Frequently, in my descriptions of the pictures of the Moon, I refer to the "lunar day" or sometimes just the "day". This can be confusing and I apologise for that confusion, but I hope that, in most cases, my meaning will be clear. The purpose of this page is to explain what I mean, and how the various values are calculated.
The Moon revolves on its axis once every 27.3 days (or about 656 hours), so, if you were on the Moon, the Sun would rise about every 655¾ hours. This would be a "day" on the Moon. This might be referred to as the "Lunar Day".
However, the interval between one New Moon and the next, seen from Earth, is 29.5 Earth days (or 709 hours), so the Moon goes through its phases in a period of about 29½ days. The phase is often referred to as the "age" and is measured in days (Earth days that is). Unfortunately there are two ways in which this age can be calculated, which adds to the confusion, but let's ignore that for the moment (see below for details). Either way, the age starts at New Moon and reaches 29.5 days at the next New Moon.
So, we have three meanings to the word "day":
1. The day on the Earth of 24 hours.
2. The day if you were on the Moon of 655¾ hours.
3. The day as a unit of time in the representation of the phase of the Moon. On this site this unit is not quite constant but averages 24 hours in length.
I use these three meanings as required, and I hope that which meaning I am using will be apparent from the context. As Humpty Dumpty said in Through the Looking Glass "When I use a word, it means just what I choose it to mean—neither more nor less". However, I hope it will always be clear. In particular, I may use the term "Lunar Day" in two ways. I may refer to a picture being taken "early in the lunar day" meaning that the Sun was low in the eastern sky of the Moon. Similarly I might say "this picture was taken in the early morning" meaning "early morning on this part of the Moon". Although I always give the date and time at which a picture was taken, I am not really concerned about the time on Earth. I know of one exception where I say "this picture was taken shortly after sunrise" and I mean sunrise here on Earth, not on the Moon. Hopefully the context makes it clear.
Finally I need to make one more point. In considering how sunlight is falling on any given part of the Moon, the libration also needs to be taken into account. The phase determines how much of the visible face of the Moon is illuminated by the Sun. But libration causes the Moon to wobble as seen from the Earth, so any given feature moves east or west, north or south relative to the central part of the visible hemisphere. So, for example at first quarter, a feature on the prime meridian (which passes straight down the centre of the Moon when in its average position) may be in sunlight or may be in darkness, depending on the libration. There are a number of examples of this on my site here, for example my two pictures of the area west of Mare Humorum (look at Gassendi).
There are two methods. Both methods start at the moment of New Moon and reach 29.5 days at the next New Moon, but the intermediate values are different.
1. In the simplest method of calculation, the interval between one New Moon and the next is divided into 29½ intervals called "days". Thus the age of the Moon at this time tomorrow will be exactly 1 day more than it is today. This sounds simple and logical, but the days so calculated do not correspond well to the phase of the Moon as seen from the Earth. The reason for this is that the Moon's orbit is not circular and the speed with which it moves is not constant, When it is nearer to the Earth it moves more quickly than it does when it is further away. So the interval between, for example, New Moon and First Quarter (when the Moon is exactly half illuminated as seen from the Earth) varies slightly through the year depending on which part of its orbit is involved. For example, if New Moon occurs a week before perigee (the point where the Moon is closest to the Earth), then the Moon is moving at its fastest and the time it takes to get to First Quarter is shorter than it is if New Moon occurs a week before apogee (when it is furthest from the Earth and moving more slowly).
2. A better way to calculate the age of the Moon is to calculate the angle Sun-Earth-Moon (in degrees) and divide that by 360 and multiply by 29.5. This gives "days" which are not of equal length, but during that time the terminator (the dividing line between the dark part and the sun-lit part of the Moon), as seen from Earth, moves the same amount. In my view this is a much better way to present the phase of the Moon as an age in days than the simpler method outlined above because it relates better to the phase as we see it. All the ages I quote on this web site are calculated this way, and are taken from MyStars!, a useful planetarium program. In this method of calculation, First Quarter is always at day 7.375, Full Moon is always at day 14.75, and third quarter is always at day 22.125.
I show pictures of the Moon at each day of a lunar cycle here and the thumbnail pictures are labelled with the age in days calculated by this second method. Unfortunately it is not practicable (where I live anyway) to take pictures at exactly one-day intervals, so the sequence is a little less even than I would have liked.
Home Back to the Moon