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» Online Classroom   » Emergency Navigation   » Public Discussion of Emergency Navigation   » Finding direction by the quarter moons

   
Author Topic: Finding direction by the quarter moons
FloridaAdventuring


 - posted December 23, 2005 09:39 AM      Profile for FloridaAdventuring           Edit/Delete Post 
In several places, I've read that one can roughly determine south by drawing an imaginary line between the horns of either the first- or last-quarter moons. That line is said to intersect with the horizon at "roughly" south.

Do you find this to be true?

From: Melrose, Florida
David Burch


 - posted December 23, 2005 11:02 AM      Profile for David Burch           Edit/Delete Post 
There is a chapter in Emergency Navigation about the use of the moon for directions. The line you refer to points to the sun, since that is what is creating the shape you are seeing. It is not a way to find south. The next time you see such a moon, check it out.

Please tell me where you read the statement you quote.

From: Starpath, Seattle, WA
FloridaAdventuring


 - posted December 24, 2005 08:22 AM      Profile for FloridaAdventuring           Edit/Delete Post 
"Survival is a Dying Art," Barry Davies, Phil Beynon. Lewis Interntional Inc.ISBN 0-9666771-3-7, page 119

"Produce a line through the horns of either of the quarter moons down to the horizon. Where it meets the horizon it will indicate South, if you are in the Northern Hemisphere. It will indicate North if you are in the Southern Hemisphere. Tests have demonstrated that this can provide a rough but very ready guide when moving at night."

"SAS Enclycopedia of Survival," Barry Davies BEM. Lewis International, Inc. 1999. ISBN 0-9666771-5-3. Page 123

"The Quarter Moons. Both waxing and waning quarter moons can be used. Draw a line through the horns down to the horizon. The point where it touches gives a rough indication of South, if you are in the Northern Hemisphere, and North if you are in the Southern Hemisphere. Although not very accurate this will at least provide you with a rough guide while traveling at night."

In Figure 7-3, the line drawn between the moon's horns would hit the horizon near south if it were projected to the horizon.

It just occurs to me that perhaps there is some confusion. A line perpendicular to a line drawn between the horns (as illustrated) does indeed point to the sun.

Quite a few times, I have projected the line between the moon's horns to the southern horizon, but I wondered if it would work at all times of night and at all times of year. If it works, it could be very handy direction indicator.

From: Melrose, Florida
David Burch


 - posted December 24, 2005 01:40 PM      Profile for David Burch           Edit/Delete Post 
ah ha! i misread your posting, thinking only of the lines we have drawn in the book.... regarding my last (now edited) post, maybe i should go back and find some reference in ancient Greece about reading carefully before responding!
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An interesting point. i can see what you mean. When speaking of "horns," I immediately thought of a new moon, since a precise quarter moon is what we often refer to as a "half moon," which may have horns of a sort on one side of the phase... (There is a nice picture of moon phases at the nautical almanac office site, i will try to find this and post it. we also have links to some moon sites i believe.)

i would guess that the horns on a new moon don't point anywhere for directions, no matter how you draw a line, but the moons you refer to certainly do have information in the lines you refer to that could be useful.

for our work it might be best to think in terms of moon age, either in days or in phase percentage, with quarter moons being 0.25 and 0.75 or days about 6-8 and 21-23.

At precisely the quarter moon phases, we see a perfect half moon. And the line along the face of that half moon should point (intersect) near due south (north from SH) at 6 am or 6 pm LMT, that is 6 hours from LAN or midnight, as the moon is then near the meridian. But I would guess the tilt of the moon even then depends on the relative declination of sun and moon. We will have to check this.

The visibility of these moons is divided between daylight and night about equally. They are especially valuable for routine cel nav, whereas other phases are not as special, as shown in Table 3-3 of The Star Finder Book.

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Near meridian passage of the precise quarter moon, the method should work reasonably well--and should be investigated before the next edition of EN, but it will take some computations to investigate the accuracy away from these ideal conditions. It may also depend on the latitude-declination difference as well, once away from the ideal case.

Clearly we already have your experiences and that of the author of the books you mention, that support the idea. The task is to learn the limits.

In any event, this is a good thing to think about and definitely should be addressed in the next version of the book. If we know the time, we can tell from the time the approximate direction of a quarter moon, but there may well be a way to incorporate the orientation of that line into the process to improve it.

Thanks for that idea, and any other new insights you might have on this subject.

--------

in passing, how would you describe the content of the two books you reference with regard to emergency navigation? Are all of the techniques of the "encyclopedia" covered in the other book?

From: Starpath, Seattle, WA
FloridaAdventuring


 - posted December 25, 2005 06:31 AM      Profile for FloridaAdventuring           Edit/Delete Post 
The celestial techniques included in both books consist of finding directions by:
1. The stick and stone (shadow stick) method
2. The watch method (presumably, the sun has to have a maximum altitude of 45 degrees although this is not mentioned).
3. Using the Big Dipper (Plough) and Cassiopeia to find Polaris
4. Using the Southern Cross
5. Using 2 sticks as a sight to observe the direction of star movement.
6. Finding south (or north in the southern hemisphere) by the "quarter moons."

