point of New Moon that sometimes it will pass directly in front of the Sun, producing a solar eclipse. If you have the privilege to stand directly in the shadow of the Moon, as I have done, you can experience the rare wonder of a total eclipse of the Sun. All this can be simulated if you have a dark room. Set up a lamp at one side of the room and whilst you stand in the middle, have someone else carry a ball in a circle around you. As with the Moon, half of it will always be lit by the lamp, but you will see varying amounts of the lit side. New Moon First Quarter Full Last Quarter The phases are caused by the orbit of the Moon around the Earth in relation to the Sun. It takes on average 29.5 days for the moon phase to go from New Moon to New Moon. Phases are man-made artificial constructs that approximate the percentage of the moon’s visible surface that is reflecting sunlight. Just like the seconds, minutes and hours are just artificial delineations along a continuum, the phases of the moon are the same. The main phases (new, first quarter, half, third quarter and full) are each between five and six days long, but the amount of the Moon’s visible surface that is reflecting sunlight is changing constantly. So, during a new moon phase, there will be times when you can see that there’s a sliver of the moon’s surface reflecting sunlight, and during the full moon cycle, at times there will be small portions of the moon that aren’t reflecting sunlight.
“How long does each phase of the moon last”
and not:
“How long does it take from each (main) phase of the moon to the next?”
One synodic month (from new moon to new moon) lasts 29.5 days, but you could argue that full moon for instance lasts only about a day or even less than that, depending on how good your eyes or telescope are, to see that it isn’t exactly full anymore.
A synodic month (29.5 days) is the period it takes for the moon to go through all phases, so from new moon to new moon for instance. Usually a month is defined as beginning on new moon. Not to be confused with a sidereal month (27.3 days), which is how long it takes the Moon to return to the same position amongst the distant stars.
Now, there are four main phases: new moon, first quarter, full moon, last quarter. Sometimes the intermediate stages waxing crescent, waxing gibbous, waning creschent and waning gibbous are also called ‘phases’.
But it doesn’t really matter if you count 4 or 8 phases: technically it is a bit strange to just divide the 29.5 by that number. Because to your eyes, each of the four main phases doesn’t last 3.5 days, let alone a whole week.
Waxing crescent: 6.375 days** (from 3.4% to 46.6% lit)
First quarter: 24 hours* (from 46.6% to 53.4% lit)
Waxing gibbous: 6.375 days** (from 53.4% to 96.4% lit)
Full moon: 24 hours* (from 96.4% via 100% back to 96.4% lit again)
Waning gibbous: 6.375 days** (from 96.4% to 53.4% lit)
Last quarter: 24 hours* (from 53.4% to 46.6% lit)
Waning crescent 6.375 days** (from 46.6% to 3.4% lit)
TOTAL: 29.5 days
* depending on how good your eyes (telescope/binoculars) are!
** depending on how generously you define each of the four main phases!
% lit is not to be confused with % length of the cycle, considering the cycle is from 0 to 100% lit and back to 0% again. So each percentage of the cycle is 2% difference, so for instance from 1% lit to 3% lit. And that way, from 3.4% lit to 0% and back to 3.4% lit again, is exactly 3.4% of the length of the cycle.
Note that for each of the additional phases there is not one exact moment that defines it. When the moon is 2% lit, or 48% lit, the description ‘waxing crescent’ fits during that entire period. After all, it is still a crescent shape, and it’s still waxing.
Jerry Stone, I have given talks on space since 1969 in the UK and abroad.
Answered Jan 16, 2018 · Author has 115 answers and 277.6k answer views
Originally Answered: Why are there phases of the moon?