Is a shadow N-S at noon?

[Wing_Z]

Lionheart mentioned the iPhone compass app in another post.
Well, it does more than your average compass: a further click makes the needle swing over to TRUE North!
With this in mind, I went outside at 12 noon and looked where a stick cast its shadow, expecting it to line up true N-S.
It did not.
Before I start complaining to out local Member of Parliament I thought I'd ask the question of the navigators (astronomers?) here.
I'd have thought the answer is "Yes" but then I also thought the sun rises in the East...

 

[Lionheart]

Sometimes you have to reset the compass. Something like walking in a figure eight path, or moving it in a wierd pattern, what ever the unit reset directions tell you to do. (I forgot).

Also, true magnetic North, if I am correct, has changed for Earth. I could be very wrong on that, but I had thought that we have had a slight change in the magnetic field in the past few years.. (please correct me if Im wrong, and its not a huge amount, only slightly).

On a side note. I put a popup fake iPhone in the Tailwind aircraft package I created, including the compass so you could navigate with it if you had a compass failure in the plane. I think we accidentally did true north instead of mag north, which came out diff then the compass in the plane. arrgh...


Bill

 

[luckydog]

True and magnetic North are a little different depending on your location..........its called declination:

http://www.magnetic-declination.com/

 

[Wing_Z]

I"m pretty sure the compass was pointing True North.
We are at 21 deg declination, and it showed up when you switched modes.
(I also did the figure 8 calibration thing, and it pointed to the same spot)
The noon shadow did not line up.
Since we're on Daylight Saving, I'd expect an hour difference before it did.
BUT, it took until 13:46 local time, for the shadow to fall N-S.
That means our time is wrong!

Unless ... a shadow does not always fall N-S at noon, solar time?
Hence the question.

 

[luckydog]

The Shadow Stick Compass Method

Shadow Stick East


Although your body casts an excellent shadow when the sun is shining, you probably do not want to stand in one position without moving for ten or twenty minutes. An inanimate object such as a straight stick will work just as well.
The shadow stick method of direction finding requires:

• The bright sun – if it is too cloudy you will not be able to observe a shadow.
• A straight stick about .5 meters (1.5 feet) or longer.
• Two pebbles or other means of marking a point on the ground.

On a sunny day plant a straight stick in the ground as vertically as possible. As shown in the first photo you need to select an area where the shadow cast by the stick is easily visible on the ground. A sandy area makes a perfect backdrop but most anywhere free of vegetation or other objects can be used.
Finding East-West


Mark the end of the shadow with a small rock or stick. Now come the waiting part.
After ten or fifteen minutes or longer, revisit your shadow stick. You will find that the sticks shadow has moved from its earlier position. Mark the end of the new shadow with another pebble or stick.
Draw a straight line between the two markers you have placed. This is your East–West line.
As you can see from the magnetic compass I have placed along the line made by the shadow stick method, the line points very closely to the true East-West direction. The first mark you made is on the west end of the line and the second mark on the shadow is on the east end.
To find the North–South directions, draw a line perpendicular to your East–West line. Looking East, North will be on the left side of the line you drew and South will be on the right side.

 

[Wing_Z]

Dog, I can't see your pictures, but I suspect strongly that this may be more of a help in marking out canine territory, than establishing whether the noon shadow is N-S :d

 

[Jagdflieger]

Magnetic Declination

True north and polar north should both point directly to the North Pole. Grid north (on military maps) is usually within a degree of true north. A properly calibrated electronic compas should also indicate true or polar north when set to true north. When set to magnetic north, it will of course indicate the same direction as a magnetic compass.

Perhaps your iPhones are set to magnetic north? You can check pretty fast if you know how many degrees (west or east) off of true north you are at 1200 hours and then compare that to the map below.

If you are in the Northern Hemisphere and you enjoy clear skies at night, you can also use the North Star (Polaris) to confirm your true north setting. The Southern Cross as an indicator of direction in the southern latitudes is less exact.

Magnetic north is ever changing and is a result of the earth's magnetic field. It currently is moving at a fairly fast rate to the north in the vicinity of 82 Deg 30 Min North 114 Deg 30 West (Appx). It has changed almost three full degrees since I started using a magnetic compass for navigation, at least at Fort Lewis. Changes will vary depending on how far you are from the line of zero declination.

By the way, Dog's use of the stick and shodow method to determine true north is a great survival method of finding direction. It is featured in the Army's survival manual, FM 21-76.

