dreamlifeof ([info]dreamlifeof) wrote in [info]astronomy,
@ 2006-01-14 00:16:00
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The moon
I have a question concerning phases of the moon, and I was hoping someone brainy and knowledgable here might be able to answer it.

Given that the phases of the moon are dependant on the position of the sun in relation to both the moon and the earth, why is it I recently read that we all see the *same* phase of the moon at any one given time, regardless of where we might be standing on the earth? Since the position of any individual on the earth is always changing relative to the sun, how can his or her view of the moon be, at any given time, the same all over the earth?

Furthermore, a full moon occurs when the sun is on the opposite of the earth from the moon, but the moon orbits the earth, and the earth orbits the sun (once daily), so surely any definition of absolute position in relation to these spheres is meaningless, and the moon should look different depending on where one is standing.

Yours, puzzled and lightly cross-posting.

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[info]artkouros
2006-01-14 12:44 am UTC (link)
The width of the earth is small compared to the distances involved. So, while you are correct that everyone sees a slightly different phase, the differences are insignificant.

Same answer for question 2. If you imagine the moon and the sun standing still while you on the earth rotate under the moon, the moon always looks the same -- because the view is esentially the same from either the east of west horizon. It's true that the moon, earth and sun are always moving, and technically, the full moon that you see is not exactly the same as the full moon someone on the other side of the earth sees 12 hours later, but the differences are insignificant.

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[info]dreamlifeof
2006-01-14 01:07 am UTC (link)
This is interesting. Thanks for your response.

You refer to the 'width of the earth', yet the earth is a sphere, with one side turned away from the moon, surely, however slight the angle of difference is be from pole to pole. Clearly this isn't the case, but I'm afraid I still don't understand how.

Or are you saying that we don't in fact all see the moon at the same time, but its phase changes so gradually that what is seen 10 or 12 hours later than the official time of the 'full moon' still looks so like a full moon as to make no practical difference?

In which case, and on a more frivolous note, werewolves and others affected by the full moon must go into wolf phase only in those parts of the world which get a view of the true full moon, which doesn't last a full 24 hours in each 29.5 day cycle.

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[info]imluxionverdin
2006-01-14 01:12 am UTC (link)
Well in at least some Hollywood movies, the werewolf turns into their wolf form only when they are bathed in the moon's light. So if they moon goes behind a cloud, they don't change. Once they're changed, of course, they stay changed even when they go indoors or if the moon went behind a cloud again (otehrwise they'd be changing back and forth all the time, which is just silly.)

So you are right, they don't stay in wolf phase for a full 24 hours.

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[info]dreamlifeof
2006-01-14 02:59 am UTC (link)
how exhausting to go through all that chopping and changing, yet what a relief for the victim about to have his throat ripped out to find that, why, his attacker was only nice Mr Jone from the bank.

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[info]kvschwartz
2006-01-14 01:19 am UTC (link)
The earth is a sphere, so people at the North Pole see different stars than people at the South Pole. But they see more or less the same Moon. That's because the stars circle the earth 360 degrees, whereas the Moon is only, if I recall, 0.5 degrees. You get different angles on it because then the Moon would be out of view.

I hope I answered your question and not some other question.

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[info]kvschwartz
2006-01-14 01:20 am UTC (link)
Oops, that should read "you DON'T get different angles."

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[info]smof
2006-01-14 01:01 am UTC (link)
I know this is me being pedantic and doesn't help with your question, but the moon orbits the earth once a month, and the earth orbits the sun once a year :)

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[info]dreamlifeof
2006-01-14 01:09 am UTC (link)
So in that case, how does the sun's position in relation to the moon affect the monthly cycles of the moon's phases?

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[info]jp7010
2006-01-14 01:28 am UTC (link)
As he said, he's being pedantic. You said "the earth orbits the sun (once daily)", which is not correct terminology. The Earth takes ~24 hours to rotate, and ~365 days to orbit the Sun.

The moon's phase is completely dependent on it's position in relation to the viewer. Any two viewers on Earth are, at maximum, only several thousand miles apart, compared with the hundreds of thousands of miles seperating both viewers from the moon itself. As has been said, the difference between what these two viewers see is roughly.. a fraction of a percent difference, in viewing an object that is only the size of your thumb held at arm's length.
Hi-resolution(relatively) telescopes would notice the difference, but your naked eye won't.

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[info]dreamlifeof
2006-01-14 02:44 am UTC (link)
I think I understand what you're saying yet I don't get the bigger picture. If the earth was flat like a coin, then I could understand that, no matter how far apart two individuals were on that plane, it would make little difference to the appearance of how the moon looked. However, if one is on the *opposite side* of a sphere (eg the earth) from an object (eg the moon), it will not be visible, surely?

