[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]

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.

OK, I think I understand more where you are coming from now, but this is still not correct. If you re-read what you are saying, you say that in order for it to point east it would have to rise after midnight. I assume you therefore also mean that if it is pointing west then it has to set before midnight. Taking the natural progression that the moon either points east or west, it must therefore either set before midnight or rise after it, therefore we could never see the moon at midnight. But this is incorrect, as we obviously can (some nights - not all nights).

I think were you are coming from is as follows:

In a first quarter moon (we see a half moon pointing west) the moon rises at noon and sets at midnight.

In a third quarter moon (we see a half moon pointing east) the moon rises at midnight and sets at noon.

Therefore, if we see a half moon and it is before midnight, we know the moon must be facing west. If we see a half moon and it is after midnight, we know the moon must be facing east.

I think this is fundamentally the argument that Bonanova put forward, but in the general case of any moon (other than full or no moon). d3k3 then confirmed that the general principle was correct because other configurations couldn't be observed.

However, this reasoning does not apply to the general case. Here is an example:

Mid way between the first quarter moon and a full moon we will see three quarters of the moon with the lit side pointing west. The moon will rise at approx. 3pm and set at 3am. The direction the lit side of the moon faces does not change at midnight.

Mid way between the full moon and a three quarter moon we will see three quarters of the moon with the lit side pointing east. The moon will rise at approx. 9pm and set at 9am. The direction the lit side of the moon faces does not change at midnight.

Now, for each of the two days presented above, at 10pm on the first day I will see a three quarter moon pointing west and at 10pm on the second day I will see a three quarter moon pointing east. At 2am on the first day I will see a three quarter moon pointing west and at 2am on the second day I will see a three quarter moon facing east. This example shows that simply knowing which side of midnight I am does not mean I know which way the moon is pointing.

Bonanova's reasoning would be correct if, instead of referring to sunrise and sunset he referred to moonrise and moonset as follows: If we see the moon and know that it rose closer to midnight than it will set, then we know the moon is pointing east. If we see the moon and know that it will set closer to midnight than it rose, then we know the moon is pointing west.

Alternatively, because of the timings of moonrise and moonset depending on the phase of the moon, it would also be true to say: If we see the moon before midnight it is more probable that it is pointing west. If we see the moon after midnight it is more probable that it is pointing east.

Finally - do an experiment and look at the moon if you aren't sure (I did last night). Unfortunately tomorrow is a full moon, so this isn't the best time to do the experiment, but if you look closely at the moon tonight you should just be able to make out that it is not quite full. You will also see that the lit side points west. You will see this if you look at the moon at 10pm, 11pm, 12am, 1am, 2am or at any other time you choose to look at the moon.

Hopefully you see that I agree that Bonanova is correct sometimes depending on the phase of the moon. However, it simply isn't true that from sunset to midnight the moon points west and from midnight to sunrise the moon points west at all times, and that is the point I am trying to disprove. If you can accept that is correct then you can not with certainty derive direction simply by looking at your watch and looking at the moon.

Note: Bonanova also said "It's least precise around noon and midnight, best around sunset and sunrise." Actually, if you look at my timings above, a first quarter moon will not be seen at any time near to sunrise and a third quarter moon will never be seen at any time near to sunset.

Please let me know if this is unclear. It isn't really a point of interpretation, deduction or logic, it is just plain fact, so if I am still not explaining it correctly then I am happy to try again.

[Guest]

OK, I think I understand more where you are coming from now, but this is still not correct. If you re-read what you are saying, you say that in order for it to point east it would have to rise after midnight. I assume you therefore also mean that if it is pointing west then it has to set before midnight. Taking the natural progression that the moon either points east or west, it must therefore either set before midnight or rise after it, therefore we could never see the moon at midnight. But this is incorrect, as we obviously can (some nights - not all nights).

Oh, yeah. I completely neglected gibbous phases. I'm suffering lately from a severe lack of caffeine...

[bonanova]

Assume observer in the northern hemisphere looking southward, seeing the moon rise to the left [E] and set to his right [W]

Beginning with new moon at the top, waxing to full and waning again to new.

Observe the lighted side of the night moon before and after midnight.

I'm interested to hear if I've sketched this correctly.

[Guest]

Assume observer in the northern hemisphere looking southward, seeing the moon rise to the left [E] and set to his right [W]

Beginning with new moon at the top, waxing to full and waning again to new.

