A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2018 Feb 25, 15:58 -0800
There's a simple, time-honored method for laying out the cardinal directions on a flat plane. You observe the azimuth of the Sun at sunrise and then again at sunset. It doesn't have to be exactly at rise/set -- you can observe the Sun at two identical altitudes close to the horizon, one shortly after sunrise, the other shortly before sunset. You then "split the difference" geometrically between these two directions to get north/south, and east/west is perpendicular. It's very good but not perfect because the Sun's declination changes during the day, and the resulting shift in azimuth is also affected by your latitude. So, for example, if the Sun rises exactly due east at azimuth 90°00' on the equator on the Spring equinox, it will set a little to the north of west. The Sun's Dec changes at a rate of just about one knot near the equinoxes so after a 12-hour day, it will set about 12' north of west at about 270°12'. The line splitting the difference between those two directions will be almost north/south but rotated by 6' towards east. If you do this at the fall equinox, the rotation would be 6' towards the west. And at the solstices there should be no rotation. How does this vary with latitude? I can picture it, but I shouldn't spend time working it out. Somebody please tell me!