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I need help understanding this images I personally took. If the phases of the moon are determined by the position of the earth, moon & sun. Then how is the moon a crescent while it is rising in the east with the sun & still a crescent while setting with the sun? What is the obstruction creating the crescent considering the moon is between the earth & sun?Sunset Crescent Moon

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  • $\begingroup$ unfortunately, the other images I took of sunrise are too large to post in this format. Regardless of how much I compress it. $\endgroup$
    – Cody Page
    Feb 26 at 13:25
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    $\begingroup$ What would you expect to see if the moon is between the sun and the Earth.? Try holding a ball so it is between you and the sun. $\endgroup$
    – James K
    Feb 26 at 13:52
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    $\begingroup$ This was in New Mexico, in the Northern hemisphere $\endgroup$
    – Cody Page
    Feb 26 at 21:09
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    $\begingroup$ Are you confusing the phases of the moon — due to the direction from which the spherical moon is illuminated by the sun — and a lunar eclipse — due to the shadowing of the moon by the earth? $\endgroup$ Feb 27 at 11:23
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    $\begingroup$ @TechInquisitor Click on the photo $\endgroup$
    – Thomas
    Feb 28 at 20:40

6 Answers 6

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What is the obstruction creating the crescent considering the moon is between the earth & sun?

The moon. The part of the moon that's dark is in the shadow of the rest of the moon. Because the crescent moon is nearly between the earth and sun, the side that's lit (the side that's nearer the sun) is mostly facing away from us.

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    $\begingroup$ This is by far the best answer. The Moon always shadows half of itself, and we're seeing only the shadowed side at new Moon, and mostly the shadowed side a few days before and after new Moon. Short and sweet. $\endgroup$ Feb 27 at 10:00
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    $\begingroup$ @TechInquisitor You presumably mean the far and near sides. We could define dark and light to match their names and thus make you wrong, but then they wouldn't be constant halves, because the Moon is tidally locked to Earth, not the Sun. This is part of why people try not to use the dark/light terms with either meaning. $\endgroup$
    – J.G.
    Feb 28 at 19:30
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    $\begingroup$ The word "dark" is a has multiple meanings, one of which means "hidden." For example, "a dark secret." When people refer to the "dark side" of the Moon, they're using this definition, not the definition meaning "lack of light." $\endgroup$ Feb 28 at 20:35
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    $\begingroup$ @TechInquisitor I wrote it the way I did to try to avoid the whole issue. :) $\endgroup$
    – hobbs
    Feb 28 at 21:02
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    $\begingroup$ Having said this, I'm not convinced people say "dark side" really thinking that the far side of the Moon is in constant darkness. It seems like most everyone who's graduated high school is going to know this. $\endgroup$ Feb 28 at 21:31
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The Moon doesn't really rise and set exactly "with the sun", it is usually somewhat north or south of it, because its orbit is somewhat tilted with respect to Earth's orbit.

In this picture the bright side of the Moon points very close to where the Sun is. As discussed below, this doesn't work as well when the Moon is fuller, but for the purposes of understanding a crescent Moon it helps us understand what's going on.

This is a flat picture so we can't see depth, but the arrow point strongly into the page because the Sun is much further away the than the Moon.

See also (though they may confuse more than help)

enter image description here

The following paper The Moon Tilt Illusion by Andrea K. Myers-Beaghton and Alan L. Myers takes us through perspective projection to show that when the Moon is much further away from the Sun this doesn't work as well, but for the purposes of this answer about a thin crescent Moon it's perfectly fine to assume the Sun is in the direction where the bright side of the Moon suggests it is.

enter image description here

Figure 11. Moon tilt illusion for waning phases in northern hemisphere. Sun is rising due east. Red line is observed slope and blue line is expected slope of moon-sun line. Azimuth measured CW from north.

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  • $\begingroup$ -1. The shadow of the Moon does necessarily not point toward the Sun as seen from the Earth. I'll add a reference shortly. $\endgroup$ Feb 26 at 21:35
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    $\begingroup$ Reference: seas.upenn.edu/~amyers/MoonPaperOnline.pdf . See figures 10 & 11. $\endgroup$ Feb 26 at 21:45
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    $\begingroup$ @DavidHammen you know darn well that in this situation it's very close. I've updated and added the U Penn paper, it's a beauty! $\endgroup$
    – uhoh
    Feb 27 at 0:40
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    $\begingroup$ @DavidHammen Your very reference proves that, with the illuminated side of the crescent pointing to the left, the photo could only have been taken at sunset in the southern hemisphere (which the author denies though). Otherwise, it must have been taken at sunrise or the photo has been flipped around. $\endgroup$
    – Thomas
    Feb 27 at 9:39
  • $\begingroup$ @Thomas The only time one can see that thin of a crescent Moon is very near sunrise or sunset. What I was disputing was the line toward the Sun. That that line doesn't always work is something flat earthers love to use as "evidence" in favor of their nonsense. It is evidence -- evidence of 3D projective geometry. $\endgroup$ Feb 27 at 9:50
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It's not a shadow. The only time the moon is shadowed is when it's full and there's a lunar eclipse. What you saw was the moon when most of the side we see was facing away from the sun. At the time of a full moon all of the side we see is facing the sun.