Observe the lighted side of the night moon before and after midnight.

I'm interested to hear if I've sketched this correctly.

I can't comment on scale, but it looks correct in principle.

EDIT: And I guess it still works for a gibbous moon too...

Edited January 13, 2009 by d3k3

[Guest]

Observe the lighted side of the night moon before and after midnight.

I'm interested to hear if I've sketched this correctly.

Not quite. The rotation of the moon will vary depending on your latitude. The second line (crescent moon) would be about right for someone near the equator, but the amount of rotation would decrease as you approach the pole (with there being no rotation at the pole).

If we assume then that we are looking at a diagram for someone near the equator who would see this level of rotation, the issue then lies in the fact that the starting and ending points for each rotation are different.

Again looking at the second line (crescent moon) you start with the crescent pointing up right and end with it pointing down right. This is correct. Similarly the last crescent moon starts with the crescent pointing down left and ends pointing up left. In each case the rotation means that we start and end with symmetrically opposite shapes through the horizontal.

Now, for some reason, when you draw the quarter moon you again start pointing up right but now end with it pointing directly down. I.e. you don't start and end with symmetrically different shapes through the horizontal. As you move onto a Gibbous moon, this rotation becomes even more distorted, so that it ends up pointing down left, which would suit the answer you provided, but this just doesn't happen. Basically you are changing the starting point of rotation as you move through the moons, which is where the error creeps in.

For ANY phase of the moon, when the moon is at its peak in the sky it will be (virtually) symmetric through the horizontal. (I say virtually, as the ecliptic orbit means there are slight variations, but nothing significant). If you are unsure about this, just take a look at the moon around midnight. You will always see that it points directly left or right as you look at it, not up or down.

Edited January 13, 2009 by neida

[Guest]

I can't comment on scale, but it looks correct in principle.

EDIT: And I guess it still works for a gibbous moon too...

[flip-flop] On second thought, that is not quite right.

Let us take latitude and other complications out of the equation, and assume that Moon's ecliptic is parallel to Earth's, Earth's axis is not tilted, and you are at the equator. That way, the sun rises at 6:00 AM and sets at 6:00 PM every day, and the moon and sun pass directly overhead every day. In that case, the moon "vector" will be normal to the horizon at moon-rise and moon-set (not at midnight, as sketched). It will point "up" at moon-rise, and "down" at moon-set, if it is waxing, and vice-versa if it is waning. When the moon is at its zenith, it will point parallel to the horizon.

So, the sketch is more-or-less correct up to the first quarter, and again from the third quarter on. Obviously, there can be no argument about the full and new moons. However, the waxing gibbous needs to be rotated CCW about 45 deg., and the waning gibbous CW about the same, to be correct. Now, let us consider again the original statement:

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.

These statements are only true between the third quarter and first quarter.

EDIT: Just read neida's post, which says the same thing.

Edited January 13, 2009 by d3k3

[bonanova]

Not that it was quick to do, but if you decide to make a correct version of my sketch, I'd be interested to see it.

I was thinking about 45^o N latitude, but equator is fine - first-order sketch is good enough.

[Guest]

For ANY phase of the moon, when the moon is at its peak in the sky it will be (virtually) symmetric through the horizontal. (I say virtually, as the ecliptic orbit means there are slight variations, but nothing significant). If you are unsure about this, just take a look at the moon around midnight. You will always see that it points directly left or right as you look at it, not up or down.

Not at midnight. At the moon's zenith.

[Guest]

Not at midnight. At the moon's zenith.

Sorry - you are absolutely correct! I said "when the moon is at it's peak" and then went on to say "midnight"! Here's me trying to make a distinction between the two and then come out with something like that!!

[Guest]

Not that it was quick to do, but if you decide to make a correct version of my sketch, I'd be interested to see it.

I was thinking about 45^o N latitude, but equator is fine - first-order sketch is good enough.

I don't have time to do it, but basically on each line of your diagram the lit portion of the moon should point directly left or right at the moon's zenith. The rotation is then equal and opposite for equal and opposite times either side of the zenith. E.g. if there is an x degree clockwise rotation 3 hours after the zenith there will be an x degree anticlockwise rotation 3 hours before.

