[Guest]

In a cold March night, you are miles away from your house. All you know that your house is at east of you. Thus you have to find out which direction is east. Of course you don't carry a compass or any magnetic object with you. There are no trees to look their moss, no ant nests. Although you see moon and stars, (as me), you have no idea about stars.

How can you find your direction without using any object, stars, or any other observable material on the ground since it is a dark night?

Confession: My answer to this question may be wrong, anyhow I want to argue it here.

[Guest]

observing the moon. You said not to use the stars, but as you've specifically stated you can see the moon, I assume you are looking for an answer that is based on that.

The dominant movement of the moon is that it rises in the east and sets in the west, so by observing the path the moon takes in the sky you could then project out to where east would be.

However, depending on the time of month/year the movement may be less visible and you may have to spend some time to observe an accurate path. Then, without any accurate instruments, trying to determine the exact orbital path the moon is taking and pinpointing exact east would be very difficult (if you are a long way away then even a slight error could lead you way off track). Finally, as I have said, east to west is only the "dominant" movement which is due to the earth's rotation, so to be really precise you would also need to take into account other causes of movement, such as it's orbit (and the fact that the orbit is elliptical).

So, to cut a long story short, by observing the moon over a relatively short period of time you could get a vague idea of which way is east. However, it probably wouldn't be accurate enough to be able to navigate and find your way home by.

[bonanova]

I'm not sure what has to be ignored here, but you seem only to explicitly exclude stars.

Use the moon.

Except for the days [nights] of full moon and new moon, the moon will have a dark side and a light side.

Not surprisingly, the light side points toward the sun.

When I say "points" I'm implying you draw a line of symmetry through the light and dark pattern you see.

In other words: the light side has a circular arc; look for the center of that arc - that's your pointer.

Now you only have to ask yourself: is it closer in time to today's sunset or to tomorrow's sunrise?

If sunset, the moon is "pointing" west.

If sunrise, the moon is "pointing" east.

[Guest]

I'm not sure what has to be ignored here, but you seem only to explicitly exclude stars.

Use the moon.

Except for the days [nights] of full moon and new moon, the moon will have a dark side and a light side.

Not surprisingly, the light side points toward the sun.

When I say "points" I'm implying you draw a line of symmetry through the light and dark pattern you see.

In other words: the light side has a circular arc; look for the center of that arc - that's your pointer.

Now you only have to ask yourself: is it closer in time to today's sunset or to tomorrow's sunrise?

If sunset, the moon is "pointing" west.

If sunrise, the moon is "pointing" east.

I also thought that if sun sets from west (in March), the circular arc of it must show west. At sunrise the opposite.

A full moon is always at east, so it is not an exeption.

[Guest]

I'm not sure what has to be ignored here, but you seem only to explicitly exclude stars.

Use the moon.

Except for the days [nights] of full moon and new moon, the moon will have a dark side and a light side.

Not surprisingly, the light side points directly toward the sun.

When I say "points" I'm implying you draw a line of symmetry through the light and dark pattern you see.

In other words: the light side has a circular arc; look for the center of that arc - that's your pointer.

Now you only have to ask yourself: is it closer in time to today's sunset or to tomorrow's sunrise.

If sunset, the moon is "pointing" west.

If sunrise, the moon is "pointing" east.

I don't actually think this is correct.

If it was correct, you would expect a moon to go through all transitions in a night. E.g. if it is a quarter moon at sunset with the lit side on the left and a quarter moon at sunrise with the lit side on the right then either we would have had to go through a full (or no) moon, or the lit side would have had to have rotated (so in the middle of the night it would have been pointing down). The former definitely doesn't happen and I don't think the latter does either.

That then leaves me with explaining why if it is "pointing" towards the sun, it is not showing east or west. I think the answer to this is because of the moons orbital path, which takes a month to go once around the earth. If you were to stand at the sun looking out at the earth, this means that over the course of one night, the moon would stay in a relatively steady position relative to the earth. I.e. if at the start of the night it was to the left of the earth, then at the end of the night it would also be to the left of the earth. This means that if you are on earth, the lit side of the moon would always point towards the sun, but it would always point down the same side of the earth (which could be to your east or west but does not change during the course of the night). The fact that it changes position in the sky during the night is primarily due to the earth's rotation (hence the rising in the east, setting in the west being the same as the sun), but does not change which direction the lit side "points" in.

