# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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From: Greg Rudzinski
Date: 2008 Jul 3, 16:31 -0700

```     A timed digital photo of the sun at low altitude provides enough
information to generate a line of position. For this task I use a
Canon PowerShot A560 set at 3 power optical zoom, 7 mega pixel, 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 degrees. Measurements are made using a ruler directly on the
laptop screen. The almanac diameter of the sun is a 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. Convert minutes of arc to
degrees and minutes. Correct for refraction, dip, semidiameter
(temperature and pressure if necessary). Perform normal sight
reduction for the GMT of the sun's photo to get an azimuth and
intercept.
Preliminary trials from the beach for altitudes between 4 degrees
and 6 degrees show the average LOP within 4 nautical miles of 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 02H 44M 00S Date 07/03/08

Laptop measurements in millimeters:

Horizon to lower limb 226mm
Sun diameter 27mm (measured horizontal to horizon to avoid refraction
differences between upper and lower limb)

(226)(31.46) = 263.3' = 4 degrees 23.3' (hs) uncorrected
27

Corrected and reduced to an azimuth 294.8 and intercept 0.4' away (by
(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.

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