How to Distinguish Between the Waxing and Waning Moon, At a Glance

DC

This is a waxing moon, meaning the sunlit portion we can see is growing. The outer curve also makes this view of the moon shaped more like the letter “D,” compared to the letter “C.” For the useful mnemonic here, remember that “D” stands for “developing.” D-shaped moons are in the waxing part of their cycle of phases, growing larger for about two weeks.

DGLater in the waxing portion of the moon’s cycle of phases, it becomes a gibbous moon — but retains its “D-like” shape. It is still slowly getting larger, approaching the full moon state.

CG

Here is another gibbous moon, but it is shaped more like the letter “C” than the letter “D,” and, in this mnemonic, “C” stands for “concluding.” This moon’s sunlit portion is shrinking, moving away from fullness, towards the new moon state — in other words, it is a waning moon. All “C-shaped” moons, as viewed from Earth’s Northern hemisphere, are waning moons.

CC

This crescent moon more closely resembles a “C” than a “D,” which is how I know, at a glance, that its phase cycle is concluding, and it is a waning crescent, soon to become invisible as a new moon.

AC

This last picture shows the most difficult configuration to figure out:  the points of the crescent near the moon’s North and South poles both point up. Having them both point down would pose the same problem. Here’s the solution, though:  check to see which crescent-tip appears higher in the sky. In this case, it is the one on the left. That shifts the curve at the bottom of the moon (the one that is an actual moon-edge, rather than the terminator) slightly left-of-center, making the visible moon-edge more closely resemble a “C” than a “D.” This crescent moon, therefore, is a waning crescent.

Later addition:  as a commenter pointed out, below, this method does not work from Earth’s Southern hemisphere — in fact, in that half of the world, the “D”/”C” rule must be completely reversed, in order to work. To accomplish this, “D” could stand for “diminishing,” and “C” could stand for “commencing,” instead.

[Image/copyright note:  I did not take these photographs of the moon. They were found with a Google-search, and I chose images with no apparent signs of copyright. I am assuming, on that basis, that these images are not copyrighted — but, if I am wrong, I will replace them with other images, upon request.]

About RobertLovesPi

I go by RobertLovesPi on-line, and am interested in many things. The majority of these things are geometrical. Welcome to my little slice of the Internet. The viewpoints and opinions expressed on this website are my own. They should not be confused with the views of my employer, nor any other organization, nor institution, of any kind.
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7 Responses to How to Distinguish Between the Waxing and Waning Moon, At a Glance

  1. Ggreybeard says:

    Cs and Ds. That’s a novel way of looking at it – but such a description will not hold if you are observing in the Southern Hemisphere.

    A global explanation is that when you see the crescent Moon in the West after Sunset then it must be waxing and if you follow it night after night it continues waxing until two weeks later it becomes a Full Moon, when it rises at Sunset. After that it must be waning and if you see a crescent Moon before dawn, that is waning too.

    🙂

    Liked by 2 people

    • I should have thought of that! In the Southern Hemisphere, though, my shortcut would work if you switched “C” and “D,” correct? Perhaps they could then stand for “commencing” and “diminishing,” to complete the mnemonic.

      Liked by 1 person

      • Ggreybeard says:

        Hi Robert,

        Yes, it is correct but I find it much simpler to see if the Moon is already in the sky before sunset, in which case it is waxing; or if it rises after sunset, in which case it is waning. This applies equally in both hemispheres.

        Maybe there are other methods as well.

        My regards from Australia.

        Liked by 2 people

  2. Jos Smits says:

    Hallo Robert ! Admiring you magical moonshape pictures very much I would like to draw your attention (probably superfluous) to the fact that the shape of the moon’s terminator is always half an ellips. (The half above or below the long axis.) ANY possible half ellips appears four times during one full moon cycle. Please see explaining drawings here: https://lh4.googleusercontent.com/zfYN5vq6d0IOFN-n1fp1O2Nk9n10pvxkLG4fqRsSO_ZwvPToQrK0AP2WIt-nGfVcyzyyTqPagdQ=w1342-h523

    Jos Smits

    Liked by 2 people

  3. Jos Smits says:

    Sorry Robert, my mistake. Try this link: https://docs.google.com/file/d/0B5rV4V5bwdoJbFhhaDg5Wm4xZXM/edit

    This ellips fenomena is not very well known. See the works of artists trying to depict the moon or pictures of the turkish flag on internet… Greetings, Jos.

    Liked by 2 people

    • Thank you. I’ve tried accessing the newer link, and it appears to be working, but your permission is needed to actually view the files there. There should now be a message there for you, requesting such permission.

      Liked by 1 person

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