I hope that all makes sense!

[Guest]

I don't actually think this is correct.

If you were to stand at the sun looking out at the earth, this means that over the course of one night, the moon would stay in a relatively steady position relative to the earth. I.e. if at the start of the night it was to the left of the earth, then at the end of the night it would also be to the left of the earth.

If moon is steady relative to earth:

Suppose, on evening you see a full moon. The moon is at east of you, so that entire moon takes lights of sun.

On sunrise, if the moon is still east of you (as you suggest), you can not see a full moon, because now the sun is behind the moon, thus you can see a tiny part of moon.

But as much as I know, if you see a full moon on sunset, you see again full moon on sunrise??????

[Guest]

If moon is steady relative to earth:

Suppose, on evening you see a full moon. The moon is at east of you, so that entire moon takes lights of sun.

On sunrise, if the moon is still east of you (as you suggest), you can not see a full moon, because now the sun is behind the moon, thus you can see a tiny part of moon.

But as much as I know, if you see a full moon on sunset, you see again full moon on sunrise??????

No, I say that at sunset if the moon is in the east then at sunrise it will be in the west.

You do see a full moon on both occasions, but that does not imply that it is "pointing" east.

I will try to draw a diagram that shows the point I am trying to make and post it shortly.

[bonanova]

If it was correct, you would expect a moon to go through all transitions in a night.

The pattern of illumination on the moon - that is, the moon's phase - changes

from full to new and back as the moon takes its 28-day orbit around the earth.

That pattern does not change appreciably at all during a single night. Nothing

at all like you describe, as "all transitions".

Something else changes during the night.

As the sun's apparent position [observed from earth] changes from below

the western horizon [just after sunset] to "beneath" the earth [about midnight]

to below eastern horizon [just before sunrise], the orientation of the pattern

on the moon's face changes. It appears to rotate. Due to the earth's rotation,

not due to the moon's orbital motion.

You see the same phase, rotating, as it maintains alignment with the sun.

Just after sunset, the crescent points horizontally [more or less] to the west.

At midnight or thereabouts, the crescent points vertically earthward.

Just before sunrise, the crescent points horizontally [more or less] to the east.

In fact, you can also judge time this way.

The rotation of the pattern has constant angular speed. So its change in azimuth

[pointing direction] at any moment - relative to the total changes in azimuth from

sunset to sunrise [about, but less than 180 degrees] - is the same fraction as the

elapsed time since sunset is of the total time between sunset and sunrise. When

the crescent points earthward [downward] the time is exactly the average of

sunset and sunrise [approx midnight.]

So the night moon provides a kind of a clock, as well as a compass.

No differently, but less distinctly, when the moon is visible during the daytime, it

can also be seen to "point" to the sun. And any clear night, the orientation of the

moon's illumination can be observed at sunset, midnight and sunrise.

[Guest]

The the pattern of illumination on the moon changes from full to new and back

as the moon takes its 28-day orbit around the earth. That pattern does not

change appreciably at all during a single night. Nothing like you describe as

"all transitions".

Something else changes.

As the sun's apparent position [observed from earth] changes from below

the western horizon [just after sunset] to "beneath" the earth [about midnight]

to below eastern horizon [just before sunrise], the orientation of the pattern

on the moon's face changes. It appears to rotate. Due to the earth's rotation,

not due to the moon' orbital motion.

You see the same moon phase, it just rotates on the moon's surface.

Just after sunset, the crescent points horizontally [more or less] to the west.

At midnight, the crescent points [more or less] earthward.

Just before sunrise, the crescent points horizontally [more or less] to the east.

In fact, you can also judge time this way.

The angular speed of the pattern is constant. The azimuth at any moment

as a fraction of the change in azimuth from sunset to sunrise is the same

fraction of time interval since sunset to the total time from sunset to sunrise.

When the crescent points earthward [downward] the time is exactly the

average of sunset and sunrise.

It's kind of like a Moon Dial clock.

A moment's thought might make this evident. But it can be checked any clear

night at sunset and sunrise.

Then I need a watch and a moon/time table, to find my direction.

Thanks to all.

[Guest]

The pattern of illumination on the moon - that is, the moon's phase - changes

from full to new and back as the moon takes its 28-day orbit around the earth.

That pattern does not change appreciably at all during a single night. Nothing

at all like you describe, as "all transitions".