The angle of rotation depends on your latitude. At the equator it will travel through a full 180 degrees. To visualise this, just think that the moon will rise directly to your east, pass directly overhead and set directly in the west. At the zenith you will be looking directly south (well actually directly up, but for any point in the northern hemisphere you look directly south when the moon is at its zenith so lets just apply that here too). When it rises you will have rotated your point of view 90 degrees to the right and when it sets you will have rotated your view 90 degrees to your left.

At the north pole (assuming d3k3's "perfect earth" where we ignore angle of rotation etc.) then you are always looking directly south no matter how much you follow the moon around (actually it will never set) and so the angle of rotation stays at zero. In between the pole and the equator that angle will vary and it is based upon the angle you need to turn from due south in order to see the moon at moon rise and moon set (an equal and opposite angle).

Hope that explains the diagram and sorry I don't have time (or probably the skills!) to draw it.

One final comment which I had thought of previously but thinking through this has helped confirm. I mentioned previously that if you knew the phase of the moon then you would know if it was pointing east or west. However, this wouldn't actually be enough to determine east or west because of the angle which you are viewing the moon from as described above - it would only be enough to give you a vague sense of which way is east (give or take 45 degrees). If you are viewing the moon at it's zenith then you know that east is in a direction perpendicular to a line between you and the moon (because you are looking directly south). However, at any other time, east will actually be at a different angle to that line and, in order to determine what angle you would need to use, you would not only need to know the phase of the moon, but also your latitude and the correct time. Tying all this in with my original comment (in my original answer) that viewing the moon isn't incredibly accurate anyway and certainly not accurate enough to navigate by, we probably see why the stars were the system of choice for navigation!

[Guest]

I haven't seen any other responses so I don't know if this is redundant. Anyway . . .

As the Eartn rotates in an Easterly direction, one could determine that direction by marking the movement of moon or stars traking in the opposite direction. Of course, this method is dependent on your distance from the poles for accuracy. Maybe?

[bonanova]

As you move onto a Gibbous moon, this rotation becomes even more distorted, so that it ends up pointing down left, which would suit the answer you provided, but this just doesn't happen. Basically you are changing the starting point of rotation as you move through the moons, which is where the error creeps in.

We have a waning Gibbous moon - 3 days past full.

At 22:00 EST last night the moon was in the eastern trees;

its dark side was seen to be about 1:00 on the moon's face - pointing down left to about 7:00.

The pattern continued to rotate clockwise, as my sketch shows.

I photographed the moon between 04:30 and now, 08:45 EST, when it set into the western trees,

In that interval the "pointing" area rotated between 9:00 and 10:00 on the moon's face [see the morning waning Gibbous sketches].

The moon pointed east after midnight.

If skies are clear tonight I'll try to photograph the earlier orientation.

[Guest]

We have a waning Gibbous moon - 3 days past full.

At 22:00 EST last night the moon was in the eastern trees;

its dark side was seen to be about 1:00 on the moon's face - pointing down left to about 7:00.

The pattern continued to rotate clockwise, as my sketch shows.

I photographed the moon between 04:30 and now, 08:45 EST, when it set into the western trees,

In that interval the "pointing" area rotated between 9:00 and 10:00 on the moon's face [see the morning waning Gibbous sketches].

The moon pointed east after midnight.

If skies are clear tonight I'll try to photograph the earlier orientation.

If it was pointing to 7:00 at 22:00 and rotated through to 9:00 and then 10:00 in the early hours of the morning, then surely it pointed to left both before and after midnight (as 7:00, 9:00 and 10:00 are all on the left side of a clock face).

[Guest]

We have a waning Gibbous moon - 3 days past full.

At 22:00 EST last night the moon was in the eastern trees;

its dark side was seen to be about 1:00 on the moon's face - pointing down left to about 7:00.

Actually, having read this I think you are agreeing with what I've previously said, it's just the highlighted bit of my comment that through me as I thought you were disagreeing with it.

My comment was referring to your diagram of a waxing Gibbous moon, saying it will not point down left. As you stated, you observed a waning Gibbous moon pointing down left before midnight (not a waxing one). Your diagram showed a waning Gibbous pointing down right before midnight. If you make the adjustments to your diagram as I had described then your diagram would instead show a waning Gibbous moon pointing down left before midnight, which would match what you observed.

The moon pointed east after midnight.

I think this bit also threw me. You said the moon pointed east after midnight, which I agree with. But, because it pointed towards 7:00 before midnight you've also observed that the moon pointed east before midnight too.

