NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: A Practical Nav Problem
From: Frank Reed CT
Date: 2006 May 24, 17:39 -0500
David, you wrote:
"Indeed, on July 1 I recorded
the sun's meridian passage at a zenith angle of 00d 00.0'. (I do remember
thinking that was pretty cool! ...)"
Sounds pretty cool to me! Myself, I've never been in tropical latitudes.
And:
"(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.)"
It's interesting to contemplate that if you had noted GMT at the exact
instant the Sun was overhead, your LOP would have shrunk down to a dot --a true
single-observation fix. This sight only works if you have a fairly good idea
where the meridian is.
"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?"
There are two ways of swinging the arc, and they both work well up to about
50 or 60 degrees altitude.
1) The original method which you'll find described in books on sextant use
up until c.1940: you swing the arc by keeping the Sun (or other body) centered
in the field of view of the instrument. The sextant is rotated about the
axis that points to the Sun. The horizon sweeps back and forth across the field
of view.
2) The later method which seems to be easier to describe and so it's taught
more often: you swing the arc by making the Sun sweep across the field of
view while the horizon behind it stays put. In this case the sextant is being
rotated about the axis that points to the horizon (same as the axis of the
telescope).
The problem with method 2 is that there is almost no curvature of the Sun's
path in the field of view when the Sun's altitude is high. So it really just
doesn't work for high altitudes. Many, many modern navigators have mistakenly
concluded that high altitudes are worthless for celestial navigation.
Meanwhile, method 1 always works, but it's a bit harded to describe verbally. This
is the sort of thing where a couple of short video files could clear up a lot
of confusion.
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
From: Frank Reed CT
Date: 2006 May 24, 17:39 -0500
David, you wrote:
"Indeed, on July 1 I recorded
the sun's meridian passage at a zenith angle of 00d 00.0'. (I do remember
thinking that was pretty cool! ...)"
Sounds pretty cool to me! Myself, I've never been in tropical latitudes.
And:
"(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.)"
It's interesting to contemplate that if you had noted GMT at the exact
instant the Sun was overhead, your LOP would have shrunk down to a dot --a true
single-observation fix. This sight only works if you have a fairly good idea
where the meridian is.
"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?"
There are two ways of swinging the arc, and they both work well up to about
50 or 60 degrees altitude.
1) The original method which you'll find described in books on sextant use
up until c.1940: you swing the arc by keeping the Sun (or other body) centered
in the field of view of the instrument. The sextant is rotated about the
axis that points to the Sun. The horizon sweeps back and forth across the field
of view.
2) The later method which seems to be easier to describe and so it's taught
more often: you swing the arc by making the Sun sweep across the field of
view while the horizon behind it stays put. In this case the sextant is being
rotated about the axis that points to the horizon (same as the axis of the
telescope).
The problem with method 2 is that there is almost no curvature of the Sun's
path in the field of view when the Sun's altitude is high. So it really just
doesn't work for high altitudes. Many, many modern navigators have mistakenly
concluded that high altitudes are worthless for celestial navigation.
Meanwhile, method 1 always works, but it's a bit harded to describe verbally. This
is the sort of thing where a couple of short video files could clear up a lot
of confusion.
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---