NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Swinging the Arc
From: Frank Reed CT
Date: 2006 May 25, 16:22 -0500
"Can you possibly differentiate those 2 methods a little better? From
reading your description, I take it as being pretty much the same procedure (i.e.
treat the view through the sextant as if the celestial body were at the
bottom of a pendulum, and swing the sextant back and forth on the horizontal
axis that points to the horizon until you find the lowest point, which means
the sextant is perpendicular to the horizon). Hope *I've* described that so
it makes sense. ;-)"
They do sound similar. A sextant is an instrument that lets you look in two
directions at once. Those two directions define two axes about which the
instrument can be rotated. Imagine you're measuring the altitude of the Sun and
just by chance a seagull is resting on the water off in the distance directly
below the Sun. Let's suppose the Sun is about 45 degrees high. Suppose I swing
the arc by method 1 (the original method). That means that I rotate the
instrument around an axis that points directly to the Sun. When I look through
the sextant, I will see the Sun always centered in the field of view. That
seagull off close to the horizon will pass across the field of view, and the
horizon will slide away from the Sun's image as I swing left and right. The
observer adjusts the instrument until the horizon just touches the Sun at one
point without overlapping it. Next suppose I swing the arc by method 2 (the
later 20th century method). This means that I rotate the sextant about an axis
that points to that spot on the horizon in front of me where the seagull is
sitting. In this case, the seagull stays centered in the field of view while the
Sun's image sweeps back and forth. The image of the Sun swings along a curved
arc, and the observer's goal is to get that arc to touch the horizon without
overlapping it. The curvature of that little arc decreases rapidly as I go
to higher altitudes which has occasionally led to the mistaken impression that
very high altitudes can't be measured correctly with a sextant.
You can experiment with this indoors. Find a ceiling light with a window
sill beneath it. The light is your Sun and the sill is the horizon. You might
want to estimate the spot directly beneath the light on the window sill and put
some small marker there, like a piece of tape. Stand in a location where the
"Sun" is about 45 degrees above the "horizon" and try swinging the arc by
the two methods. You don't actually have to think about axes of rotation and
all that. Just look through the instrument and swing the instrument from side
to side in such a way that the "Sun" remains exactly centered in the field of
view at all times (method 1). Next rock the instrument from side to side so
that your marker on the window sill remains exactly centered at all times
(method 2). You should find that both methods work and yield the same angle for
altitudes from zero up to maybe 50 or 60 degrees. So far so good. Now try it
all again when you're standing almost directly underneath your stand-in Sun.
That is, stand in a location where the "Sun" is, say, 80 degrees high. Under
these circumstances, method 1 (where the Sun is centered) still works, but
method 2 (where the marker on the window sill is centered) doesn't work at all.
While you're keeping the Sun centered in the field of view with the Sun
almost straight up, you should find that you are rotating your sextant almost
about a vertical axis [note that since the real Sun and other celestial objects
are extremely far away, it's easier to do this than it may appear to be at
first when you're using a ceiling light as a pseudo-Sun]. So method 1, the
original method, always works, and once you've seen it done, it's easy.
If this doesn't do it, I'm gonna have to make a video!
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
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From: Frank Reed CT
Date: 2006 May 25, 16:22 -0500
"Can you possibly differentiate those 2 methods a little better? From
reading your description, I take it as being pretty much the same procedure (i.e.
treat the view through the sextant as if the celestial body were at the
bottom of a pendulum, and swing the sextant back and forth on the horizontal
axis that points to the horizon until you find the lowest point, which means
the sextant is perpendicular to the horizon). Hope *I've* described that so
it makes sense. ;-)"
They do sound similar. A sextant is an instrument that lets you look in two
directions at once. Those two directions define two axes about which the
instrument can be rotated. Imagine you're measuring the altitude of the Sun and
just by chance a seagull is resting on the water off in the distance directly
below the Sun. Let's suppose the Sun is about 45 degrees high. Suppose I swing
the arc by method 1 (the original method). That means that I rotate the
instrument around an axis that points directly to the Sun. When I look through
the sextant, I will see the Sun always centered in the field of view. That
seagull off close to the horizon will pass across the field of view, and the
horizon will slide away from the Sun's image as I swing left and right. The
observer adjusts the instrument until the horizon just touches the Sun at one
point without overlapping it. Next suppose I swing the arc by method 2 (the
later 20th century method). This means that I rotate the sextant about an axis
that points to that spot on the horizon in front of me where the seagull is
sitting. In this case, the seagull stays centered in the field of view while the
Sun's image sweeps back and forth. The image of the Sun swings along a curved
arc, and the observer's goal is to get that arc to touch the horizon without
overlapping it. The curvature of that little arc decreases rapidly as I go
to higher altitudes which has occasionally led to the mistaken impression that
very high altitudes can't be measured correctly with a sextant.
You can experiment with this indoors. Find a ceiling light with a window
sill beneath it. The light is your Sun and the sill is the horizon. You might
want to estimate the spot directly beneath the light on the window sill and put
some small marker there, like a piece of tape. Stand in a location where the
"Sun" is about 45 degrees above the "horizon" and try swinging the arc by
the two methods. You don't actually have to think about axes of rotation and
all that. Just look through the instrument and swing the instrument from side
to side in such a way that the "Sun" remains exactly centered in the field of
view at all times (method 1). Next rock the instrument from side to side so
that your marker on the window sill remains exactly centered at all times
(method 2). You should find that both methods work and yield the same angle for
altitudes from zero up to maybe 50 or 60 degrees. So far so good. Now try it
all again when you're standing almost directly underneath your stand-in Sun.
That is, stand in a location where the "Sun" is, say, 80 degrees high. Under
these circumstances, method 1 (where the Sun is centered) still works, but
method 2 (where the marker on the window sill is centered) doesn't work at all.
While you're keeping the Sun centered in the field of view with the Sun
almost straight up, you should find that you are rotating your sextant almost
about a vertical axis [note that since the real Sun and other celestial objects
are extremely far away, it's easier to do this than it may appear to be at
first when you're using a ceiling light as a pseudo-Sun]. So method 1, the
original method, always works, and once you've seen it done, it's easy.
If this doesn't do it, I'm gonna have to make a video!
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
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To unsubscribe, send email to NavList-unsubscribe@fer3.com
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