Finding True North
Using the Moon

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For a long time I used the sun to determine true north. The procedure is well documented in many places. It involves determining the exact moment that the sun will be at true south (known astronomically as "local noon" - Note this is almost always not noon on your watch) and then observing the shadow cast by your tower. The shadow at that moment will be cast, of course, in the direction of true north.

For the last 30 years or so, however, I've lived on a heavily wooded lot where distinct shadows are not cast by my towers. I had to modify the procedure described above by standing north of the tower to observe the alignment of the sun and the tower when the sun was true south. I'd go out to the tower a few minutes before local noon and start occasionally observing the position of the sun behind the tower, standing at point where the sun was visually directly behind the tower. I'd continue to adjust my position as local noon approached to account for the movement of the sun, until at the moment of local noon, I knew I was at a point exactly true north of the tower. I would mark the spot and then have a permanent true north reference.

The problem with this approach is that it violates one of the basic teachings of our mothers, namely "DON'T LOOK AT THE SUN". Even taking brief glances of the sun, using my procedure above, is irritating, painful, and potentially dangerous. If you must use this method it must be done with proper protective eyewear or equipment to protect your vision.

Faced with this dilemma, one day it dawned on me (pun intended) to use the moon, rather than the sun to determine true north. Why hadn't I thought of this before? Using readily available resources I could just as easily calculate when the moon would be true south of my tower and use my procedure above without worrying about eye protection, sunburn, etc.

Well, I can report that this method works wonderfully. In addition to saving your vision, the procedure is more accurate because you can watch the position of the moon for extended periods of time and carefully note it's position at the moment it is true south of the tower.


DETERMINING WHEN THE MOON WILL BE TRUE SOUTH

The U.S. Naval Observatory has a nice web site than can generate custom tables showing the exact position of the sun or moon as a function of time at your particular geographical location. Click on http://aa.usno.navy.mil, then click on "Data Services", and then "Altitude and Azimuth of the Sun or Moon During One Day". You should now be at a screen that looks like the one below.

USNO Web Site

Enter data for your location. Be sure to check "Moon" and enter a tabular interval of 1 minute. You can get times in UTC by entering 0 in the Time Zone box, or you can get your local time by entering your local offset from UTC (Be careful not to get confused by Daylight Savings Time). When you press "Compute Table" you'll be presented immediately with a very nice table showing the azimuth, elevation angle, and illumination of the moon for every minute of the day you entered. You're interested in finding when it will be at an azimuth of 180 degrees (true south). On some days the moon is not true south at convenient times, so you may need to find a date when it will be in that position between 8pm and midnight local time.

Here's an excerpt from the resulting table for my location on August 7, 2006 UTC.

Astronomical Applications Dept.
U.S. Naval Observatory
Washington, DC 20392-5420

W3YY (UTC)

W 77  23, N 38 47

Altitude and Azimuth of the Moon
Aug 7, 2006
Universal Time

HH:MM Altitude Azimuth Fraction Illuminated
03:02 22.6 179.3 0.92
03:03 22.6 179.6 0.92
03:04 22.6 179.8 0.92
03:05 22.6 180.0 0.92
03:06 22.6 180.3 0.92
03:07 22.6 180.5 0.92


The table indicates that at my location on August 7, 2006, the moon was at an azimuth of 180 degrees (true south) at 0305 UTC or 11:05pm EDST (August 6th local) at an elevation angle of 22.6 degrees. If 0.92 means that 92% of the moon is illuminated, it looks like this would have been a good night for viewing with the moon almost at full-moon stage.

I hope you give this method a try sometime and find it as useful as I did.

If I'm correct, I believe you will have to reverse this procedure somewhat if you are in the southern hemisphere.




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