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  • $\begingroup$ you can add those animations and diagrams to make this answer look more complete $\endgroup$ Feb 26 at 15:19
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    $\begingroup$ I guess one could stretch the definition and it is a shadow, but the moon is shadowing itself $\endgroup$ Feb 26 at 15:31
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As you said in your question, the phases of the moon are determined by the positions of the Sun, Moon and Earth in space (it is easy to test out the principle at home using a light bulb and a ball and changing their relative positions to yourself). The dark parts are just those parts of the Moon that are on the backside as seen from the Sun. And these thin crescents appear when the Earth, Moon and Sun are close to being in one line (in that order) i.e. at a time close to New Moon. But you will never see the crescent both at sunrise and sunset on the same day. You will either see it a couple of days before New Moon shortly before sunrise or a couple of days after New Moon shortly after sunset (it is in this order as the Moon appears to move slightly slower in the sky than the Sun (the orbital motion of the Moon is opposite to the Earth's rotation)) .

Based on these insights, one can conclude that, if your picture was taken in the northern hemisphere, it must actually have been taken at sunrise (not sunset as it claims). This is because because the Sun and Moon are both moving from East to West, which means in the northern hemisphere from left to right. So if your photo would be a sunset photo, the crescent would be moving from the top left to the bottom right. The Sun would have already done that and would be ahead of the crescent below the horizon. So the illuminated side of the crescent should broadly point to the right. But it is in fact broadly pointing to the left, which means the crescent was rising from the bottom left to the top right, with the Sun about to rise behind it.

This is also confirmed by the figures in the scientific publication mentioned in some of the comments here. The possible phases at sunset are not consistent with the photo at all

moon phases at sunset

whilst the possible sunrise phases contain instances that are consistent with the photo;

moon phases at sunrise

If the photo was indeed taken at sunset, it must either have been taken in the southern hemisphere (where everything would be reversed) or flipped horizontally.

Here are the sunset/moonset and sunrise/moonrise views visible from Albuquerque, New Mexico around New Moon on 1 Feb. 2022 (courtesy of Sky and Telescope Skychart )

Sunset 3 Feb. 2022 Sunset 3 Feb. 2022

Sunrise 29. Jan. 2022 Sunrise 29. Jan. 2022

And below again the photo taken by the OP (obviously showing the scenario slightly later/earlier than the charts, with the Sun below the horizon)

photo

So if the photo indeed taken at around the last New Moon (which is not confirmed by the OP as yet) it could only have been taken at sunrise not sunset. The chart shows a few planets near the Moon on this occasion as well, but they may have been too close to the horizon to show on the photo given the probably shortish exposure time. If that would be a sunset photo, the illuminated side of the crescent would not only point opposite to the sun (which is impossible, and which I have never seen in 50+ years observing the sky) but it should have been considerably higher in the sky as well)

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  • $\begingroup$ I'm not sure your conclusions are supported yet. The link is to a very nice looking tutorial, not a scientific publication (there's no evidence of peer review) and the Moon's orbit does precess every 18 years. I'll go off and do some Skyfield simulations but I have a hunch that just after sunset in Arizona the Moon can be either left or right of the Sun. $\endgroup$
    – uhoh
    Feb 28 at 23:09
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    $\begingroup$ No, “altitude” is exactly “altitude”. Check for instance the sentence “For example, in Figure 10 for a waxing moon, the horizontal “boat” moon at high altitude in the west is observed near the equator but not in temperate zones” (and the figure shows the “boat” as corresponding to 75°): hence, they actually refer to a moon high in the sky and seen from a latitude near to 0° (equator). $\endgroup$
    – DaG
    Mar 1 at 7:35
  • $\begingroup$ @DaG Yes, you are right. I removed this note from my answer. $\endgroup$
    – Thomas
    Mar 1 at 19:06
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    $\begingroup$ @uhoh The paper was published in "Journal of the Croatian Society for Geometry and Graphics" . See also the skycharts I added to my answer, which also prove that this can not be a sunset photo in the northern hemisphere (not to mention that I have never seen such a scenario in 50+ years observing the sky) $\endgroup$
    – Thomas
    Mar 1 at 22:14
  • $\begingroup$ @Thomas Thanks for all of that! Please don't equate "I'm not sure your conclusions are supported yet" with suggesting that anything is wrong here. It's just me, and I'm simply not yet sure. $\endgroup$
    – uhoh
    Mar 1 at 22:19
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What you need to consider is, that the fastest moving object in this story is you moving from west to east in half a day, while standing on a spinning planet. Compared to you, the sun, the moon and the earth hardly move, so why would it look much different?

