NavList:
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Re: Cugle - longitude at sunrise and sunset
From: Gary LaPook
Date: 2013 May 2, 01:27 -0700
From: Gary LaPook
Date: 2013 May 2, 01:27 -0700
Here is a simple way
to see why Noonan would never have planned to use a sunrise
observation. I think we have all been on a beach watching the sun go
down. Think back to your own observations. Were you able to actually see
the upper limb of the sun disappear behind the sea horizon? Hmmm?
Probably not unless you have looked for it many times. The vast majority
of the time there are clouds between you and the horizon and even more
clouds beyond the horizon and the sun sinks behind the clouds, not
behind the sea horizon. The clouds prevent you from accurately
determining the time the sun is actually aligned with the true horizon
and every four seconds of error in this timing causes a one nautical
mile error in the derived position line. It is an even bigger problem if
trying to do this in flight because the horizon is much farther away so
providing a much greater opportunity for clouds to block the view. Also in flight the horizon is so far away that the air over the ocean is rarely clear enough to see all the way to the horizon. For example, from 10,000 feet the horizon is 130 NM away, how often do you have 130 NM visibility? (Over the desert, where the air is very dry, you might get this kind of visibility but not over the ocean where the air in moist.) In 2009 I sailed across the Atlantic on the Royal Clipper and posted about it before, see http://www.fer3.com/arc/m2.aspx?i=110827&y=200911 We were 10 days at sea from Tenerife to Barbados so there were ten sunrises and ten sunsets and I was up every morning before sunrise to shoot the stars and everybody was on deck to catch the sunsets. So, how many sunsets and sunrises was I able to see clearly and record the time so that I could work out a line of position as recommended by van Asten as the sun set or rose from the sea horizon? Just three times out of a possible 20! Clouds blocked the observations 17 out of 20 times. And that was from sea level, on a ship where the horizon was only 5 NM away, not in an airplane. You want to bet your life on these odds? gl --- On Tue, 4/30/13, Gary LaPook <garylapook@pacbell.net> wrote:
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