It gets worse Greg. The new system to advance your license includes a
large number of classes which included practical examination elements. We
now take two tests, one for 3rd mate, and the second for Chief mate. The
3rd mate celestial navigation requires you to observe regular sun lines, LAN,
and star lines among other things. The "advanced" celestial navigation
practical require you to shoot a upper transit of a body other than the sun and
an Ex-meridian of the sun.
You talk about the tables. I was at my union school (Ft Lauderdale,
FL) in the third week of June and we went to the beach and shot our ex-meridian
of the sun. It was well over 86 degrees Hs. We got back to class and
I flipped open Bowditch only to discover something: If you look closely at
the tables, when the zenith distance is too small (< 4 deg), the tables fall
apart. The teacher knew this would happen and had a USCG exam question for
us to reduce because our shots were unable to be reduced using the ex-meridian
method. That was about the only thing that I learned in that class.
My CM/Masters test had an ex-meridian of a LOWER transit to work out.
That was a fun one.
I agree that the whole process of ex-meridian is pretty pointless when you
have an accurate timepiece. It takes longer to do a ex-meridian than a
standard reduction, when using tables for both. I find it strange that
ex-meridians are one of the things that IMO focuses on for the advanced
navigation practical.
One last thing, I never use the second table. I find the altitude
factor "a" from the first table and then used the formula C=(a*t^2)/60. a
is from the table, t is meridian angle in minutes of time. C is your
correction to Ha.
Jeremy
In a message dated 12/26/2010 2:05:29 A.M. Central Asia Standard Tim,
gregrudzinski@yahoo.com writes:
The strangest Nav exam question that I can remember involved the
ex-meridian. This required table 29 and 30 lookups in Bowditch to correct an
observed altitude near meridional passage to get the actual meridian altitude
(Bowditch calls this reduction to meridian). Never used this method outside
the exam room and felt that it was a curious antiquated celestial technique.
Table 29 is entered with the nearest whole latitude and declination to get the
one minute altitude rate of change(a). The one minute rate of change(a) and
meridian angle(t) or minutes to transit are used to enter table 30 for the
correction in minutes of arc to be added to the observation(Ho) to get an
upper transit altitude. Table 30 covers observations made within 28 minutes of
time either side of meridional passage.
Greg
Rudzinski
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