NavList:

Herewith a set of six star to star distances, corrected for refraction, used in this case to investigate arc errors in a plastic sextant. The spreadsheets used for the calculations are attached to this post. All measurements were made using the same sextant, from the same place, on different dates, with index errors used on each occasion that were taken immediately after the star to star distance itself. All measurements were made by adjusting the sextant in the same, increasing angle, direction.

The overall results come out as:

Star1                Star2               H         Arc Error

                                                Degs    Mins

Procyon           Pollux              23        0.8 Low

Arcturus          Alkaid              31        1.0 Low

Capella            Pollux              34        0.8 Low

Arcturus          Alioth              40        1.5 Low

Altair               Eltanin             49        1.0 Low

Arcturus          Dubhe             54        1.1 Low

These tests were the final result from an over winter project comparing the arc errors of two Ebbcos against Two Davis Mark IIIs and one Davis Mark 25, all bought opportunistically using eBay. The quoted results are for the Ebbco which I kept, the other sextants were nowhere near as good and were sold back into eBay. The star to star distance technique worked exceedingly well for me, thank you. I still have the selected Ebbco and have been well pleased with practical on board results from it. Please use, abuse, modify or disregard the spreadsheets as you wish. I have no commercial interests in this area, except as a moderately discerning purchaser of second hand sextants.

It's all part of the fun....

Good navigating

Michael Bradley