Crescent moon in the daytime
Discussion
I'm sure this is a basic question but it has been bugging me.
As I understand it (I hope this is right!!) a crescent moon at night is caused when the earth lies between the moon and sun therefore when viewing the moon from the dark side of the earth only part of the moon is illuminated i.e. the crescent bit.
So....how come I can sometimes see a crescent moon during the day ?
The earth isnt between the sun and moon because I can see them both from a single point on the illuminated side of the earth - right ??
As I understand it (I hope this is right!!) a crescent moon at night is caused when the earth lies between the moon and sun therefore when viewing the moon from the dark side of the earth only part of the moon is illuminated i.e. the crescent bit.
So....how come I can sometimes see a crescent moon during the day ?
The earth isnt between the sun and moon because I can see them both from a single point on the illuminated side of the earth - right ??
It's about the relative positions of sun, earth and moon. The sun (nearly) always illuminates a full face of the moon (the side facing the sun obviously). The position of the moon relative to the earth determines how much of that lit face you can see at any time (and whether it is daytime or not for the observer).
It's all about triangles
Edit: there must be something on wikipedia about phases of the moon with nice diagrams.
It's all about triangles
Edit: there must be something on wikipedia about phases of the moon with nice diagrams.
Edited by ewenm on Wednesday 27th June 17:00
Its to do with angles and the distance the sun is from the moon or something.
ETA - http://en.wikipedia.org/wiki/Lunar_phase
ETA - http://en.wikipedia.org/wiki/Lunar_phase
The closer the moon is in the sky to the sun - as seen from earth, the thinner the crescent will be.
The new moon is when the moon is between the earth and the sun. Therefore the side of the moon facing the earth is completely in darkness and is invisible.
As the moon progresses in its orbit around the earth, more of the side of the moon that faces the earth gets exposed to sunlight so gradually more and more of the earth facing side gets lit up. This is called "waxing".
When the moon is completely on the opposite side of the earth to the sun, you will have the earth-facing side of the moon in complete sunlight - so you get a full moon.
The moon then continues on its path around the earth and gradually less of the earth -acing side receives sunlight. This is called waning and continues in this way until the next new moon.
The cycle is run roughly every 28 days, the time it takes for the moon to complete one full orbit of the earth.
The new moon is when the moon is between the earth and the sun. Therefore the side of the moon facing the earth is completely in darkness and is invisible.
As the moon progresses in its orbit around the earth, more of the side of the moon that faces the earth gets exposed to sunlight so gradually more and more of the earth facing side gets lit up. This is called "waxing".
When the moon is completely on the opposite side of the earth to the sun, you will have the earth-facing side of the moon in complete sunlight - so you get a full moon.
The moon then continues on its path around the earth and gradually less of the earth -acing side receives sunlight. This is called waning and continues in this way until the next new moon.
The cycle is run roughly every 28 days, the time it takes for the moon to complete one full orbit of the earth.
Simpo Two said:
pia1361 said:
As I understand it (I hope this is right!!) a crescent moon at night is caused when the earth lies between the moon and sun
Nope, that's an eclipse. The effect you're describing is simply to do with where the sun's light is coming from.pia1361 said:
The earth isnt between the sun and moon because I can see them both from a single point on the illuminated side of the earth - right ??
Hold a tennis ball directly in front of you and shine a torch on it from the side. Will the whole of the front of the ball be illuminated like the side? No, because that's not where the light is coming from.
Johnnytheboy said:
Depends on your definition of "almost". You know exactly what I mean.
Bloody PH pedants.
Johnnytheboy is confusing a crescent with an eclipse because he thinks that a crescent is caused by the sun's light being blocked by the earth, as happens during an eclipse when one celestial body blocks the light of the sun casting a shadow on another celestial body. Bloody PH pedants.
A crescent is not caused by blocking, and is not an eclipse. A crescent is caused by the light being shone at an angle so that only part of the moon is illuminated as seen from the earth.
This bugged me as a budding young astronomer for years but I couldn't get my head round it!
It's actually really simple but many answers on here are perhaps too detailed. (or maybe I'm just thick!)
A crescent moon is when the moon is illuminated by the sun but we're seeing it from the side and therefore see the light and dark sides and the join is crescent shaped because of the curvature of the moon. It's just like night and day on the earth just viewed from the side. The moon maybe!
The curvature of the light and dark is what used to confuse me as I assumed it was the shadow of the earth cast on the moon rather than the curvature of the moon itself.
It's actually really simple but many answers on here are perhaps too detailed. (or maybe I'm just thick!)
A crescent moon is when the moon is illuminated by the sun but we're seeing it from the side and therefore see the light and dark sides and the join is crescent shaped because of the curvature of the moon. It's just like night and day on the earth just viewed from the side. The moon maybe!
The curvature of the light and dark is what used to confuse me as I assumed it was the shadow of the earth cast on the moon rather than the curvature of the moon itself.
sebdangerfield said:
A crescent moon is when the moon is illuminated by the sun but we're seeing it from the side and therefore see the light and dark sides and the join is crescent shaped because of the curvature of the moon. It's just like night and day on the earth just viewed from the side. The moon maybe!
Not quite, we always see the same face of the moon so we don't 'see it from the side'. The light coming from the sun is shining on the side, but we're seeing the same face. The dark bit is just because of where we are in relation to the light coming from the sun.moleamol said:
sebdangerfield said:
A crescent moon is when the moon is illuminated by the sun but we're seeing it from the side and therefore see the light and dark sides and the join is crescent shaped because of the curvature of the moon. It's just like night and day on the earth just viewed from the side. The moon maybe!
Not quite, we always see the same face of the moon so we don't 'see it from the side'. The light coming from the sun is shining on the side, but we're seeing the same face. The dark bit is just because of where we are in relation to the light coming from the sun.Eric Mc said:
Funny, but it NEVER confused me.
Excellent. I'm not sure why I struggled with it, just a young mind not thinking logically I suppose. I guess things just click as you near adulthood and it seems silly in hindsight. I was exactly the same with understanding time dilation during my A levels.
Our school syllabus had fairly good geography books in Ireland in the 1970s so most of this was covered quite well with fairly decent pictures and diagrams. Also, I was a keen astronomy book reader when I was 10-11 years iold so no misconceptions ever really intruded into my understanding of what I was seeing.
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