NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Mer. Pas.,Lat at Lan etc.
From: Dick Savage
Date: 2003 Dec 30, 08:59 -0800
From: Dick Savage
Date: 2003 Dec 30, 08:59 -0800
This is Doug Royer.I'm at a friend's house in San
Pedro for the next few days and am useing his terminal.This post is in responce
to Jim Thompson's request for differant practical Mer Pass procedures.The
following are actual sights taken on a transit in the last few
days.
Jim,I wouldn't be overly concerned with hitting the
time of Mer. Pass exactly as the Sun hangs in the sky for around 20 -30 secounds
during this time frame.For practical navigation purposes on a moving vessel
these will serve the sight taker fine and give resonable results.The sight taker
must be on deck and takeing cuts 15 min. before and after M.P. in case
something goes wrong such as clouds blocking the horizon or sun for a few
minutes at M.P.In your studies you may be beyound these simple techniques.I
don't know what you are being taught.If these are not what you're looking for
just disreguard.
The equipment 1st.
1.Time stamps useing WWV 5-10.0 Mhz S.S.B.
radio
2.Hamilton Watch Co. stop/lap watch
3.Tamaya MS-733 sextant .1' off
arc
Get the base GMT time from wwv and use the watch to
time each cut.
The methods :
Depature : 33*20.0'N;118*5.0'W 0823
zt approach : 31*45.6'N;116*48.4'W Date 12-27-03 Temp.
50*F,29.79'HG. speed 17 kt. course 154*T transit time 7h 32'
ETA 1555zt ht. 28 ft.
Calc. EP Lat. = Lf + - e [s/60
cosT*] long. = lon.f +- e [s/60(sin T*/cos Lf)] + e
after L and lon f. - e before lat and lon.f
Calc. the Lan EP pos. useing the above.Ship
heading south - from Lf.Heading north + to Lf. Heading east - from lon.f.Heading
west + to lon.f
e=1200-0823=3.6167
1200zt Ep = 32*27' 3"N 117*32'45"W
The 2 methods for determaning Mer. Pass: from the
N.A.
Time method
M.P. LMT=1201
center long. of zd=120*0.0'W
1200 zt EP lon.=117*32'45"W
delta*=2*27'15"
delta t=9'49" [delta */15]
M.P. LMT=1201
delta t= 9'49" subtract delta t from M.P. LMT
because the vessel is east of center of zd.If west of center add delta t to
LMT
zt of LAN=115111
GHA method
M.P. LMT=1201
1200 EP lon.=117*32'45"W
next lower whole GHA in N.A.=104*44'42" note
the GMT of the GHA
delta GHA=12*48'3
delta t=51'12" [delta GHA/15]
GMT=1900
corr. GMT=195112 GHA will always be east of
vessel useing this method so always add delta t to GMT
zd=+8 rev.
zt of LAN=115112
LAN observation: on this occassion I used a timed
cut, not averaging cuts, due to other duties.This is where the lap function of
the watch comes in.You know the zt of LAN so use the lap function along with the
time stamp to time the sun touching the horizon and read off the
hs.Experiance and practice my friend.
CO ALT method = LAT at LAN=90*-Ho+ -
dec
hs=34*3.4'
Ho=34*13.2'
90*-(34*13.2' - 23*19.2')=32*27'36"N
Intercept method = calc. Hc=sin-1[cos(EP Lat.+
- dec.)] + dec cn - dec sn
Hc=sin-1[cos(32*27'3"N +
23*19.2')]=34*13.75'
Sun is south of vessel so :
1.aA : + a to EP Lat.
2.aT : - a from EP Lat.
Hc=34*13.75'
Ho=34*13.2'
a = 0.45 m A
32*27'3"N + 0.45=32*27'42"N
12-27-03 1151zt Lat :
GPS : 32*27'11"N
Co alt. : 32* 27'36"N
Int. : 32*27'42"N
You can see all results are good enough for
government work.These reduction results are quick to obtain,with
practice,if one has the proper tools to do the calculations.
There is a way to find one's pos.(both Lat. and
Long.)to a resonable degree useing transit cuts and the 6 min. rule.If
interested I will show it to you.
