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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.
