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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: angles betwqeen lines of position
From: Bill B
Date: 2012 Jul 18, 01:09 -0400
From: Bill B
Date: 2012 Jul 18, 01:09 -0400
On 7/17/2012 11:53 PM, Alan S wrote: > Possibly I should have mentioned it, I was shooting using a Davis > Artificial Horizon. Does that effect the thing? Alan No dip for the AH. Assuming you corrected for IE and refraction and remembered the altitude is 2x what you would measure from the horizon, (IE and random error are cut in half) you are good to go. Assuming we were looking at the same sun on the same day, or close to it, declination will be similar, so time of observations from LAN (or any other time) and latitude tell the tale. On Tuesday of this week at lon 86d 14!9 W and lat 41d 45!5 N LAN was 13:51:17 EDT. From 1 hour before LAN to LAN the azimuth changed from 144d to 180d. A change of 36d per hour. Nominally the same on the other side of LAN. Sunrise was 6:51:21. From 20 minutes after sunrise to 1hr 20 minutes after sunrise the azimuth changed from 64d to 73d. A change of only 9d. If I change the date to near the winter solstice, the above figures change quite a bit. A 2D thought experiment. Imagine a horizontal line passing through the hub of a bicycle wheel. We are not measuring along the circumference of the wheel/tire, we are measuring a along a vertical from the outside of the tire to the imaginary horizontal line. The best 3D mental image I give you--imagine your celestial horizon. It meets your horizon due east and ends due west. Its peak is 90d minus your latitude. At 0 declination the sun follows that arc. At +23d the the sun describes an arc 23 larger than the 0d arc/circle. In winter at -23 declination the sun forms a circle smaller your celestial horizon's circle/arc. Three concentric circles. End result--sun is higher in the summer, sunrise and set are north of east or west. Sun is lower in the winter, sunrise and set south of east or west. To the crux of the biscuit (as Frank Zappa might say): We are not directly measuring the movement of the sun along its arc/circle. We are measuring as distance between the sun and the horizon along is meridian, which appears as a straight line from directly overhead through the sun/body to the visible horizon when facing the body. In the time it took me to be verbose, Brad posted a brief and concise answer. Hope one or the other syncs up with you perception. Bill B