NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Sun squash- was Green Flash and Longitude
From: Paul Bryans
Date: 2006 Jan 20, 09:46 -0000
From: Paul Bryans
Date: 2006 Jan 20, 09:46 -0000
Questions:
Why did he not use the GPS for its normal
purpose? Perhaps he was becalmed and bored.
Why upper limb not lower?
My digital cameras do not record seconds so the
time will never be accurate.
I think the image of the sun would be burnt out so
the limbs will not be clear if the horizon can also be seen.
How high was the sun - would a decent photo be
possible (wide angle - he says zoom?)
If he got longitude was this near sunset/sunrise so
sun close to the horizon?
What has the focal length, sensor size and number
of pixels across the image got to do with the calculation?
"....line length and divided by 60" - I thought he
was counting pixels?
Sounds like a semi-plausible joke to impress
gullible friends!
Paul Bryans
----- Original Message -----From: frank swiftSent: Friday, January 20, 2006 1:51 AMSubject: Re: Sun squash- was Green Flash and LongitudeFor those of us lucky enough to be on the west coast, the January edition of "Latitude 38" in the column "Max Ebb: photo finish", p.p. 158 - 161 is of interest.
The gist of the article was that on a race to Mexico the Navigator forgot his sextant. He determined his position using photoshop and a digital camera.
He measured the number of pixels that told him the angle. He knew the focal length of the lens, the size of the sensor in the camera and the number of pixels across the image.
Using photoshop he determined the sun to be 303 pixels wide at the zoom setting.
He then checked the Nautical Almanac data: Semi-diameter of the sun is 16.3 minutes at that time of year which came out to 0.176 minutes per pixel.
He then measured the distance from the top edge of the sun to the horizon; checked the line length and divided by 60 and he got the "observed altitude" of the sun's upper limb in degrees.
He then checked the clock time of the camera with the GPS time for the chronometer correction and used the PC-Nav DOS program from Davis and cranked out the program.
He got the longitude position that was within 0.4 miles.
Frank Swift
KG6WZA S/V "Is A Bella"
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