Something else changes during the night.

As the sun's apparent position [observed from earth] changes from below

the western horizon [just after sunset] to "beneath" the earth [about midnight]

to below eastern horizon [just before sunrise], the orientation of the pattern

on the moon's face changes. It appears to rotate. Due to the earth's rotation,

not due to the moon's orbital motion.

You see the same phase, rotating, as it maintains alignment with the sun.

Just after sunset, the crescent points horizontally [more or less] to the west.

At midnight or thereabouts, the crescent points vertically earthward.

Just before sunrise, the crescent points horizontally [more or less] to the east.

In fact, you can also judge time this way.

The rotation of the pattern has constant angular speed. So its change in azimuth

[pointing direction] at any moment - relative to the total changes in azimuth from

sunset to sunrise [about, but less than 180 degrees] - is the same fraction as the

elapsed time since sunset is of the total time between sunset and sunrise. When

the crescent points earthward [downward] the time is exactly the average of

sunset and sunrise [approx midnight.]

So the night moon provides a kind of a clock, as well as a compass.

No differently, but less distinctly, when the moon is visible during the daytime, it

can also be seen to "point" to the sun. And any clear night, the orientation of the

moon's illumination can be observed at sunset, midnight and sunrise.

The moon does appear to rotate (counter-clockwise) during the course of a night, but not very much at the best of times (solstice), and not at all at equinox. I would have to think upon it a some more to calculate how much it would rotate in 12 hrs, but it's nowhere near 180 deg. In any case, the OP says it is March, so you're completely out of luck.

You would do much better to just lie still for a few minutes, pick a reference point, and observe which way the sky is moving.

[Guest]

Haven't read the replies but here's how I would do it.

MOON. Assuming moon is visible (more later), the moon will move East to West, rising and setting like the sun due to Earth's rotation. Once you can tell which direction it's moving, head in the opposite direction.

The moon phases (full to new) are result of moon revolving around earth which is slow compared to earth rotation which causes rise/ set east to west. The phase of moon is due to shadow of the earth blocking sun and drives the moon cycle which is about monthly (time it takes for moon to revolve around earth)

[Guest]

The pattern of illumination on the moon - that is, the moon's phase - changes

from full to new and back as the moon takes its 28-day orbit around the earth.

That pattern does not change appreciably at all during a single night. Nothing

at all like you describe, as "all transitions".

Something else changes during the night.

As the sun's apparent position [observed from earth] changes from below

the western horizon [just after sunset] to "beneath" the earth [about midnight]

to below eastern horizon [just before sunrise], the orientation of the pattern

on the moon's face changes. It appears to rotate. Due to the earth's rotation,

not due to the moon's orbital motion.

You see the same phase, rotating, as it maintains alignment with the sun.

Just after sunset, the crescent points horizontally [more or less] to the west.

At midnight or thereabouts, the crescent points vertically earthward.

Just before sunrise, the crescent points horizontally [more or less] to the east.

In fact, you can also judge time this way.

The rotation of the pattern has constant angular speed. So its change in azimuth

[pointing direction] at any moment - relative to the total changes in azimuth from

sunset to sunrise [about, but less than 180 degrees] - is the same fraction as the

elapsed time since sunset is of the total time between sunset and sunrise. When

the crescent points earthward [downward] the time is exactly the average of

sunset and sunrise [approx midnight.]

So the night moon provides a kind of a clock, as well as a compass.

No differently, but less distinctly, when the moon is visible during the daytime, it

can also be seen to "point" to the sun. And any clear night, the orientation of the

moon's illumination can be observed at sunset, midnight and sunrise.

Another great discussion!

I still do not agree with you though, and here's a diagram to explain why...

Please excuse poor quality - done very quickly in paint...

Think of this as a top down view of the earth orbiting the sun and the moon orbiting the earth (obviously not to scale). As the sun (and moon) rises in the east, the earth is rotating anticlockwise (when I say top down, think of us looking directly down on the north pole). Now if you imagine standing on the North Pole, then at any time in the diagram on the left the moon is a half moon with the lit side of the moon on the right as you look at it, but in the diagram on the right the half moon is lit on the left side.

Now pick a point slightly away from the north pole and away from the sun. If we plot the same point in each diagram then it will be the same time of day at that point in each diagram (the middle of the night). However, in each diagram, the lit side of the moon will be on opposite sides. Now move slightly away from the middle of the night position and you get exactly the same thing.