So, I think we're all agreed on this one now??

[bonanova]

Well, so far the moon hasn't taken issue with my sketches.

If the sky is within walking distance, one can check that at 1:45 am ET it looks very like my 3:00 am waning Gibbous.

And in a couple weeks, should anyone still care, more of the various theories can be checked out.

Sketches aside, the point remains: the lighted pattern on the face of the moon points toward the sun.

That's relevant to the OP because, season and latitude taken properly into account,

information regarding the compass points can be drawn from the position of the sun.

That position can come either directly, from observation during daylight; or indirectly, by observation during moonlight.

I'm happy to stay with that.

Truce.

[Guest]

Well, so far the moon hasn't taken issue with my sketches.

I'm not sure why you say this. You said yesterday that at 10pm the moon pointing towards 7 on a clock face, whereas for a waning Gibbous your sketch says it should point towards 5. As we are talking the difference between pointing east and west (or left and right) that's really the major area where a difference matters. I'm in the UK and had written my last comment about 11pm and, just to make sure I wasn't losing my sanity, I looked at the moon and, true enough, it was pointing east before midnight.

Sketches aside, the point remains: the lighted pattern on the face of the moon points toward the sun.

Agreed.

That's relevant to the OP because, season and latitude taken properly into account,

information regarding the compass points can be drawn from the position of the sun.

Agreed.

That position can come either directly, from observation during daylight; or indirectly, by observation during moonlight.

I think this is where the argument falls down. Observing the moon does not give you enough information to determine the position of the sun. I acknowledge that it points towards the sun, but that's just not enough. All it is telling you is where the sun is in relation to itself. Let me try to explain this with a final diagram...

Let's forget about the earth and moon for a second and just imagine that you are at point A. You don't have a view of the space above and to the left of you because of a brick wall directly in front of you. However, you know that there is an object there somewhere and you want to work out where it is. So you ask a friend at point B to point towards it, as in the diagram. Now, from this information, can you tell me where the object I am looking for is? In particular, can you tell me if it is to the left or right of the marker line half way along the wall?

If you transfer this exact situation to you being on the earth, B being the moon and the wall being the earth, then the problem is exacerbated due to the curvature of the earth increasing the room for error in determining the angle of view towards the sun.

Now you may argue that you know the distance from B to the object as well as the direction, so you can pinpoint the object. But in the example of the earth there are two problems with this:

1. The distances are so great it would be almost impossible to be accurate.

2. As the moon moves around the earth the distance changes. It is only small (relatively) but big enough to introduce errors.

These combined with the curvature of the earth make it extremely difficult to pinpoint where the sun is.

As a side point, the diagram may also explain why, on any given night (or in any given phase) the moon always points east or west as it always points around the same side of the earth.

I do agree that, given the season, time, latitude and phase you would be able to work out east or west from which side of the moon is lit, but this is not by determining where the sun is, it's just by knowing how the moon should look and where it should appear in relation to your position.

[Guest]

If you transfer this exact situation to you being on the earth, B being the moon and the wall being the earth, then the problem is exacerbated due to the curvature of the earth increasing the room for error in determining the angle of view towards the sun.

Sorry, I should have added that the curvature of the earth is also why you can't see B after midnight in a first quarter moon or before in a third quarter moon and why you can see it either side of midnight in Gibbous moons, which is why Gibbous moons are the ones that cause contention.

I'm sure anyone already following this discussion will already know this, but just in case anyone else is reading this and trying to make sense of it!

[bonanova]

I do agree that, given the season, time, latitude and phase you would be able to work out east or west from which side of the moon is lit....

Thank you. With regard to my answer to the OP I declare truce.

I did not say the moon would be as directly informative as looking at the daylight sun.

Only what you say; which is all that I needed to answer the OP.

------------------

The "at the moon's zenith" statement is still not clear to me.

Unless it also needs to be adjusted for season, time, latitude, phase, kitchen sink, what have you:

For one thing, the full moon is a degenerate case, not pointing left or right; OK no problem with that.

From one day before full to one day after full, it switches exactly 180 degrees, at all latitudes?

Or just at the equator, ignoring the ecliptic, or ... ?

Here are some data:

This morning [41^o N latitude], 4 or 5 days after full moon:

At 3:00am, prior to zenith, the waning gibbous moon pointed downward, to about "7:00" as shown in my 3:00 am sketch.

