NavList:

   

Reply

I'm delighted to see that Jeremy (Anabasis) is setting a series of
navigational problems, and would like to add my own words of encouragement
to those of others.

It will add a certain extra spice that they are coming from a professional
in the big-ship world. I hope that some of them can be based on real
experience, though presumably others will be invented scenarios.

His first problem raises, from me, one question, and in the traditions of
this list, a minor pedantic quibble.

Here's my question. I see that the navigator is working on zone time. Is
that standard practice, for navigators in the merchant marine? In my
experience, small-craft navigators, particularly if they are using astronav,
usually keep a timekeeper for the purpose which is always set to GMT, winter
or summer, and always work on that. Of course, you need zone time as well,
from another clock, to tell you when the pubs will be open and to work the
tide tables.

And here's my quibble. Jeremy has calculated the time of Local Apparent Noon
to be at 12h 13m 42s. And yet, he says-
"I shot the sun at 12hr 13m 40s as I observed start to descend from its
highest altitude."
But it's unlikely he could have observed it starting to descend, at that
moment, for three reasons-
1. It wasn't even quite due to get to Local Apparent Noon then, if only 2
seconds short of it.
2. Around noon, the altitude changes so slowly that it's a significant time
before any descent can be noticed, even by the most skilled navigator.
3. At that date in May, the Sun's declination is increasing Northwards by
about half a minute each hour. So at the moment of LAN, the Sun's altitude
will still be increasing at that rate, and it will reach its maximum
somewhat later than LAN, before any fall can even commence.

So, if the Sun really was observed to have started descending at the time
Jeremy says, it throws some doubt on his quoted longitude.

George.

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.


--~--~---------~--~----~------------~-------~--~----~
Navigation List archive: www.fer3.com/arc
To post, email NavList@fer3.com
To , email NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---

   

Reply