I.e. from any point on the surface of the earth, the side of the moon that is lit is dependant on the moon's position in it's orbit rather than the time of day (nearer sunrise or sunset).

Having gone through this example, I do acknowledge that the moon will appear to rotate from some points on the earth to a greater or lesser degree. Obviously I also acknowledge that the earth actually rotates at an angle so isn't quite as simple as I have represented here. However, I believe this example roughly shows that the pattern on the moon will vary dependant on it's position in it's orbit.

Therefore, although you could work out which way is east in this way, you would have to know at what point in the moon's cycle you are and exactly what time of day it is. If you have all the knowledge and instruments to do this, then just navigate by the stars as it will be far more accurate!

[bonanova]

The phase of moon is due to shadow of the earth blocking sun ...

That's an eclipse.

[Guest]

Haven't read the replies but here's how I would do it.

MOON. Assuming moon is visible (more later), the moon will move East to West, rising and setting like the sun due to Earth's rotation. Once you can tell which direction it's moving, head in the opposite direction.

The moon phases (full to new) are result of moon revolving around earth which is slow compared to earth rotation which causes rise/ set east to west. The phase of moon is due to shadow of the earth blocking sun and drives the moon cycle which is about monthly (time it takes for moon to revolve around earth)

the phase of the moon isn't due to the shadow of the earth blocking the sun as that happens very rarely (and is an eclipse). Instead it is based on the angle at which we are viewing the moon in relation to the sun - i.e. when the moon is (mainly) behind the earth we have a full moon, when it is (mainly) between the earth and sun we have no moon, when it is to one side (as in my diagram) we have a half moon.

[bonanova]

Comments in red inside spoiler

Another great discussion!

I still do not agree with you though, and here's a diagram to explain why...

Please excuse poor quality - done very quickly in paint...

Think of this as a top down view of the earth orbiting the sun and the moon orbiting the earth (obviously not to scale). As the sun (and moon) rises in the east, the earth is rotating anticlockwise (when I say top down, think of us looking directly down on the north pole). Now if you imagine standing on the North Pole, then at any time in the diagram on the left the moon is a half moon with the lit side of the moon on the right as you look at it, but in the diagram on the right the half moon is lit on the left side.

If I'm standing on the north pole, I won't see sunrise or sunset. Think "Land of the Midnight Sun".

The sun moves 360 degrees around me at horizon level.

Should the moon be visible at any time, the "side" of the moon that is illuminated will still be the side that faces the sun.

Also if I'm standing at the North Pole, I have absolutely no problem with directions. Every direction is South.

Which also makes it problematical for my house to be east of me.

Now pick a point slightly away from the north pole and away from the sun. If we plot the same point in each diagram then it will be the same time of day at that point in each diagram (the middle of the night). However, in each diagram, the lit side of the moon will be on opposite sides. Regardless, the lit side of the moon will be the side that faces the sun. Near the North Pole, directions [especially east and west] and time zones become nebulous things. But at any latitude, the lit side of the moon is as reliable an indication of east and west as the sun is when the sun is visible in the sky. Now move slightly away from the middle of the night position and you get exactly the same thing.

I.e. from any point on the surface of the earth, the side of the moon that is lit is dependant on the moon's position in it's orbit rather than the time of day (nearer sunrise or sunset). I think you'd have to draw me a picture that shows the moon illuminated on a side that is not the side the faces the sun in order for me to understand your point here. The light that impinges on the moon's surface comes from the sun -- during all of the moon's phases -- and light, [ignoring gravitational influences], travels in straight lines. I do not ever recall shading my eyes from the sun when I while I was facing away from the sun.

Does the lighted side of the moon help me discern east from west? That's the question that relates to OP. And, roughly to the same degree that seeing the sun in the sky during daytime helps me do that, seeing the lighted side of the moon at night - whatever its phase - does as well.

Having gone through this example, I do acknowledge that the moon will appear to rotate from some points on the earth to a greater or lesser degree. Obviously I also acknowledge that the earth actually rotates at an angle so isn't quite as simple as I have represented here. However, I believe this example roughly shows that the pattern on the moon will vary dependant on it's position in it's orbit. I agree the pattern [phase = new, quarter, half, full, etc....] depends on the moon's orbital position.

