NavList:

"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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