Here is a map of the Earth's magnetic declination as of 1995 and also a map of the wondering magnetic pole as of 2000.

 

[TeaSea]

This is interesting...but somewhere in our school system in the states we have neglected to mention to children that True North and Magnetic North are not the same thing, and that Magnetic North is constantly changing. I've asked my own kids this and they were a little surprised to discover that "North" is not always "North".

Incidentally Jagdflieger, my squad actually used the North Star for navigation one night in an overland movement. When the compass guy started having difficulty the grader on our exercise lane called a halt and asked us why we just didn't follow the stars.

We ascertained which was the North Star, and simply tracked slightly to the right of it...and after a 12 KM walk through the desert, found ourselves within 500 Meters of our intended objective. Using terrain association we came out right on top.

Of course the lane grader knew we would, having used that same technique repeatedly on that lane. He just used this as a teaching point.

 

[PRB]

Lionheart mentioned the iPhone compass app in another post.
Well, it does more than your average compass: a further click makes the needle swing over to TRUE North!
With this in mind, I went outside at 12 noon and looked where a stick cast its shadow, expecting it to line up true N-S.
It did not.
Before I start complaining to out local Member of Parliament I thought I'd ask the question of the navigators (astronomers?) here.
I'd have thought the answer is "Yes" but then I also thought the sun rises in the East...

To acually take a stab at your question about the shadow: Since the Earth's rotational axis is inclinded 23 or so degrees to the plane of the solar system, I would expect our shadow, at noon, to be about 23 degrees off from north (true north in this case)...

 

[leroy10]

Hi All,

As a survival tip you can also use an analog watch to navigate through the day. Point the hour hand at the sun. Slice an angle between the hour hand and 12 o'clock and you have North.

Other tips for getting your bearings:
The bark grows thicker on the southern side of trees in the Southern Hemisphere, and on the northern side in the Northern Hemisphere.

In the Australian outback magnetic termite nests point mag N/S.

Cheers :ernae:
Lindsay

 

[Haining]

By definition local noon occurs when the Sun is at it's highest point in the sky on that day. Shadows will be cast North-South depending on which hemisphere you're in. If they aren't, it isn't local noon.

 

[dogknot]

By definition local noon occurs when the Sun is at it's highest point in the sky on that day. Shadows will be cast North-South depending on which hemisphere you're in. If they aren't, it isn't local noon.

You have to consider the time of year, or season, as well. Summer here, at noon, the sun actually is straight overhead, thus the sun dial, or stick, casts no shadow at all.

 

[Wing_Z]

All these "Sun" arguments usually rely on:
1. Your location,and
2. The season

I know one could look this stuff up, but a bit of string and stuff can make it an interactive experience.
So...

So far I see:
1. The sun is only directly overhead at specific times and places:
- on the equator, at the equinox
- at latitudes of 23deg N and S, at the solstice.
2. The locus of the stick's shadow is not an E-W line, except at the equinoxes.
You could be 20deg off if you drew a straight line between 2 shadow points at lat 40deg at the solstice.
Also you should do this in the middle of the day - it is exaggerated at the ends of the day.
Taking a shadow an hour each side of noon would give a better result.
Mmm this may be the clue I've been looking for to answer the original question...

 

[Jagdflieger]

Tea Sea,

I've always taught my teams how to navigate with the stars at night, especially in the desert, artic or mountains where the air is clear and visibility good. Of course sand storms and blizards can effectively put paid to that technique!

Often cloud cover, trees or terrain will obscure the North Star, so you can then use the constellations to help. Orian, Ursis Major, and Caseopea are quite useful. They change positions throughout the night (from east to west), but they will give you an accurate north sensing within a few degrees with experience. I've been able to use Orian when as far south as 15 degrees south latitude. Of course in the southern latitudes, the Southern Cross is useful too.

Leroy,

Those magnetic termite nests sound interesting.

 

[GT182]

Also, true magnetic North, if I am correct, has changed for Earth. I could be very wrong on that, but I had thought that we have had a slight change in the magnetic field in the past few years.. (please correct me if Im wrong, and its not a huge amount, only slightly).


Bill

The earthquake in Chile has moved the magnetic field again.... by 33 feet, according to scientists. And has caused the duration of daylight to decrease... tho only by milliseconds each day.