What I'm discussing is the orientation of that pattern. What I assert is that when the moon is visible, the portion of the moon's surface that is lighted [our 2-dimensional projection of the moon's observed surface] will provide a vector to the sun's position. At different altitudes and at different seasons of the year, the sun's observed path - and the moon's observed path in the sky will change. Nevertheless, it is always the sun that illuminates the moon for us, and my assertion is a simple consequence of that fact.

Therefore, although you could work out which way is east in this way, you would have to know at what point in the moon's cycle you are and exactly what time of day it is. Not at all. See previous statement. If you see anything other than a full moon or a new moon [the only two phases with no preferred azimuth], the moon provides a vector to the sun. If the sun's position gives you a clue to east/west, then a partially lighted moon does, as well. [sun on horizon could be east or west - you need know whether it's morning or evening.] At night, you only need to know whether the time of day is nearer to sunset or to sunrise to know west from east. If you have all the knowledge and instruments to do this, then just navigate by the stars as it will be far more accurate!

Maybe this helps summarize what I'm saying.

If it's soon after sunset, then east is to the left.

If it's almost sunrise, then east is to the right.

If the arrow points downward, it's near midnight - grab a snack and check it again later.

And note which way it moved in the sky, as a check.

[Guest]

I was going to say

until daybreak and watch the sun rise in the east. But then I realized that the sun is a star and has been explicitly excluded.

But now I see that all the

that mention the moon suffer from the same flaw - the moon itself is not visible unless illuminated by a star (the sun). I'd say that these are thus excluded by the phrase 'using and object, stars'.

Also, can it truly be a 'dark night' if there is a moon?

-Satan's attorney

In a cold March night, you are miles away from your house. All you know that your house is at east of you. Thus you have to find out which direction is east. Of course you don't carry a compass or any magnetic object with you. There are no trees to look their moss, no ant nests. Although you see moon and stars, (as me), you have no idea about stars.

How can you find your direction without using any object, stars, or any other observable material on the ground since it is a dark night?

Confession: My answer to this question may be wrong, anyhow I want to argue it here.

Edited January 9, 2009 by xucam

[Guest]

Regardless, the lit side of the moon will be the side that faces the sun. Near the North Pole, directions [especially east and west] and time zones become nebulous things. But at any latitude, the lit side of the moon is as reliable an indication of east and west as the sun is when the sun is visible in the sky.

I am not trying to argue at all that the lit side of the moon isn't the side that faces the sun. That goes without saying. In fact my diagram and argument is based on the fact that it is the side nearest the sun but that, depending on which side of the earth the moon is on, that means that the lit side would be direct opposites from our viewpoint.

In both cases in my diagram, the moon provides a vector to the sun, but the lit side is different depending on where it is in it's rotation and so without knowing lots more info, we can't deduce east or west.

Your comment that at the north pole east and west become nebulous things I think is where things are misleading. If you are "just" off the north pole (say a mile) then there is a technical east/west and day night (assuming no angle of rotation of earth, perfect symmetries, etc.) In this case both diagrams show a different view of the moon even though it is the same time. If you do think that this is too nebulous, then keep on walking away from the north pole in the same direction. From what you are saying, at some point you will get to a place where east and west is no longer nebulous and one (or both) of the views flip so that they become the same. Where does that happen and, what happens if I step a few metres back again?

When the sun is visible in the sky, we get an indication of east and west based on the fact it rises in the east and sets in the west. I have said that the moon can give you the same information based on it also rises in the east and sets in the west (although not as accurately), but I don't think it gives you anything simply because it points at the sun.

In fact, let's look at what "rising in the east" can tell us...

We know that the moon rises in the east. On a night of a half moon, as the moon comes over the horizon, if we look directly at the moon we know we are looking east. So, if we were to follow the path of the lit side of the moon, it would point in another direction.

Alternatively, think of looking at the moon exactly half way through the night. We know from phases of the moon that sometimes there will be a half moon with the left side lit and sometimes there will be a half moon with the right side lit. So if we stand in the same spot and wait about 14 days, we will see both happen, which would suggest that both directions are east. Now, you may argue that exactly in the middle of the night, you can't tell, because we are at that changing point where the sun is actually moving from our west to our east. But again, just go a couple of minutes either side of the middle of the night - the moon doesn't suddenly flip.

