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Hi Bill

At Lunch, I took the following 10 readings, first off, then on, then off, etc.  Vernier Type Sextant, 10x Scope;  Upper and Lower limbs of the Sun brought into contact.  Apparent SD for today, 18Feb2010, is 32 minutes 21.8 seconds (source: http://sob.nao-rozhen.org/content/apparent-solar-diameter-and-basic-heliographic-data-2010 )

From these 10 readings we can readily create 9 index error measurements.  That is, we can group 1+2, 3+4, 5+6, 7+8 and 9+10.  Additionally, we can easily see our way to group 2+3, 4+5, 6+7 and 8+9, as any of these pairs would be a natural pairing in that the on/off arc reading follows each other.

Here are the 9 index error measurements

The average of these 9 index error measurements is 0 arc seconds, with a standard deviation of 31.6 arc seconds. 

Extending the group/pair concept, we can permute every On the Arc against every Off the Arc reading.  That is, I can pair 1 with 2,4,6,8 and 10.  Next pair 3 with 2,4,6,8 and 10. Etc, yielding 25 IE measurements.  In this case the average of all permutations is 2 arc seconds, with a standard deviation of 26.9 arc seconds.

The IE equal to zero outcome is a function of properly setting up the sextant in the first place.  The sextant was carefully set to read zero when the index mirror angle is zero.  Of course, I did not get 1/10 minute (6 seconds) every time.  That wasn’t possible as the smallest division on my vernier is 10 seconds.  Nor did I get 10 seconds every time (although clearly sometimes). 

For this reason, I don’t bother to check my IE before and after a round of sights.  Firstly, I find this same result every time I make a serious attempt to measure the IE.  Secondly, I treat my sextant with kid gloves (in a gentle fashion for those not familiar with American slang).  Essentially, as Frank has noted in his high magnification IE trick, the IE is stable for metal sextants.  I have found the same thing and count on it.

No need to warm up the congratulation machine Bill.  However, I would claim a very small IE for my sextant, certainly less than 6 seconds!

Best Regards

Brad

Bill, you wrote:

…if they can get the same result within a tenth of a minute each time, I will be among the first to congratulate their instrument.

Kind regards

Bill Morris
Pukenui
New Zealand

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