NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: A Practical Nav Problem
From: David F. McCune
Date: 2006 May 24, 10:47 -0500
Frank:
> Historically, navigators did not mind getting latitude when the
> Sun was very
> close to the zenith. In the logbooks they even seem to get a
> kick out of it.
> I think the modern aversion to high altitude sights is due to a specific
> method of "swinging the arc" which became popular around 1940
> +/-10 years.
> There's really no reason why a navigator should not be able to
> get good altitudes
> even when the object observed is close to 90 degrees altitude.
Your comment got me to dig out my old log book from a trip to from
California to Hawaii in June/July. In the period around June 28 to July 3 I
was at 24d N (heading WSW) and the sun's declination was just over 23d N.
So I had a zenith angle of less than a degree. Indeed, on July 1 I recorded
the sun's meridian passage at a zenith angle of 00d 00.0'. (I do remeber
thinking that was pretty cool! Though there's not a single note to that
coolness in my log. I think I was tired and lonely by that time and just
wanted to get on with the sailing, so the log just show's the numbers.) I
did several sights each day, using the sun, moon, planets and stars.
Looking back on the plotting sheets for those days, I seem to have had no
trouble with my latitude. On the other hand, I have a memory of being
troubled. But I can't really tell you why. So it seems you are right.
Somehow anxiety crept into my navigation when the zenith angle was less than
a couple of degrees, but that anxiety does not seem to have been supported
by the facts. And anyway, on July 4 I found the Alenuihaha Passage right
where it was supposed to be.
(On reflection, I think some of my anxiety came from being a bit unsure how
to plot those sun sights just around noon when the sun's GP was so close to
my own. After all, the LOP curves noticeably when the GP is just a degree
or so away. I vaguely remeber thinking I could draw the GP and then draw an
arc. But looking at the plotting sheets, I just drew straight LOPs,
including straight E/W LOPs for the meridian passage sights, and went about
my business.)
So I suppose you are right. My concern about the sun's declination being a
few degrees from my latitude is unjustified.
What is the "specific method" of swinging the arc that came into practice
around 1940? Prior to that, was there a different or better technique for
assuring perpendicularity?
David
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From: David F. McCune
Date: 2006 May 24, 10:47 -0500
Frank:
> Historically, navigators did not mind getting latitude when the
> Sun was very
> close to the zenith. In the logbooks they even seem to get a
> kick out of it.
> I think the modern aversion to high altitude sights is due to a specific
> method of "swinging the arc" which became popular around 1940
> +/-10 years.
> There's really no reason why a navigator should not be able to
> get good altitudes
> even when the object observed is close to 90 degrees altitude.
Your comment got me to dig out my old log book from a trip to from
California to Hawaii in June/July. In the period around June 28 to July 3 I
was at 24d N (heading WSW) and the sun's declination was just over 23d N.
So I had a zenith angle of less than a degree. Indeed, on July 1 I recorded
the sun's meridian passage at a zenith angle of 00d 00.0'. (I do remeber
thinking that was pretty cool! Though there's not a single note to that
coolness in my log. I think I was tired and lonely by that time and just
wanted to get on with the sailing, so the log just show's the numbers.) I
did several sights each day, using the sun, moon, planets and stars.
Looking back on the plotting sheets for those days, I seem to have had no
trouble with my latitude. On the other hand, I have a memory of being
troubled. But I can't really tell you why. So it seems you are right.
Somehow anxiety crept into my navigation when the zenith angle was less than
a couple of degrees, but that anxiety does not seem to have been supported
by the facts. And anyway, on July 4 I found the Alenuihaha Passage right
where it was supposed to be.
(On reflection, I think some of my anxiety came from being a bit unsure how
to plot those sun sights just around noon when the sun's GP was so close to
my own. After all, the LOP curves noticeably when the GP is just a degree
or so away. I vaguely remeber thinking I could draw the GP and then draw an
arc. But looking at the plotting sheets, I just drew straight LOPs,
including straight E/W LOPs for the meridian passage sights, and went about
my business.)
So I suppose you are right. My concern about the sun's declination being a
few degrees from my latitude is unjustified.
What is the "specific method" of swinging the arc that came into practice
around 1940? Prior to that, was there a different or better technique for
assuring perpendicularity?
David
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To post to this group, send email to NavList@fer3.com
To unsubscribe, send email to NavList-unsubscribe@fer3.com
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