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    Re: Testing sextant arc error
    From: Greg Rudzinski
    Date: 2017 Jan 18, 04:59 -0800


    On a DIY sextant or plastic sextant there will be arc errors measured in full minutes through the arc. Production metal sextants will have just tenths of a minute arc error through the arc. The fastest way to generate an arc error table is by using a metal sextant as the standard as Frank has described. I was able to check arc error on Ebbco's and Davis plastic sextants through 120° of arc in less than an hour. Then to verify the table use artificial horizon, lunar, horizontal angles, and star to star observations. I now own four plastic sextants and have arc error tables for each. 0 to 25° seems to have small errors (0 to 2') with max errors ( 5' to 8') above 45°. There must be a reason for this ? Once the table is verified then tape it to the sextant for future use. A big improvement in accuracy should follow.

    Greg Rudzinski

    From: David Pike
    Date: 2017 Jan 17, 06:07 -0800

    I’ve been thinking how one might calibrate the arc of a DIY sextant while living inland without specialised equipment.  Is there anywhere on the www a list of the angular difference on the celestial sphere between the 57 navigational stars, or would you have to start off with up-to-date SHA and declination and do the spherical trig? Not my favourite cup of tea.  I am aware that you would have to choose your stars to minimise differences in refraction.

    Another possible solution would be to compare horizontal angles of ground features with those measured with a real sextant of known good quality, but the features would need to be quite a way away for repeatability.  DaveP  


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