When speaking of "quarter moons," each book depicts what I suppose is properly called a "waxing quarter moon," that is, it's not yet half full, but well past the new-moon phase.

I've tried this method with the "half-full moon" and it doesn't always work. Yet, with a waxing quarter moon (the crescent-shaped moon one would find cut in an out-house door), it seems to give a rough, southerly direction (south, to maybe within 25 degrees or so).

If the sun illuminates the side of the moon opposite the horns, and a line perpendicular to a line between the horns would point to the sun (which should be in an easterly or westerly direction), then a line between the horns when extended to the horizon should point in a southerly direction, I suppose. As the moon moves through the sky in relation to the illuminating sun, maybe the moon "tilts" to maintain the southerly direction of the horns.

I'd like to know what tests the author is referring to when he says that "tests have demonstrated that this can provide a rough but very ready guide..." Perhaps the British Special Air Service (SAS) of which he was a member has done studies.

From: Melrose, Florida
David Burch


 - posted December 25, 2005 04:41 PM      Profile for David Burch           Edit/Delete Post 
Thanks for the details. For the record here is the line we are talking about. I have written to the photographer to ask if he has any specs for these pics, which i presume are not in order, nor necessariy systematic. But i guess someone does have a sequence of pics as the moon crosses the sky.

Our job now is to start making some observations with real measurements. I am guessing the answer can be computed, but this will take some head scratching, and then still we need real data.

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From Michael Myers photography

It is an interesting idea, but if we are talking about ± 25 or 30° then we can likely do better with the time, and that method applies to either side of the quarter to the extent we can guess it.

Nevertheless, anything that can be done by just looking is better than something that requires a computation.

It is a good exercise when seeing a quarter moon or ± a day of such when driving around, to make a guess of its direction relative to the time. Often streets are N-S etc to help with this. Or just make your best guess, note the time, then go to the AA website to get the true bearing at that time for a check.

From: Starpath, Seattle, WA
David Burch


 - posted December 25, 2005 07:27 PM      Profile for David Burch           Edit/Delete Post 
Some sample photos — or, one way to spend Christmas.

These are from fixed camera locations, taken sequentially with the times marked. We will come back later for more specific analysis. At least we can see that for a fairly low moon, the line does point to some fairly consistent direction, though we do not yet know what direction that is.

Note that even though the bearing to the line intersection does move a lot in the pictures, the total shift is not much in real degrees. Use the width of the moon as a reference since it is only 0.5° diameter.

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From: Starpath, Seattle, WA
FloridaAdventuring


 - posted December 27, 2005 09:59 PM      Profile for FloridaAdventuring           Edit/Delete Post 
I'd be pleased to learn that the lines drawn across the moon's horns do always point roughly south. In any case, I'd be pleased to learn your conclusions on the topic. Thanks.

I've learned a lot from EN and still have a lot left to learn. I've never learned to be a mariner (not by design, that's just the way things turned out)but I know I'd enjoy the thrill of navigating by the stars on the open sea, where the entire sky is in plain view.

I admire the ingenuity, the skill, the and courage of the Polynesian wayfinders. Imagine sailing a fairly small wooden craft across thousands of miles of ocean with no compass, no chronometer, no sextant, and no written notes (not to mention, no weather reports). Apparently, their only guides were starpaths, winds, swells, sun, marine mammals, and birds. I do hope that art is not lost in this day of GPS.

From: Melrose, Florida
FloridaAdventuring


 - posted January 04, 2006 09:35 PM      Profile for FloridaAdventuring           Edit/Delete Post 
I just went out and looked at the moon (about 2030 EST)and noticed the horns pointing up, with a slight tilt to the south, still creating a line that would hit the horizon at a southerly point. The horns are pointing up because--it seems to me--the moon is fairly near the horizon and is being illuminated by the sun, which is of course way below the horizon.

Earlier in the evening, around 1830, when the moon was higher in the sky, the horns were NOT pointing up, and an imaginary line between them went at a much steeper angle to the horizon, hitting the horizon within 30-45 degrees or so of south, at least indicating a rough southerly direction.

What I think is going on is this: As the moon nears the horizon a different part of it is illuminated, making the horns point increasingly upward (e.g. the moon continues to tilt until its points tilt upward, with a slight tilt to the south). I think that's what's going on.

On a clear sky, the stars plainly indicate the direction, so finding direction by the moon is not necessary. But sometimes on a cloudy night, the moon may be all that is visible from time to time, so it's good to be able to reckon direction from it.

From: Melrose, Florida
David Burch


 - posted January 04, 2006 11:41 PM      Profile for David Burch           Edit/Delete Post 
very good, we are started with some observations. can you plese fill in the details of what you observed.ie, lat lon of position, time (and time zone) date and your preception of the bearing to the point on the horizon as specifically as you can remember. as i read this you have two obsevations... and may get more in the future.

it would be good to get these as well as any others you observe. we can then analyze the results more specifically. i cannot work on the math problem at the moment, but we can collect data.... in FL you likely have more opportunites this time of year than we do here in Seattle!

by the way, you can see the moon rotating as you explained in several of the pictures above.

From: Starpath, Seattle, WA


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