At 6:50 am, after zenith, the sun being just below the horizon, it still points somewhat downward, as in my 6:00 am sketch.

At zenith, therefore, [perhaps 4:00am], the sun being further below the horizon than now, La Luna also must have pointed downward.

[Note added at 9:30 am a couple hours after sunrise]

Moon now points to about 10:00 on its face, in agreement with my 9:00am waning gibbous sketch.

[/Note]

My sketch should show zenith somewhere between moons 3 and 4 - not at moon 3.

Also my 1Q sketch should mirror the 3Q sketch.

• For waxing crescent and gibbous phases, the sun is pre- and post- setting, at moon zenith; so the light/dark boundary isn't vertical.
  [my waxing crescent sketch should be changed in that regard]
  
• For waning crescent and gibbous phases, the sun is pre- and post- sunrise, and the same conclusions obtain.
  
• At 1Q and 3Q, the moon's zenith is at sunset and sunrise, creating a vertical light/dark boundary.
  

When I was an experimental scientist I loved theories and I loved data. When they didn't agree I always sided with the data.

[Guest]

Thank you. With regard to my answer to the OP I declare truce.

I did not say the moon would be as directly informative as looking at the daylight sun.

Only what you say; which is all that I needed to answer the OP.

Your original answer to the OP was:

Use the moon.

...

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.

which was later clarified on several occasions, most succinctly as:

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.

Now, if we take:

When I was an experimental scientist I loved theories and I loved data. When they didn't agree I always sided with the data.

and...

At 22:00 EST last night the moon was in the eastern trees;

its dark side was seen to be about 1:00 on the moon's face - pointing down left to about 7:00.

your first answer says that the moon points west before midnight, but your data says that you observed it pointing east (assuming you were in New York or any other place in the northern hemisphere) before midnight. (Note that all these are comments that you have posted yourself.)

Surely then, you do not still argue that the moon always points west before midnight and east afterwards?

Also, if we look at my full quote (which you only took half of):

I do agree that, given the season, time, latitude and phase you would be able to work out east or west from which side of the moon is lit, but this is not by determining where the sun is, it's just by knowing how the moon should look and where it should appear in relation to your position.

Now I must admit I am at fault here of not wording this correctly and I should not have said "from which side of the moon is lit", as the words in bold were intended to negate this but clearly didn't. My point is that, if you know the season, time, latitude and phase then, without looking at the sky, you can work out what the moon looks like (including what side is lit) and where it should be in the sky. You need to look at the moon in order to know where it appears - you already know what it will look like. It is where it appears that could accurately give you your direction using this method, not what it looks like.

Now, if we weren't looking for "exact" east, but just approximate, the simplest summary would be:

"Viewed from the northern hemisphere, the left side of the moon points to the west and the right side of the moon points to the east. Sometimes the left side is lit (during a waxing moon) and sometimes the right side is lit (during a waning moon)."

From this we see that the lit side actually makes no difference and was just a red herring all along that was complicating matters unnecessarily.

To try to put this one to bed, if you still believe in some way that your original answer was correct, then can you please either explain why the data contradicts it or rewrite it in simple terms without caveats (or reference to other information such as moon phase tables, etc.) so that, simply by looking at the lit side of the moon, I can tell east from west?

The "at the moon's zenith" statement is still not clear to me.

Unless it also needs to be adjusted for season, time, latitude, phase, kitchen sink, what have you:

Apologies - I think you are referring to my comment that the moon will be horizontally symmetric at it's zenith. I was wrong to make that statement - I was still thinking about the "perfect" earth which isn't rotating at an angle to the sun. In the real earth, this "tilt" of the earth will have an impact on the rotation of the image on the moon and so, yes, it should be adjusted for season, etc. However, this does not change the statements above about it pointing east/west before/after midnight.

I'm sorry if I sound like I'm going on about this, but as the original poster of the question agreed with your original answer (moon points west before midnight and east after midnight), I think it's important to clarify that this is actually wrong and that, if you are still standing by your original answer, it must be in some rewritten, clarified and caveated form. I have a lot of respect for you and have rarely known you to be wrong on this forum but, largely because of that, I think it is important to admit when you are. At the moment it feels like you are trying to cling on to your original answer in some way and I think that will just confuse people.