So in other words, at any one point in real time, regardless of time zones, not all the earth can see the same moon. Or if so, I still don't get how.

And anyway, how does the sun (however often orbited by the earth) affect how we see the moon? Maybe you're sick of discussing this by now.

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[info]vulture23
2006-01-14 04:50 am UTC (link)
True -- the moon is not seen identically by everyone on earth, per se... only by those people who can actually *see* the moon at a given time. If the moon is below the horizon for a given observer, then it's not visible, so only half the Earth can see it at one point in real time. However, the change in the phase of the moon during the course of the 12 hours it takes for the other side of the Earth to be exposed to it is not really noticeable to the naked eye.

The sun affects how we see the moon because the moon does not give off light itself, it only reflects sunlight. The half of the moon facing the sun is visible and bright; the half of the moon facing away from the sun is dark and normally not visible to the naked eye. This is what gives the moon a noncircular shape; how much of the shaded half you see depends on your position relative to both of them. Think of trying to look at some object by using a flashlight in a dark room. You point the flashlight at the object. If you're on the same side of it as the flashlight, you can see it pretty well. But if the flashlight is on the opposite side of it from you, then you're looking at its shadowed part, and you can't make out any detail. As the moon orbits the earth, we go from being on the same side as the flashlight -- that is, the sun -- at which time the moon is full, and being on the opposite side, at which time the moon is new.

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[info]dreamlifeof
2006-01-14 12:31 pm UTC (link)
Right, defining the moon as being in the same phase for everyone in the world simultaneously really refers to its apparent appearance, not its literal state of phase. 'Full moon', for instance, has a precisely defined day and time of appearance (universal time, not local time) every month from now until infinity. We may think we're seeing the full moon, but unless our nighttime coincides with its exact appearance, we are not. Some in the world, on any given month, will see the true full moon, others will not. I know I am being pedantic, but I just wanted to get the facts sorted in my head.

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[info]trane608
2006-01-17 12:53 am UTC (link)
I'd suggest that pretty much any telescope is suitable for seeing differences in phase. One of my favourite pastimes is to spend an hour "walking the terminator" and observing how crater illumination (and, therefore, phase) changes. I do that with a small NexStar 4 GT.

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[info]countrycousin
2006-01-14 02:44 am UTC (link)
There is a moving diagram here

BUT - the real distances between the three bodies are very large compared to the bodies. Furthermore the moon's phases change slowly (full cycle per month) versus the Earth's rotation (1 day). So, as has been mentioned, different people in different places on Earth do see slightly different phases, but the difference is small. It is hard enough for one person to observe the difference on successive nights.

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[info]dreamlifeof
2006-01-14 02:52 am UTC (link)
I raised a point in answer to a comment above, saying: 'are you saying that we don't in fact all see the moon at the same time, but its phase changes so gradually that what is seen 10 or 12 hours later than the official time of the 'full moon' still looks so like a full moon as to make no practical difference?'

Previous commenters have seemingly stated that we do all see the *same* moon, just from very slightly different angles which make a negligible difference to its appearance. According to what I think you're saying, that is not the case, although the *appearance* will be very similar to all of us looking at the moon over roughly the same period of hours, since the phases change so slowly. However it must be impossible for everyone in the world to see the moon, in whatever phase, at the exact *same* time (real time I mean) since half the world will have its back turned on the moon at one given point in time. Or if I am wrong, I still don't understand how.

Thanks for the diagram!

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[info]countrycousin
2006-01-14 03:40 am UTC (link)
1. Two people looking at the moon at the same time, but several thousand miles E-W apart see the Moon at slightly different angles. The Moon is 250,000 miles away - the maximum spread on the Earth is 8,000 miles - that will give a different perspective that could be observed with a telescope and camera, but would not be noticeable to the unassisted eye.

2. Consider two people who look at the Moon at midnight, local time, but they live on opposite sides of the Earth. During the 12 hours between their viewings, the Moon's phase will have changed by approximately 1/56 of its cycle, or 1/28 of the change between New and Full. This is easily noticed if one takes pictures and compares, but practically, both observers will describe the Moon the same way - crescent - full or nearly full - half - etc.

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[info]iwp
2006-01-14 12:11 pm UTC (link)
It's true, the view of the moon is very slightly different depending where on Earth you're standing.

Read through the Lunar Parallax Demonstration Project for a discussion of this and some fantastic photos that demonstrate this. This image shows the effect quite clearly.

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