h a l f b a k e r yRomantic, but doomed to fail.
add, search, annotate, link, view, overview, recent, by name, random
news, help, about, links, report a problem
browse anonymously,
or get an account
and write.
register,
login
|
|
|
So in my 20's I was hiking with friends and one of them told me something I, to this day, can't wrap my head around and thought; hey, this might be something one of you guys might now.
I was telling him how you can take a skim coating of ice from a cup of water and turn it into a magnifying glass
between your palms to start a fire and he told me that; no matter the state of a crescent moon, drawing a line from the top through the bottom is always south.
I've been trying to disprove it for decades now and can't.
Does it work in reverse south of the equator?
<shrugs>
I don't get it. It shouldn't work... but it works every time.
Check it yourselves.
How to Navigate using the Moon
https://www.natural...our-way-using/moon/
[xaviergisz, Dec 03 2025]
|
| |
Seems clear enough to me, but hard to draw because its a genuinely 3D geometry problem. |
|
| |
The line c1-c2 from the ends of the crescent is easy enough to imagine. The centre of the moon M is halfway along of course. |
|
| |
Perpendicular to c1-c2 is the line M-S from the centre of the moon to the centre of the sun, that seems obvious enough. |
|
| |
There is a triangle U - M - S which connects the moon, the sun, and you. |
|
| |
I think we assume that this triangle is in the orbital plane of the solar system, the same plane as the orbit of the earth round the sun and also the orbit of the moon round the earth (they are not quite coplanar but I think we can ignore that for now). |
|
| |
Therefore because of how we generated that triangle, the line c1-c2 is perpendicular to the orbital plane of the solar system. |
|
| |
On the Earth, North and South are approximately perpendicular to the orbital plane of the solar system |
|
| |
So your line between the tips of the crescent will point approximately South. |
|
| |
I think that is right, and I think it is clear, but I am not sure. |
|
| |
You have to ignore all the local stuff like the fact that you are attached to the curved surface of the earth and can only see half of the sky etc. |
|
| |
Thought this was going to be a complex physical instrument that you preset for the required phase of the moon, then use it to draw the two intersecting arcs as one continuously line. |
|
| |
If you're in the southern hemisphere, drawing a line from "top to bottom" I guess will instead point north. No idea what would happen at the equator, presumably the moon looks like it's going sideways? |
|
| |
[pocmloc] I had to read that through several times to grok it. |
|
| |
Like swinging a basketball, which is swinging a golf ball around itself, around a light bulb and they all spin, more or less, in the same plane, then the shadow cast will always be vertical. |
|
| |
Change any one of those factors though and Moon compass doesn't work. |
|
| |
Cool that at the equator the moon's crescent shows both north and south.
I've never heard of any historical account of using this effect to navigate at sea. |
|
| |
I'm curious, tell me straight.
Has anyone here ever heard of this phenomenon before this posting? |
|
| |
Well if you can see the crescent moon you can probably also see the north star which is more accurate. |
|
| |
imagining a line from the tips of the crescent is going to be quite inaccurate - maybe 20˚ uncertainty because the "tips" are pretty fuzzy and are also close together. |
|
| |
Also the tips of the crescent create a line that is vertical to the ecliptic, i.e. the celestial north pole not the terrestrial north pole. The earth is tilted about 24˚ I think. The difference between terrestrial north-south, and the line joining the tips of the crescent, will vary from -24˚ to +24˚ over the course of the month. It will be zero when the moon is in line with the direction of the earths tilt but of course this changes over the month and year as the earth orbits the sun. It will be zero at new & full moon at midsummer/midwinter, and it will be zero at half moon at the equinoxes. |
|
| |
Obviously at quarter moon it will be the same difference in November and February - whether this is zero difference or maximum difference is left as an exercise for the reader. |
|
| |
If you know the hour, midway between the hour hand and 12 is due south. |
|
| |
You can use the moon to tell time, If you don't know the time, and its the middle of the night. You yell: Hey I don't see the moon. Somebody will probably yell back: Hey! Shut up! Its 2:15 am! |
|
| |
//If you know the hour, midway between the hour hand and 12 is due south.// |
|
| |
I think this might have been garbled, or at least need further explanation, because it can't possibly be true as it stands. |
|
| |
//no matter the state of a crescent moon, drawing a line from the top through the bottom is always south.// |
|
| |
Where are you supposed to stop drawing the line? The horizon? |
|
| |
[Loris], you point the hour hand at the sun. Of course if you have a true 24h dial then you point the hour hand at the sun, and 12 points due south (actually you need to correct for local offset from the time zone meridian and also for daylight savings time as well. But we are in approximate world right?) |
|
| |
As for drawing the line through the crescent points, you draw the line on the inside of the sky, i.e. the celestial sphere, and it forms a great circle, intercepting the local horizon at approximately north and south |
|
| |
//you point the hour hand at the sun. Of course if you have a true 24h dial then you point the hour hand at the sun, and 12 points due south (actually you need to correct for local offset from the time zone meridian and also for daylight savings time as well. But we are in approximate world right?)// |
|
| |
I looked this up for a clearer description: |
|
| |
In the Northern Hemisphere
Hold the watch horizontally and point the hour hand directly toward the sun. A thin stick's shadow can help align the hour hand more accurately with the sun's direction.
Find South: Imagine a line that bisects (cuts in half) the angle between the hour hand and the 12 o'clock mark. This line points South. |
|
| |
What if I'm on the equator? |
|
| |
Can we define the top and bottom of the moon? |
|
| |
"There's a moon in the sky .... It's called the moon!" |
|
| |
//Can we define the top and bottom of the moon?// Easy. The top is the highest point and the bottom is the lowest point. |
|
| |
I'm assuming you mean the point closest to high noon and the point furthest away; then why refer to top and bottom? Trace "down" from literally any point on the moon and you're looking at about the same direction as a compass point. I'm not being deliberately obtuse here, I genuinely don't get it. |
|
| |
As the moon travels across the sky the tilt of the shadow changes and always points due south in northern latitudes. |
|
| |
The difference between this and something like the north star for navigation is that the moon is quite often visible in the day time and I have been in many prairie forests where, unless you know other tricks, this one still works day or night. |
|
| |
OK, this time of year you're going to have to do this in your memory, because the best time to visualize it for as far north as you are is in midsummer. |
|
| |
As you know, at high latitudes and the long days of the year, the sun sets quite a ways to the right of west. And it's skidding along at such an angle that by the time it's been down an hour or so, it's really even further north - and only reluctantly letting the sky get dark, as it's not dropping below the horizon very much. Now the moon in crescent phase is not too far from it, but following at that same angle; if you imagine it being at a point where the moon is due west of you, the sun substantially north of that, then the line across the cusps will be pretty much up & down; yes, the line will hit the horizon south of due west, but it won't be anywhere near "due south". |
|
| |
So the success of your idea depends greatly on how much error you're willing to permit. (Not saying it's a failure - I mean, we define "east" and "west" by sunup & sundown - which are also markedly variable). |
|
| |
(For some more interesting discussion - and folklore - connected to this, look up "Wet moon" on Wikipedia.) |
|
| |