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Even if we see a crescent moon, always the lower circumference is visible. Why we never see this? enter image description here

PS: this image is vertically inverted.

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The moon has no "lower" or "upper" half. Up and down are local coordinates and point in different directions for people at different places on Earth. –  Walter Jan 28 at 23:15
    
Can you suggest a better wording? –  BlueFlame Jan 30 at 19:24
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3 Answers

up vote 12 down vote accepted

Actually, you sometimes can see the Moon illuminated by the Sun from the top, and therein lies the answer to your question - during day. When looking at the Moon during night time, the side of the Earth where you're standing and looking at it is pointed away from the Sun, while this is reversed during the day. The illuminated side of the Moon is also facing the Sun, of course. So we have:

  • Night time on Earth: Side illuminated by the Sun is "below us" and that's where the Moon is illuminated from too.
  • Day time on Earth: Side illuminated by the Sun is "above us" and that's where the Moon is illuminated from too:

    enter image description here
    Blue Moon in the daytime sky (Photograph source: LPOD)

So, the direction from which the Moon is illuminated from is the same as the direction the Earth is, both illuminated by the same celestial body - our Sun. So, when you see the Sun high above the skies, and if the Moon is visible during daytime, it's going to be illuminated from the same direction from which you are, and during night time, when you'd expect the Sun "below where you're standing", it's illuminated from that direction.

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You're awesome! –  BlueFlame Jan 27 at 23:56
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I removed some distracting comments. @Envite if you still believe that the correct answer is not represented or misrepresented feel free to post your own answer. –  called2voyage Jan 28 at 14:24
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To add to TidalWave's answer, here's another way to think about it. Here are some facts:

  • The "horns" of the crescent always point away from the sun.

  • The sun and the moon follow roughly the same path through the sky on any given day; they rise in the east and set in the west.

  • The crescent is very thin only when the moon appears close to the sun in the sky.

  • A thin crescent is very dim compared to a full moon, and extraordinarily dim compared to the sun.

If you don't understand why any of these facts are true then think about the relative positions of the earth, moon and sun in space until you do.

So how then can we possibly have a downward-opening thin crescent? These facts imply that this only happens when the moon appears to be very close to the sun and farther west than the sun. Which means that the moon is going to set before the sun does. So therefore the only time when such a moon is visible is during the day, and during the day the sun is much, much brighter than the thin crescent moon.

By contrast, the upward-opening crescent happens when the moon is close to the sun and farther east, which means that there is time after the sun has set but before the moon has set when it is still in the sky and bright enough to be seen in the dusk.

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It very much depends on where on the globe you are in terms of your latitude. From mid-northern or mid-southern latitudes the terminator (line between dark and light parts of the moon) will appear roughly vertical around midnight local time at first and third quarter ('half full'). If you observed the same phase on the same night at the same local time from the equator the terminator would be roughly horizontal. Same goes for the constellations, it took some getting used to when I visited the southern hemisphere to see Orion (and all the other constellations I was familiar with) 'upside down'.

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