# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Re: watch as compass**

**From:**Frank Reed CT

**Date:**2007 Jul 30, 17:40 -0700

I wrote earlier: "Anyone have a quicker method? Something you can do in fifteen seconds instead of fifteen minutes? I've been working on a few..." So here's one. As I've mentioned the principal problem with the 'watch as compass' trick is that it confuses azimuth with hour angle. We can partially fix that by tilting the face of the watch, but I think it would be better to go a slightly different route and build a "pocket hour angle model" since many watches are no longer analog. The concept of using local time to get compass direction is fine. It's basically a sundial in reverse. A sundial when aligned properly for compass direction and latitude yields local apparent time. Therefore if we have local apparent time already and align ourselves properly for latitude, we can get compass direction. I assume I have a timepiece of some sort and it is set to local zone time. The first step is to convert to local apparent time --time by the Sun. You can get as accurate as you want here, but the key step is to subtract an hour for Daylight Saving Time if it's in effect (and more days than not, it is in effect, by current laws). Equally important, if you are at the western end of a time zone, you may need to subtract another hour or a good fraction thereof. For example, if you're wandering around in the U.P. of Michigan, most of it is on Eastern Time even though it's fairly close to 90 degrees West longitude, the middle longitude of the Central Time Zone (for those not familiar with it, the "U.P." of Michigan is the "Upper Peninsula", a detached, mostly rural section of the state of Michigan north of Wisconsin). With your estimate of local apparent time, you calculate the Sun's local hour angle by counting the number of hours from Noon and multiplying by 15. As an example, if it's 7:20pm CDT in Chicago, the local apparent time is close to 6:20pm so the Sun's local hour angle is close to 95 degrees. It's important to remember at this point that the Sun is always east of the meridian in AM hours, always west in PM hours, everywhere on Earth. Now it's time to build a model... To model the geometry of the Sun's local hour angle, we need a piece of stiff cardstock or maybe a strip of bendable metal. A matchbook cover will work nicely. Let's imagine using an ordinary index card. Fold the card cleanly in half. You now have two planes and an "axis" (the axis is the fold in the card). Bend the card along the fold until the angle between the two sides is roughly equal to the Sun's hour angle (15 degrees times the number of hours since noon). Hold the bent card in front of you and tilt it so that the axis (the fold) makes an angle with the horizontal equal to your latitude. Also turn the card so that one side is vertical. Now turn slowly around without changing the tilt of the card until the Sun is exactly aligned in the plane of the other side of the index card. This is an easy condition to meet by looking at the shadow of the tilted side cast on the vertical side. When the shadow just disappears, the Sun is lined up in that plane. And you're done: the vertical side of the card is now aligned north-south (you're facing south in the northern hemisphere, north in the southern). This method works because the planes of the card are aligned with the abstract geometric planes that we require. The vertical side of the card corresponds to the plane of the observer's meridian. The tilted side of the card corresponds to the plane containing the observer, the elevated pole, and the Sun. Note that you can re-construct the rules easily if you understand the basic geometry of local hour angle and the altitude of the elevated pole. This method for determining north with known watch time will work anywhere on Earth, and it will give compass direction within ten degrees if you're careful aboud setting the angles. It can be done with a card as small as a matchbook cover, or if required, by making a larger model of the geometry and by being more careful with the estimated angles and the number of hours since Noon, the accuracy can be increased. After a few trials, it takes only a few seconds to set up the angles and do the observation repeatedly during the day. It's as good as a magnetic compass and naturally requires no correction for magnetic variation/declination, but of course it does depend on having a functioning timepiece. -FER --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To unsubscribe, send email to NavList-unsubscribe@fer3.com -~----------~----~----~----~------~----~------~--~---