NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Pre-setting sextant; Wulf's Grid.
From: Frank Reed CT
Date: 2004 Oct 20, 01:13 EDT
From: Frank Reed CT
Date: 2004 Oct 20, 01:13 EDT
Alex E wrote:
"In certain observations, like lunar distances, or
altitudes with art horizon, it is not easy to catch
both objects in the field of view (and easy to catch
some wrong star instead of the right one!)
Pre-setting the sextant on the approximate angle
helps.
I know the following methods of quickly calculating
this approximate angle.
Lunar distances experts: how do you do it?"
For lunars the historically recommended procedure, and the one that I use today, is nothing more complicated than looking up the distance in the almanac (or equivalent software). A navigator is never completely lost so you know GMT within a few minutes. You open the almanac to the right date, eyeball interpolate for your GMT and then preset your sextant to that angle. Then you look through the sextant and, unless you're dealing with an object well off the ecliptic, you rotate the instrument so that a line through the horns of the Moon is perpendicular to the frame of the instrument. In most cases, the other body will pop right into the field of view, and you'll be within one degree of the correct apparent distance. One could get fancy and make a rough guess for parallax. This would give a more accurate preset distance but it's not usually necessary.
And:
"2. By measuring the needed angle directly on a star globe.
This seems to be the fastest and most convenient method.
But a star globe is bulky and expensive."
I have access to a planetarium with a thirty-foot dome to play with and sometimes, just for fun, I read off angles directly from the dome, but yeah, it's a little inconvenient aboard ship. <g>
Of Wulf's Grid:
"The device consists of two sheets of paper, one permanent
and one replacable. On the permanent sheet a spherical coordinate
system is drawn in Stereographic Projection,
with circles for each even
number of degrees.
The diameter of the picture is 20 cm.
The replacable sheet is made of transparent paper (you can draw
on it), a picture of the circle of the same diameter is made
on it and the center of the circle is marked. The sheets
are connected by a pin passing through the centers of the circles
so that one sheet can be rotated with respect to another,
so the thing looks very much like the Rude starfinder."
This sounds fun. I'm having a little trouble picturing it. Can you post a photo?
Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois
"In certain observations, like lunar distances, or
altitudes with art horizon, it is not easy to catch
both objects in the field of view (and easy to catch
some wrong star instead of the right one!)
Pre-setting the sextant on the approximate angle
helps.
I know the following methods of quickly calculating
this approximate angle.
Lunar distances experts: how do you do it?"
For lunars the historically recommended procedure, and the one that I use today, is nothing more complicated than looking up the distance in the almanac (or equivalent software). A navigator is never completely lost so you know GMT within a few minutes. You open the almanac to the right date, eyeball interpolate for your GMT and then preset your sextant to that angle. Then you look through the sextant and, unless you're dealing with an object well off the ecliptic, you rotate the instrument so that a line through the horns of the Moon is perpendicular to the frame of the instrument. In most cases, the other body will pop right into the field of view, and you'll be within one degree of the correct apparent distance. One could get fancy and make a rough guess for parallax. This would give a more accurate preset distance but it's not usually necessary.
And:
"2. By measuring the needed angle directly on a star globe.
This seems to be the fastest and most convenient method.
But a star globe is bulky and expensive."
I have access to a planetarium with a thirty-foot dome to play with and sometimes, just for fun, I read off angles directly from the dome, but yeah, it's a little inconvenient aboard ship. <g>
Of Wulf's Grid:
"The device consists of two sheets of paper, one permanent
and one replacable. On the permanent sheet a spherical coordinate
system is drawn in Stereographic Projection,
with circles for each even
number of degrees.
The diameter of the picture is 20 cm.
The replacable sheet is made of transparent paper (you can draw
on it), a picture of the circle of the same diameter is made
on it and the center of the circle is marked. The sheets
are connected by a pin passing through the centers of the circles
so that one sheet can be rotated with respect to another,
so the thing looks very much like the Rude starfinder."
This sounds fun. I'm having a little trouble picturing it. Can you post a photo?
Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois
