How to find true north

The following discussion about how to find the true north took place in the Moon-Net reflector. I found the subject so interesting that I decided to put together all the messages in this page.

On 9-Feb-1999 JR1EDE wrote:
Hello folks,

As the subject says, my friend needs how to find true north in daytime for his rover system.
He knows the method of using a telescope etc but he needs more accurate and faster way.

This is necessary to aim the target within 1 to 2 degree for his 24GHz or above contact.
He says he uses a GPS but it fails in the range of hundred meters and still further needs the true north for aiming.

This is not a matter for eme but think it's good place to ask.
Hope your good understand, thanks.

On 10-Feb-1999 K9EK wrote:
One way to find true North, outside during the day: Put a fairly tall, thin pole in the ground, vertically, on a flat area. In the morning, mark the end of the stick's shadow on the ground. In the late afternoon, mark the end of the shadow again. A line through those two points is East-West, so, a line at right angles to the line is North-South.

On 10-Feb-1999 WB4APR wrote:
Seems like you would have to do this at preciesly the same time before and after local-apparent-noon for this to work?

On 10-Feb-1999 ON7RB wrote:
Hi, here's my frank's (or euro's) worth on this matter:

No it is not true north, even if you keep precisely the same time before and after noon (the true noon = culmination of the sun)

There is a difference between the length of a sun-day and a star-day 23h56m4.091s. As the sun is moving along the ecliptica, the sun-day is not always 24h, in fact 24h is an average over a whole year. If you calculate the mid-day time for your longitude, you will find the culmination of the sun to be somewhere between 15 minutes fast (early November) or slow (early April); it will be exact 4 days only (April 16, June 15, September 2 and December 26).

I have not figured out in how far the simple pole-shadow method would be influenced by this time equalization phenomenon.

My idea: point a telescope to Polaris and you've got true north for the next century or so.

On 10-Feb-1999 W8MQW wrote:
  I seem to remember working this out once --- the shadow scribes a perfect E-W line. It is a question of geometry, not of time.  I'm rechecking it now

On 11-Feb-1999 K9EK wrote:
Doesn't have to be exactly either side of noon. If you plotted a LOT of points all day long, they all would lie on the same line; east-west.

On 11-Feb-1999 PE1OGF wrote:
Why not doing it much easier.. If you have Instant-track you can find out at what (local) time exactly the sun is at 180deg for true south. In this case you don't have to calculate.

On 11-Feb-1999 K7RR wrote:
John, that is EXACTLY how I determine local apparent noon!  With some effort using Instantrack one can narrow down the time window to less that a minute.   It takes some patience, but the results are confirmed using IT and normal navigational methods.


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