By the way, I may well be wrong here, but I can't see why my logic is wrong at the moment.

[Guest]

This was bothering me so I had a quick check to see if I could find out definitively anywhere.

I found the following:

"You can tell when the moon is first quarter because the sunlit side is to the west, on the right side. A third quarter moon also shows half of the moon, but the sunlit side of the moon is on the left, facing east."

on this site: http://www.astrotulsa.com/Learn/lesson2.asp

Hope this helps!

[Guest]

Interesting that you all want to use a certain object lit by a certain star - is it a paradox?

Camera SLR, mirror up for long exposure and take a star trail picture to find true nth/sth... if you know which hemisphere you live in

[bonanova]

When the sun is visible in the sky, we get an indication of east and west based on the fact it rises in the east and sets in the west. At night, the vector given by the moon's illumination gives that same indication.

I have said that the moon can give you the same information based on it also rises in the east and sets in the west (although not as accurately), I haven't said this yet, but of course I agree; your method works - I'm really just defending mine. but I don't think it gives you anything simply because it points at the sun.

1. Well, we know the sun is to the west [between noon and midnight, more or less]
   
2. And the sun is toward the the east [between midnight and noon, more or less]
   

So, if we know which half-day we're in, pointing to the sun gives an idea of west and east.

That was the question posed in the OP.

In other words, during the day you can look at the sun to know where it is.

During the night, you can look at the moon to know where the sun is.

Whatever information the sun gives about east and west during the day

is given by the moon during the night.

Let's make a rough table.

From Sunrise to Noon: Sun can be seen in the EAST

From Noon to sunset: Sun can be seen in the WEST

From Sunset to Midnight: Moon "points" to the WEST [where the sun is, out of direct sight]

From Midnight to sunrise: Moon "points" to the EAST [where the sun is, out of direct sight]

This method gives an instantaneous reading of east/west.

It's least precise around noon and midnight, best around sunset and sunrise.

Tracking the sun and the moon as it moves across the sky also gives a reading of east and west;

And it does very well around noon and midnight.

But it does require a certain time interval to perform.

And both methods suffer, of course, from the fact that the moon is not always visible every hour of the night.

Depending on its phase and our latitude.

[Guest]

Duh! (slaps forehead)

First, forget everything I said in my last post. I misunderstood Bonanova's post, picturing the moon spinning like a record as it moved through the sky (it does have a monthly libration cycle, but that is irrelevant). Obviously, what he was referring to was the fact that whatever part of the moon is pointing down (toward the horizon) just after moon-rise will be pointing up (away from the horizon) just before moon-set, and vice-versa. I don't understand the bit about "pointing downward" at midnight though. The part of the moon that is in shadow always points east(ish) or west(ish), because the moon's orbit is almost in the same plane as Earth's ecliptic.

Having said all that, you could use the moon to estimate the position of the sun, as Bonanova suggests, and if you knew the time, you could then figure out which directions were east and west (approximately). You wouldn't need to know the moon's phase, and it would still work if the moon were full. The problem is that just because it is night-time, doesn't mean you can see the moon. It can rise or set at any time of the day or night, depending on the phase (e.g. you will never see a new moon in the night sky, so you would have to wait until morning anyway).

Finally, I think I see the source of confusion between Bonanova and Neida. Basically, you are both right. However, some of the situations you both describe could never actually be observed. For example, if the figure Bonanova provided is of a waxing crescent (north is "up"), it is only visible in the night sky from around sunset to sometime before midnight (when the moon sets). A waning crescent (north is "down") rises sometime after midnight, but sets after sunrise, so it will probably be too right for you to see it. In essence, you will never see a waxing moon rise, or a waning moon set.

[Guest]

From Sunrise to Noon: Sun can be seen in the EAST

From Noon to sunset: Sun can be seen in the WEST

From Sunset to Midnight: Moon "points" to the WEST [where the sun is, out of direct sight]

From Midnight to sunrise: Moon "points" to the EAST [where the sun is, out of direct sight]

This method gives an instantaneous reading of east/west.

It's least precise around noon and midnight, best around sunset and sunrise.

And both methods suffer, of course, from the fact that the moon is not always visible every hour of the night.

Depending on its phase and our latitude.

Right. I didn't see that before posting...

I agree, except for the highlighted portion, which depends on the phase of the moon for the latter two cases. For example a waxing quarter at midnight will give a very good indication of where the sun is. Of course, it will give you an even better indication of where west is...

