NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Camera Bris Sextant Exercise
From: Peter Monta
Date: 2018 Jun 26, 23:40 -0700
From: Peter Monta
Date: 2018 Jun 26, 23:40 -0700
Hi Greg,
Using the uncompressed crop, the Sun's diameter is 223 pixels, and the angular diameter of the Sun at this time is 31.4 arcmin, giving a plate scale of 0.141 arcmin/pixel. The lower limb is 165 pixels above the horizon, or 23.2 arcmin after scaling.
I would consult the handwritten table on the device, but very unfortunately the value for Sun 8 is not visible :-). So instead, we start with the apparent topocentric altitude of the center of the Sun for this location and time, 33d 7.7' (using the nominal refraction model). The horizon has an apparent altitude of -2.5' (the dip), plus the 23.2' gap from above, plus the solar SD of 15.7', gives 0d 36.4' as the apparent altitude of the center of the lowest Sun. The difference between these two is the desired calibration constant for Sun 8, 32d 31.3'.
Cheers,
Peter
1. What is the gap in minutes of arc between lowest lower limb Sun and horizon ?
Using the uncompressed crop, the Sun's diameter is 223 pixels, and the angular diameter of the Sun at this time is 31.4 arcmin, giving a plate scale of 0.141 arcmin/pixel. The lower limb is 165 pixels above the horizon, or 23.2 arcmin after scaling.
2. What is the calibrated fixed angle reflection Hs for the Sun nearest the horizon ?
I would consult the handwritten table on the device, but very unfortunately the value for Sun 8 is not visible :-). So instead, we start with the apparent topocentric altitude of the center of the Sun for this location and time, 33d 7.7' (using the nominal refraction model). The horizon has an apparent altitude of -2.5' (the dip), plus the 23.2' gap from above, plus the solar SD of 15.7', gives 0d 36.4' as the apparent altitude of the center of the lowest Sun. The difference between these two is the desired calibration constant for Sun 8, 32d 31.3'.
3. What is the name of the oil platform pictured under the Suns ?
The azimuth of the center of the platform is 277.0 degrees by offset from the lowest Sun (though this is somewhat uncertain because swinging the sextant may not get it perfectly plumb), and it can't be more than 5 or 10 km away. Probably Platform Gilda. I think Platform Grace is also visible in the larger image, just barely: about 2.3 degrees (248 pixels) to the left of Gilda and almost below the horizon.
Peter