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    $\begingroup$ Your answer could be improved with additional supporting information. Please edit to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers in the help center. $\endgroup$
    – Community Bot
    Mar 2 at 13:25
  • $\begingroup$ Are you seriously telling me to explain why the earth is round ? $\endgroup$
    – user45371
    Mar 3 at 20:26
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   Sorry if you've received your satisfactory answer already but this caught my eye so here's my analogy. (Dang it! sorry this got so long)
(*When* your photo was taken will be helpful as to the "which side is lit" question but that's in my second part. So I'd love to know that!) 

To address your question:

The Set up: We're walking single file into the low setting sun and I am directly in front of you. You are following behind me looking right at the back of my head and the low setting sun is likely blocked from your view by my head. So you, me, then the sun all in alignment. (Our shadows would align stretching behind us)

You would see zero directly-lit, sun-reflecting parts of my face. (My head is the "moon" here)

The Change: Then you quicken your pace and are behind me still but just off to the side a bit (as if to catch up to walk along side say. We're not aligned anymore) The sun would be in your view now, as you've sidestepped a little, and almost immediately, once you move out from behind me, you would catch a sliver of my face and it reflecting the sunlight. That sliver of reflecting face would grow as you caught up to me.

The Conclusion: So when there is a crescent moon that means the earth (you) and the moon (me) and the sun (the sun) are very closely in alignment to get only a sliver of the moon to be lit up and in your view. When there is a crescent moon and you are pointing at it in the sky you would then need to move your finger only a little bit to redirect and point at the sun. They always appear near each other in the view of the sky.

Crescent Direction explained...maybe, mostly:

As to the direction of your crescent photo some were discussing (not sure they'll get to see this but): Oddly enough there are times when the moon sets north of where the sun sets. So the sun would be on its left as we face it by the time the moon is all the way down. This might be why the lit part was on the bottom left. So I am not agreeing about the northern hemisphere crescent moon "always" being lit on the bottom right as one suggested (I mean, I believe you anyway) But for most crescent nights I believe it will be bottom right. Just a rare few that aren't (and only at the horizon).

Its arc in the sky changes over the course of a few days even. As you know the sun's arc crossing the sky is higher in the sky in summer and lower (more southern) in winter. (Think tilted hulahoop) This similar arc change for the moon happens much faster. I remember seeing the moon rise one night on the south side of a local mountain from my view. The very next night it rose on the north side of the mountain. So I knew it was setting similarly. I never really put together how much it moved like that before. (Helpful website down below)

I can't quite pinpoint the date of your photo but I'll leap at Feb 3rd 2022? This was one of those times where the moon set after the sun, and was north of where the sun went down (in NM). You can see this effect on mooncalc .org -That's a link so in good practice google search it- You can change the date to around Feb 7th and see its arc set really north vs Feb 28th. Click and move the dot along the top timeline too. A similar site is called suncalc .net just for the sun instead. Also helpful is time and date .com

I'm more photographer but night photoing got me to dabble in astronomy too, timing for moonless nights.

Anyway, hope this helps!

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    $\begingroup$ Your conclusion about the direction of the crescent when setting is wrong. I agree the moonset was farther north than the sunset, but that does not matter. What matters are the positions at the time the photo is taken. At anytime after sunset and before moonset, the sun was farther north (and below the horizon of course) by approximately 15 to 20 degrees. Therefore, the crescent would be facing to the right. In late spring the crescent can be pointing more "straight down" toward the horizon, but it will never be facing to the left as shown in the photo. The photo is sunrise, IMHO. $\endgroup$
    – JohnHoltz
    Mar 1 at 13:48
  • $\begingroup$ @JohnHoltz See the added skycharts in my answer, which prove as well, that this can only be a sunrise photo $\endgroup$
    – Thomas
    Mar 1 at 22:08

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