[Guest]

Sorry, I'm still going to have to disagree.

In other words, during the day you can look at the sun to know where it is.

During the night, you can look at the moon to know where the sun is.

I think your reasoning would work if the sun revolved around the earth, but not the earth revolving around the sun. The point is that the sun stays pretty much in one place, it is just our view of it that changes. Also, in one night, the moon stays pretty much in one place relative to the earth and it is just our view of it that changes. At all times the moon is pointing towards the sun, but the moon never changes the direction it is pointing in, simply the angle of our view of the moon changes very slightly as the earth revolves. Now, if the view of the moon from the earth (note that you do not have to pick a particular point) shows the left side of the moon is lit, then any point will show the left side being lit, i.e. any time zone will show the left side being lit.

The problem isn't that you want to know where the sun is - the sun doesn't move. In the day, provided we know the time, we know where in our rotation we are, so can work out the angle at which we should be able to see the sun. By lining that up, we can then identify other directions. At night we can not see the sun and so we have no angle to line up. Observing how one distant object points at the sun is not enough information for us to be able to deduce that angle.

From Sunset to Midnight: Moon "points" to the WEST [where the sun is, out of direct sight]

From Midnight to sunrise: Moon "points" to the EAST [where the sun is, out of direct sight]

Again, this would only happen if the sun revolved around the earth. I've attempted another diagram to show this:

moontriangle.bmp

On the left we see the situation during a particular night. Although the earth is revolving (which gives us day and night), the triangle that is represented stays roughly the same - i.e. the earth, moon and sun all stay in roughly the same position relative to each other. This diagram shows that, at any time and viewed from anywhere on earth, the moon "points" towards the sun down the SAME side of the earth - in this case as we look at it it always points down the left side. Night time runs on the opposite side of the earth to the sun, from one side to the other. Again, we see that regardless of whether you are on the left or right side of the earth (or in the morning or at night) the moon still always points to the sun down the same side of the earth.

Where you would be correct is in the diagram on the right, where the sun moves around the earth. In this scenario there is a time where the moon would stop "pointing" down one side of the earth and start "pointing" down the other side. In any case we know that the sun doesn't revolve around the earth so this case isn't valid.

I hope this all makes sense and explains why for half a month the moon points towards the east and for the other half it points towards the west.

[Guest]

Just taking a better read of that link I posted earlier, if you know the phase of the moon and the time of day then you could work out rough east by where the moon is in the sky (not where it is pointing). This is because of moonrise and moonset being at different times of day depending on the moon phase.

However, without any external information (moon phase or time), observing the path the moon takes appears to be the only way to be sure of which way is (approximately) east.

[Guest]

I hope this all makes sense and explains why for half a month the moon points towards the east and for the other half it points towards the west.

That is true, but you do not need to know the moon's phase to estimate the direction of the sun relative to Earth. And, it doesn't matter what body is orbiting what other body. They could all be completely stationary, and the method would still tell you what direction the moon is illuminated from. It is the same as pointing a flashlight at a ball in a dark room. By looking only at the ball, you can judge which direction the flashlight is in, from any position or angle.

The fact that certain configurations can never actually be observed is irrelevant. If you read Bonanova's original post, it says:

Now you only have to ask yourself: is it closer in time to today's sunset or to tomorrow's sunrise?

If sunset, the moon is "pointing" west.

If sunrise, the moon is "pointing" east.

I had initially assumed that he meant that it will change from pointing west at sunset to east at sunrise during the night. It doesn't say that though. It says (essentially), if it is before midnight, the moon points west. This is always true, because if it were pointing east, it would not rise until after midnight.

Also, here's a fun link: http://www.forgefx.com/casestudies/prentic...se/eclipses.htm

[Guest]

Also, here's a fun link: http://www.forgefx.com/casestudies/prentic...se/eclipses.htm

well that was fun - the sun initialy went around the earth

The Earth looked flat

Then the earth appeared in a new video and looked like half a flat disc > half disc-overy wait, so did the moon

I'm sure that I read they are globes - oh yeah and if that red stick was at there it would be easier to find the pole - perhaps more chance of a dinosaur?

That said - it's been fun to read, you all get a way-finder scout badge. Next we will do tracking and finish with knots

Edited January 10, 2009 by Lost in space