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Oct 3, 2008

Use a digital camera image, instead of a sextant, to get a sun line of position. Left, the sun seen through sextant shades.  (Photos: Greg Rudzinski)

To the editor: Even if you have no sextant, you can use a timed digital photo of the sun at low altitude to generate a line of position. For this task I use a Canon PowerShot A560 set at three power optical zoom, seven megapixel, and ISO 800. The high ISO setting helps reduce camera shake on a moving platform.

The digital image of the sun (using a shade filter held out in front of the camera covering the sun) is downloaded to a laptop, cropped, rotated, and enlarged so that the sun has at least a 20 millimeter screen diameter on the laptop. This limits altitudes to about 6°. Measurements are made using a ruler directly on the laptop screen. The almanac diameter of the sun is given at approximately 32 minutes of arc. Knowing this will allow the ratio of the laptop screen measurements to yield an altitude of the sun’s lower limb above the horizon in minutes of arc. You need to convert minutes of arc to degrees and minutes and then correct for refraction, dip, semidiameter (and temperature and pressure, if necessary). Perform normal sight reduction for the GMT time of the sun’s photo to get an azimuth and intercept.

Preliminary trials from the beach for sun altitudes between 4° and 6° show the average line of position to be within four nautical miles of position derived from GPS. Precision is limited by rounding to the nearest millimeter on laptop screen measurements.

Example:
Full diameter 31.46’
GPS latitude 34° 09.8’ N, longitude 119° 14.0’ W
Height of eye 8 ft.
GMT 02:44:00, Date 07/03/08

Laptop measurements in millimeters:
Horizon to lower limb 226mm
Sun diameter 27mm (Measured horizontally to avoid refraction differences between upper and lower limb in a vertical measurement.)

(226)(31.46)/(27mm) = 263.3’ =
4° 23.3’ sun hs

Corrected and reduced to an azimuth 294.8° and intercept 0.4’ away (by navigation computer) (not corrected for temperature and pressure). The Nautical Almanac A4 table shows an additional +0.5’ correction for a pressure of 29.9 in. and a temperature of 70° F

iPhoto was used to enlarge and crop sun images.

— Greg Rudzinski is a retired merchant mariner who lives aboard his Offshore 38, Nightcap, in Southern California.

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