NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Remember your first time?
From: Fred Hebard
Date: 2005 Oct 31, 11:49 -0500
From: Fred Hebard
Date: 2005 Oct 31, 11:49 -0500
On Oct 31, 2005, at 11:19 AM, Paul Flint wrote: > ...How do I use the different colored filters? The orange one seems > good for cloudy days. The blue for bright days? Do you use them all > at once? > > Is there a slick way to check index error with an artificial horizon? > > Nice story Paul! I'm not sure about all the colors, but I believe you just put filters down until the image is quite dark (you don't want any black spots as after images!). You can check index error by looking directly at the sun, touching the two images just like in the horizon. Touch the lower limb of the movable image (through the index mirror) to the upper limb of the fixed image (through the horizon mirror) and record the reading. Then reverse, touching the upper limb of the movable image to the lower limb of the fixed image, and record the reading. Half the difference between the readings is the index error. Always adjust in the same direction. There's a trick in recording the second reading, when the upper limb of the movable image touches the fixed image's lower limb. If the index error is remotely correct, it will be negative. Record it as a positive number, the number of minutes below zero. This will reverse the readings on the scale. For instance, if the micrometer is between 24 and 25 and is 1/5th torward 25, that would be 35.8 seconds off the arc, not 24.2 seconds. The semi-diameter is one fourth of the sum of the readings and provides a good check on your technique. Semi-diameters of the sun are catalogued daily in the Nautical Almanac. By the way, for artificial horizons, I like to set the two images slightly apart and let them converge, then mark the time. If you're reading through the window, open it.