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## Re: [TowerTalk] And now for something completely different (TrueNorth)

 To: "Rajiv Dewan, N2RD" , towertalk@contesting.com Re: [TowerTalk] And now for something completely different (TrueNorth) Jim Lux Mon, 28 Aug 2006 09:46:39 -0700
 ```At 07:15 AM 8/28/2006, Rajiv Dewan, N2RD wrote: >The stick at high noon is a very good method and a lot more useful >than stars at night. You do not have to live exactly at the time >zone meridian. You just have to find the sunrise and sunset for >*your* location and date. So here are the steps: > >1. Find the sunrise and sunset time for the location and day you are >going to do the experiment. You can find that in your local >newspaper or visit >http://aa.usno.navy.mil/data/docs/RS_OneYear.html and type in the >state and city. (For even greater accuracy and greater generality, >you can enter the exact latitude and longitude instead of using the >city/state combination) The web site has the times in ST without the >daylight correction. >For example it lists the sunrise and sunset times are 0530 and 1852 >EST for Rochester, NY on August 28, 2006. With the daylight >correction the times are 0630 and 1952 EDT. > >2. Compute the mid point which is high noon for the day and location >specified. The duration of the day is 19:52 - 06:30 = 13 h 22 min. >Half of that is 6h 41 min. Adding that to 6h 30 min (sun rise time), >I get 1:11PM EDT. Meridian transit (when the shadow points north) is NOT always halfway between sunrise and sunset (although it's certainly good enough for 1 degree kinds of accuracy) a) most (good) calculations of sunrise and sunset make corrections for atmospheric refraction, which may be different in morning and afternoon (the temperature profile is different) b) The sunrise time is calculated for a position of the earth that is 12 hours or so earlier than the sunset time. For example, if you're in the spring, when sunrises are getting earlier and sunsets are getting later, by a couple minutes per day, the time from meridian transit to sunset will be a minute (or so) longer than the time from sunrise to meridian transit. You're basically interpolating between two points on a sinewave. Near the solstices, this is less of an issue, because the slope is low, likewise on the equinox, because the sine is close to linear As long as you're going to the usno website, why not just use the transit time, which is given as well as sunrise and sunset times. By the way, the same site gives az and el for the sun at 1 minute ticks, which provides a nice illustration of why the "wait til the shadow is shortest" approach is difficult: For today for my location, the elevation of the sun is above 65 degrees from 11:38 until 12:15, reaching a peak of 65.3 degrees from 11:47 until 12:06. I doubt that anyone could accurately discern the difference of 0.3 degrees in a shadow length, especially considering that the shadow itself has a 0.5 degree wide border. (the shadow difference is a few mm if you have a 1 meter high stick) >3. The shadow of a vertical stick (use a plumb line for greater >accuracy) at 1:11PM on August 28, 2006 at Rochester, NY points due >North. The same site gives the following interesting information (for today, Rochester): Time El Az 12:09 56.4 178.8 12:10 56.4 179.3 12:11 56.4 179.7 12:12 56.4 180.2 12:13 56.4 180.6 So you can see that if your clock is off by a couple minutes, you're still within a degree. --- Just to get a feel for the impact of screwing up your position: Here's 119W, 34N Sunrise 05:27 Sun transit 11:57 Sunset 18:27 and here's 119, 10'W (roughly 8-10 miles west..) Sunrise 05:27 Sun transit 11:58 Sunset 18:28 So it looks like a 10 mile position error at mid latitudes in late summer is worth about a 1 minute error in timing (probably less.. I think there's a roundoff/truncation thing too), which, in turn, is about 1/2 degree azimuth error. Jim, W6RMK _______________________________________________ _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
 Current Thread Re: [TowerTalk] And now for something completely different(TrueNorth), (continued) Re: [TowerTalk] And now for something completely different (TrueNorth), Rajiv Dewan, N2RD Re: [TowerTalk] And now for something completely different (TrueNorth), Jim Lux <= Re: [TowerTalk] And now for something completely different(TrueNorth), Mark Re: [TowerTalk] And now for something completely different (TrueNorth), Jim Lux Re: [TowerTalk] And now for something completely different (TrueNorth), Tom Osborne