Hanno seems to be re-inventing the method, described by Chauvenet in
"Spherical and Practical Astronomy", (1863), as his 4th method for finding
the longitude (by moon culminations). Also relevant is his 5th method, by
azimuths of the moon, or transits of the Moon and a star over the same
vertical circle. Both highly accurate when observed from on land, but not
possible otherwise.
Used for determining the longitude in the ice of James Bay, an appendix of
Hudson bay, accurately by Thomas James, who overwintered there in 1630-31.
This was the first successful use (that I know of) of lunar methods for
longitude determination.
As opposed to William
Baffin, who is often credited with the first use of
such lunar measurements 40 years earlier, but actually got it hopelessly
wrong.
There may be more to be said, but it's time for bed.
George.
contact George Huxtable, at
george@hux.me.ukor at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
----- Original Message -----
From: "Hanno Ix" <
hannoix@sbcglobal.net>
To: <
NavList@fer3.com>
Sent: Friday, August 07, 2009 12:38 AM
Subject: [NavList 9393] Re: Working a lunar - a PS
Brad:
Thank you for the time and thoughts you spent on this.
We both
agree that - if it worked - this approach to find position/GMT by
the difference between the times of culminations, i.e. DT, is only practical
from land. But that itself would be rather desirable and probably quite
within the technical means of a traveling ship.
Frankly, I don't really know much about the kinematics of culminations of
moving Heavenly Bodies. I suspect, though, that there might be significant
deviations from a first order analysis. Even though these deviations might
seem small for now, they might also end up a little cumbersome in practice
because of the small size of the observed phenomenon itself - the movement
of the moon.
Nevertheless, the simplicity of the concept would make the approach
attractive because it seems easy to understand and to remember. And applying
some corrections is a common part of celestial navigation. So, it might
perhaps end up useful - if it is
correct!.
So, let's submit this concept to further critique of the group members.
Thanks again and regards
H
--- On Thu, 8/6/09, Brad Morris <
bmorris@tactronics.com> wrote:
From: Brad Morris <
bmorris@tactronics.com>
Subject: [NavList 9392] Re: Working a lunar - a PS
To: "
NavList@fer3.com" <
NavList@fer3.com>
Date: Thursday, August 6, 2009, 1:34 PM
Hi Hanno
I have been considering your statement “If I see things right, there must be
a LOP which
connects
all those locations on Earth that have a given, fixed difference DT between
the meridian passages of
sun and moon”
Just to be sure I understand your statement, I will re-write it. The first
object crosses your meridian. Let us assume that it is the sun. While this
is
LAN, we don’t care about that, you merely start your timepiece stopwatch.
Next we wait for the second object to cross your meridian. When it does, you
stop your timepiece. We observe the delta time. From this one data point, we
are expecting a LOP.
This has nothing to do with the altitudes of the objects, just the Delta
Time of the meridian crossing.
One problem (not insurmountable) is that the two celestial objects have
apparent diameters. As such, we must perform a few more measurements. That
is, assuming
you are using an older theodolite with 5 wires, you would record the 5
times that the leading limb
crosses each wire and the 5 times that the
trailing limb crosses each wire, and then mathematically determine when the
object was on your meridian.
The next problem (not insurmountable) is to align the theodolite to your
meridian. Even Bowditch in the 1800’s knew how to do this. However, the
requirement
to align the theodolite to the meridian precludes any use of this method
whilst at sea.
Now which LOP corresponds to the Delta Time (DT)? I suggest to you that it
is your MERIDIAN. Anyone, at any other latitude, that is on your longitude
will
measure the same precise value that you do. Now there’s an interesting
outcome! Sure, the altitudes will be different, but the DT will be the same.
Can we tell which meridian? Considering that the moon essentially travels it’s
diameter in an hour, we run right into the resolution problem, very similar
to the Lunar lack of resolution.
Since we will be measuring with a
theodolite, we will have a very good measurement for the time of meridian
crossings, assuming that the theodolite is aligned to the meridian to
perfection. Assuming you measure precisely
and accurately, then the answer is yes.
Best Regards
Brad
"Confidentiality and Privilege Notice
The information transmitted by this electronic mail (and any attachments) is
being sent by or on behalf of Tactronics; it is intended for the exclusive
use of the addressee named above and may constitute information that is
privileged or confidential or otherwise
legally exempt from disclosure. If you are not the addressee or an employee
or agent responsible for delivering this message to same, you are not
authorized to retain, read, copy or disseminate this electronic mail (or any
attachments) or any part thereof.
If you have received this electronic
mail (and any attachments) in error,
please call us immediately and send written confirmation that same has been
deleted from your system. Thank you."
