NavList:

   

Reply

George H, you wrote:
"Earlier, I questioned that second sentence, suggesting that the 6 miles
given should have been 3."

SIX was correct for a lunar LOP (at known GMT). THREE is correct if you're
finding GMT from a lunar distance and converting that to longitude (but
since GMT is never unknown anymore except in highly contrived circumstances,
that only applies historically).

And you wrote:
"It calls for some clear thinking about what we are discussing, a lunar
distance observation. Does that, in itself, generate a line of position? Not
on its own, no. A lunar distance itself is nothing more than a measurement
of the position of the Moon in its orbit. From that, you get Greenwich time,
nothing else. You can set your watch from it, that's all. It tells you
NOTHING about your position, just the same as reading Greenwich time from
your chronometer tells you nothing about where you are."

I guess you've forgotten that long discussion I started back, when was it, a
year and a half ago? When you shoot a lunar distance at a known instant of
GMT, it generates a "cone" of position which intersects the Earth in an arc,
and locally that arc is an LOP.

And you wrote:
"It is generally true that in order to correct a lunar it is normal to
measure the altitudes of the two bodies involved as well as the lunar
distance itself. But that's not necessary, and it's quite possible (but a
bit complicated) to correct a lunar by computing where the two bodies must
be in the sky, without making any altitude observations at all."

Of course, you can do it without the altitudes. That's why entering that
data is optional in the lunar clearing software on my web site. I wasn't
talking about LOPs generated from the altitudes. I was talking about LOPs
from the raw lunar distance observation itself.

And you concluded:
"So what I am saying here is that a lunar observation itself may provide no
position line at all, or the observations taken with it may provide two
position lines, and so a fix. Which is why I question Frank's statement that
"each lunar generates a line of position, which you could plot on a chart
just like any other LOP". It doesn't. If it did, which direction would that
LOP run?"

This is one of several things that I will be talking about at Mystic Seaport
on Sunday, so I don't want to go on at length right now. For the moment,
I'll just offer this example: suppose I see the Moon 70 degrees high very
close to the meridian in the South. Suppose the planet Jupiter is very close
to directly below the Moon, also nearly on the meridian. I measure the angle
between the Moon and Jupiter (note that a lunar distance would never have
been meaured in this direction, basically perpendicular to the Moon's
motion, in the historical usage of lunars). For this measured lunar
distance, I get 10d 00.0', just to make the numbers simple. Now, this
distance implies a line of position. Why? Because at this moment in time,
many other places on the Earth's surface would also see exactly that angle
 --ten degrees. Where would I have to go to see that angle increased by a
tenth of a minute of arc? Where would I go to see it decreased by a tenth?

See how it works now? Traditionally, measuring a lunar distance gave
navigators a means of finding time. Today, some of us use them to test our
sextants. But in addition, for a modern navigator, lunar distances can still
provide a small amount of genuine navigational information, even if GMT is
assumed to be known at all times. Each observation of the angle between the
Moon and another celestial body determines a line of position. And a tenth
of a minute error in the measured distance typically shifts the LOP by at
least six nautical miles (in about half of cases, if the Moon is low in the
sky, then the potential error in the LOP is increased --but not always).

 -FER


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

   

Reply