And I predict, as a result of said earthquake...... Global Warming will reverse it's self, totally p!ssing off Al Gore. :d

 

[PeteHam]

I"m pretty sure the compass was pointing True North.
We are at 21 deg declination, and it showed up when you switched modes.
(I also did the figure 8 calibration thing, and it pointed to the same spot)
The noon shadow did not line up.
Since we're on Daylight Saving, I'd expect an hour difference before it did.
BUT, it took until 13:46 local time, for the shadow to fall N-S.
That means our time is wrong!

Unless ... a shadow does not always fall N-S at noon, solar time?
Hence the question.

This may help with some of the time difference .....

http://www.dia.govt.nz/diawebsite.n...We-Provide-About-Daylight-Saving?OpenDocument

It seems that midday here is not solar midday ......

Just so long as the sun shines when I'm flying

Pete.

 

[Wing_Z]

So I ran the torch battery down eventually, but it helped to visualise what happens to the shadows.
The answer to the question is: Yes! But...

And so to the internet, which turns up 3 things which make it Yes! But...

First: the world is chopped up into 24 1-hour time zones. If your position is near the longitude that defines the time zone, noon will pretty much be noon, and the shadow will fall N-S. But at the far end of the time zone, our clocks stay the same, then suddenly take a 1-hour step into the next zone.
The sun shadow doesn't do that kind of thing...

Second: Obviously, if you are on Daylight Saving, there's an hour time difference, and the noon shadow will not happen at noon clock time.

Third: A bit of a curved ball - the earth's axis is tilted, and its orbit around the sun is elliptical, not circular. So there's a little wandering around of the sun in the sky. There are only four times in the year when the sun behaves properly! At other times it is a little slow, sometimes a little fast, like this:

Perhaps the right answer would be "Sometimes!"
Thanks for coming along for the ride...

 

[Jagdflieger]

Time Zones and Geo Coordinates

The time zones have a lot to do with our geo coordinates as well, at least they are measured from the same math that any globe is measured.

Each time zone is 15 degrees wide at the equator. 15 into 360 degrees (a circle) = 24 and thus 24 time zones, one for each hour of the day.

Turn the degrees into minutes (60 per degree) and each minute into seconds (60 per minute) and you now have the basis for our geographical coordinates.

 

[mfitch]

The causes for noon not being so high are a source of fun for me.

If my classes are in a room with east facing windows, we can track the position of the sunlight on a north wall each day. Once it appears it climbs each day (earlier sunrise) providing a sundial style calendar. Around this time of year (close to equinox) the day to day change is of course greater (marks on this sun calendar are not uniform).

Because time zones are affected by politics, we also experience noon is not solar noon rather badly. Stuffing four time zones into one out of convenience and choosing the east most zone for the time (for business reasons), means that solar noon is over two hours off actual not counting daylight savings time (a real joke here outside of business calls to the east coast).

Of course "high" noon never happens. This one shows up in a calculus project (they learn to handle non-perpendicular projections).

Is Canada or any other northern region any better with the time zones?

 

[Ken Stallings]

The reason why techniques from antiquity don't appear to match our understandings today is because today we are so much more precise in our measurements.

When an accurate navigation across the Atlantic Ocean meant you arrived within 500 miles of your destination and then navigated along the shoreline to reach your destination, using a sun compass was a very accurate means of navigation!

Back in the pre-GPS days, I used celestial navigation combined with pressure line of position techniques to navigate C-130's across the ocean. My standards were to be within 10 nautical miles of course and 5 minutes of time from my most recently passed ETA in the position report to the ADIZ entry point.

Today, if you achieved an accuracy better than half that with GPS pilots would be screaming bloody murder that their equipment was screwed up!

If you were in the Northern Hemisphere and created a sun compass, you would start with a white disc with a pin in the center. In the morning you would hold the compass vertically, with a consisten orientation of the disc, and measure the tip of the shadow formed on the disc.

Then, at various increments of time, you would keep this process going until you constructed the points as forming an arc over the disc from one side to the other. At the point where the arc reached its apex, this would be true north. If you were in the southern hemisphere, the point where the arc reached its apex would be true south.

Indeed, the point you first measured the shadow in the early morning would be approximately true west and in late afternoon would be true east. At least for mariners several hundred years or more ago, it was accurate enough to enable them to get within reasonable distance to their desired destination and back home again.

Using modern measurement standards, we can determine that it isn't all that accurate. But, I know of people who decided to sail boats hundreds of miles using just a sun compass and it worked just like it worked in antiquity.

And for those wondering, when I used celestial and pressure line I normally arrived at the ADIZ point (and associated radar coverage) within five miles and two minutes of time. That was pretty good back then!

Cheers,

Ken