|
|
From: Bill (no email)
Date: Fri Jul 09 2004 - 17:38:58 EDT
Refraction and sunset
As I make progress inHO229 celestial navigation, I am beginning to get a
"feel" for the numbers rather than plugging and chugging with a cheat sheet
on standby. Thanks for all your help and understanding to date.
The idea struck me that at sunrise and sunset I could get a LOP without a
sextant. One barrel of my binoculars, a heavy-duty neutral-density filter,
a watch, Nautical Almanac and HO229 tables would suffice.
Because of refraction, the sun is already below the horizon when I observe
the upper limb kiss the horizon. Corrections would be -49.6 to -49.8 for an
upper-limb observation at 0d 00.0 Hs. Throw in dip (-2.4 for me standing at
the waterline with a smooth sea--or subtract wave height from height of eye
to arrive at dip when I am on shore and there are waves). Factor in the
always controversial temp/pressure adjustment <G> and I wind up with a
negative Ho in the 0d 50'-60' range. As I should I think.
I set up several sunset scenarios in the Northern Hemisphere using the
middle date on the almanac page and the sunset time given. I used a DR
position of N40 and W90 to keep it simple. As the sunset time seems to
change about 3 minutes between pages, I surmised I should subtract 1 minute
from the stated time for the first date on the page and add 1 minute for the
last in practice. I thought even though the times were rounded, the
worst-case scenario error would be 30 seconds--nominally 7.5 miles off.
After working through the HO229 forms I headed for the Sight Reduction
Tables. This is where things got sticky. For the January problem with a
negative declination, the declination value fell above the line on the
upper-right (LHA 75d) contrary page. OK
For the months where the Sun had a positive declination the usual were the
90-270 LHA range Same table on the lower right page. Here, I crossed over
from the 90-270 LHA range Same table into the 0-90, 270-360 Contrary BEFORE
I hit my declination in most cases.
By inspection, I noticed the Hc seemed to bottom out at the division line
and then start off in the opposite direction. The trend of "d" seemed to
continue but changed signs. Z appeared to be 180- the Z below the line.
When my declination fell in the wrong (upper) table I decided to try
reversing the signs of Hc and d, and subtract Z from 180 then convert to Zn
in the usual manner. In working out adjustments, declination increment
corrections seemed to move in the correct direction. Hc was a negative
number, which given Ho was negative, made sense. I figured since "a', the
length of the intercept, was just a distance, I would leave it positive.
Also decided even though Ho -52.2' looked like a larger number than Hc
-33.8', it was actually smaller on a number line, so Ho -55.2' was less than
Hc -33.8, so I should plot the intercept away as usual.
When plotting my exercises things worked out very well.
Did I just get lucky in the range of problems I defined for myself and
arrive at the right destination by the wrong means, or is the method sound?
If yes to the above question, Will it also work in the Southern Hemisphere?
Thanks
Bill
|