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Frank -

Is there a picture of this anywhere on line? Being self-taught (or
more accurately having learned from 20th century books) I had never
heard of this way of swinging the arc.

To take the sun as an example: when you have it at the right altitude,
does the horizon appear to slide around the edge of the sun's disk,
just touchng it; as it were, stroking it?

Thanks.  Hewitt



On 1/5/11, Frank Reed  wrote:
> Greg Rudzinski, regarding your Jupiter sights, you wrote:
> "I did use Byron's vertical sextant technique of rocking the body from
> horizon to horizon through the dark back ground of the sea then splitting
> the difference to get the mid point."
>
> Note that what you are describing here is essentially the original (and
> preferred) method of "swinging the arc".
>
> Sometime in the middle of the 20th century, probably involving the big
> changes of navigation education during the Second World War, navigators
> started swinging the arc by rocking the instrument about the axis of the
> telescope that points to the horizon. This makes the Sun or star shoot from
> side to side across the field of view and, while it does make an arc, it's
> not much of an arc and especially for high altitude objects it can be hard
> to say whether that arc is really hitting the horizon at its low point. This
> is definitely the most widely taught method of rocking the sextant, and it's
> not literally wrong, of course. It works very well for low altitudes.
>
> The original method for swinging the arc is described with some cumbersome
> words in most older books on navigation going at least as far back as
> Maskelyne in the 18th century. It involves rocking the sextant by rotating
> it about an axis pointing to the body in the sky, and necessarily that means
> that the observer has to rotate from side to side while the instrument is
> being rocked. By doing so, the Sun or star remains centered in the field of
> view while the horizon shifts up and down beneath it (that's what you see
> looking through the instrument). If you're having trouble picturing this,
> try holding a sextant horizontally, face up, with it set to the approximate
> altitude of the Sun. Point the direct "horizon" view through the instrument
> to a point in the sky left of the Sun at about the angle from the Sun that
> you have pre-set. Scan about until you see the Sun (reflected image,
> properly shaded! don't point the horizon side at the Sun). Get the Sun
> reasonably close to the field of view. The horizon side should show nothing
> but sky. Now slowly rotate the instrument down to a vertical orientation
> keeping the Sun centered in the field of view. Then continue rotating until
> the sextant is horizontal again but face down on the other side. This is an
> exaggerated 180-degree sweep through the motion of the original method of
> swinging the arc. In practice, you would normally only sweep through a small
> fraction, say 30 degrees, of this motion. When you use this method of
> swinging the arc, if you start with an altitude that is a bit too large, the
> Sun will cut the horizon on one side descending, then pass a low point in
> the middle where its image is partly superimposed over the sea (and that low
> point naturally is vertical position), and then the Sun will cut the horizon
> again rising on the other side. The vertical orientation is, of course, in
> the direction halfway between the two azimuths where the Sun crossed the
> horizon during the rocking motion.
>
> The mid/late-20th century method of rocking the arc is the one described in
> nearly all modern textbooks, and if you see animations, they almost always
> show the horizon remaining fixed with the Sun swinging back and forth across
> the field of view (there's an animation just like this on the Wikipedia
> "Sextant" article). There are some exceptions. For example, in John
> Letcher's "Self-contained Celestial Navigation with H.O. 208" there's a very
> fine diagram illustrating the original (correct) method where the Sun stays
> centered in the field of view for the whole operation (Letcher's book is
> filled with interesting little gems and I always recommend it. It's still
> available from used book dealers. John Letcher is yet another former
> physicist who found great pleasure in celestial navigation. Yes, we are a
> pox on the subject, aren't we?).
>
> A number of people, including, so it would seem, Byron Franklin, have
> re-discovered the original method of swinging the arc at some point in the
> past few decades and done their best to recommend its virtues. I'm in the
> same boat. Unfortunately, navigators tend to be "mildly confident" in the
> veracity of those things which they learned at their first navigation
> instructor's knee, and there's tremendous resistance against doing things
> the "right way". How, after all, could it be wrong if so many navigators
> have used it successfully for so many decades?? And that's the thing, it's
> not "wrong" --it's just less effective and definitely worse, to the point of
> being useless, for higher altitudes. BOTH methods of swinging the arc work
> very well for low altitudes, and both work well enough for middle altitudes
> (though the original method has a real advantage here), but ONLY the
> original method works at all altitudes, including those times when the Sun
> or star are very high in the sky. The original method is universal --there
> are no cases where it fails-- and it's no more difficult to learn than the
> more popular late-20th century method of swinging the arc.
>
> -FER
>
>
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