NavList:

Of course you must determine the index error first.
The way to do this is to set the index to about zero and then look look directly at the sun through the sextant. Adjust the index arm until the lower limb of the sun's image in the index mirror just touches the top of he sun as seen through the horizon glass (and filter!) Take the reading. If the index error is zero you should read 32 minutes, the diameter of the sun. Next reverse the images and touch the the top of the sun in the index mirror to the bottom of the sun as seen through the horizon and take the reading. With no index error it should be a minus 32 minutes but, since there is no negative scale on the sextant, the reading should be 28 minutes on the micrometer or vernier and the degree index will be below the zero mark. It is unlikely that you will start with a perfect sextant with no index error. So to compute the actual index error just add the two micrometer (or vernier) readings together, subtract 60 and then divide by two. Using a perfect sextant the readings would be 32 and 28 (see above) which added together makes 60. Subtract 60 equals zero which divided by two equals a zero index error. Looking at a more normal case as an example, the first reading is 37 and the second is 33. Add them together you get 70, minus 60 equals 10 which divided by 2 gives you the index error of 5, meaning that all readings with the sextant will be 5 minutes too high so you would subtract 5' from all sextant readings. A second example, the first reading is 20 and the second is 16. Add them together you get 36, subtract 60 gives you minus 24 which divided by two gives the index error of minus 12 meaning all readings are 12 minutes too low so you would add 12 minutes to all readings. (If, in an extreme case in which the second reading is not "off the arc", with the degree index below zero, then do the same computation but omit the "subtract 60" step.)

gl

Andrew Corl wrote:

All,

I want to thank everyone for the advice and support.  I took my little artificial horizon our this evening and got everything lined up.  Took a sun sight but overlaid the images of the two suns (did not know to have the limbs touch).  I did go online to Celnav.de and used Henning's sight reduction program and was within 22 miles of my house lat and long which I measure using a GPS.  I WAS THRILLED!!!! Now that I know how to do this I will probably practicing on a regular basis.

Again thanks to everyone for your help and guidance. 

Andrew

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From: Bill Morris <engineer@clear.net.nz>
To: NavList@fer3.com
Sent: Thu, September 2, 2010 4:24:00 PM
Subject: [NavList] Re: Using an artificial horizon

glapook wrote:

"Lining up the bottom edge of the
sun's image in the index mirror with the top edge of its inverted image
in the liquid gives you a lower limb observation."

This is something other advisors seem to have missed. Always line up the edges(limbs) of the sun's images. Trying to superimpose one on top of the other is more difficult and less accurate, though if you did get it right, you would not then have to apply a correction for semi-diameter.

I began practice 30 years ago using the reflection off the surface of a swimming pool on a calm day. It worked at night too with stars, and made identification of images easier, given the wide "field of view". I graduated to old sump oil in a roasting tin.

Bill Morris
Pukenui
New Zealand
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