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----- Original Message -----

From: Alexandre Eremenko

To: NAVIGATION-L@LISTSERVWEBKAHUNA.COM

Sent: Saturday, October 09, 2004 6:33 AM

Subject: Pointers

Can anyone explain me that the "Pointers" are?
I mean this is something in the sky,
probably a group of fixed stars.

I just read a funny paper:
Joel Brenner, Determination of latitude in an emergency,
American Math. Monthly, 51, N 6 (1944) 343-244.

The author recommends a method of determination of latitude
without almanach, without any tables, using only a sextant
and Polaris (and the "Pointers").

In general to find lalitude from Polaris altitude with accuracy
of few minutes, one only needs its LHA, (or LHA Aries, or LHA of
any fixed star).
(See the table in the top on p. 274-276 of the 2004 almanac).
One can neglest the a_1 and a_2 corrections in the
middle and bottom of this table, which are very small.
And knowing LHA Aries with only 10 degree accuracy may be OK
"in emergency". This is because Polaris is so close to the
North Pole that its altitude changes very slowly with LHA.
The author
proposes to use the position of
the "Pointers" in the sky to find LHA Aries and thus LHA
Polaris. Instead of using the almanac table, he proposes
(in emergency) to find the correction graphically,
just by drawing a picture as explained below.

Then, of course, refraction remains. On this the author remarks:
"A good navigator knows the refraction corrections.
For observed altitudes 45d, 30d, 20d, 15d, the corrections are
-1', -2', -3', -4', respectively.

Here is an Example from this paper:

"As sea, charts lost, clock stopped, tables illegible,
sextant working, navigator takes altitude of Polaris, 44d30'20",
IC -0'20", no dip. The refraction can be neglected.
After consultation
with the crew, it is decided that the figure shows the approximate
direction of the pointers [the author includes the figure].
A circle is sketched and drawn free hand (or by the use of
two holes on a
card, or by the use of a coin);
the hour angle of Polaris next estimated [according to the author,
Polaris is 135 degrees anticlockwise from the Pointers]
(an angle of 135d=90d+45d is easy to draw) and last the value of cos
of the hour angle of Polaris can be read to one place
of the decimals from
the figure.
The latitude equals 43d42' plus or minus 4'

[End of citation].


So what are the Pointers?
The article does not explain this, apparently assuming that this was
common knowledge in 1944:-)

Alex.