Greetings. fellow dialists,
Astronomical purists may feel uneasy but the easiest way to find south
if there are any buildings about is to look at the satellite dishes.
Around here they point a little east of south and they presumably point
near to south throughout the northern hemisphere.
Frank 55N 1W
-------- Original Message --------
Subject: Re: Using the moon to find south [SEC=UNOFFICIAL]
Date: Mon, 12 May 2014 11:31:36 -0400
From: Bill Gottesman <billgottes...@comcast.net>
To: Hank de Wit <h.de...@bom.gov.au>
CC: sundial@uni-koeln.de <sundial@uni-koeln.de>
Thanks, Hank. Â Very helpful observations. Â The ecliptic north pole
lies in the curve of Draco's (The Serpent) neck. Â From your
explanation, it seems that the line connecting the crescents of the moon
should always point approximately to that location, and this should be
something easy to test in the night sky. Â Is there a way to use this
information (perhaps the time of year) to help refine finding south by
the moon's crescent? Â -Bill
On Mon, May 12, 2014 at 3:07 AM, Hank de Wit <h.de...@bom.gov.au
<mailto:h.de...@bom.gov.au>> wrote:
HI Steve,
I think I can answer this one approximately. The maths is also
beyond me, but we can get an intuitive answer without causing too
much brain strain.
The first point to remember is that both the Sun and the Moon travel
on paths nearly along the Ecliptic. The Sun sits exactly on the
Ecliptic, and the Moon, deviates plus or minus 5 degrees, because
it's orbit is inclined by 5 degrees to the Ecliptic. This means that
that shadow of the terminator between light and dark on the moon
must be aligned nearly perpendicular to the path of the Ecliptic in
the sky - they are in the same plane. So the problem reduces to the
angle that the path of the Ecliptic makes in the sky.
To reduce variables even more, let's just think about the Moon when
it is highest in the sky, along the meridian through South (North in
the SH).
We need a planisphere to visualise, and I found a nice online one here:
    http://drifted.in/planisphere-app/app/index.xhtml
This planisphere has the Ecliptic marked as a blue line in the sky.
If you rotate the outer disk to move through the months, and imagine
the Moon along the Ecliptic and sitting on the north-south meridian
you can clearly see the tilt of the Ecliptic line, and therefore the
line through the horns of the Moon if it were located at that point.
You can see that this line is not directly through north for most of
the year, and can be either side. The biggest deviations are at the
two equinoxes. It is pointing south (north) at the solstices. I
wonder if the amount of maximum deviation from due south (north) is
plus and minus 23.5 degrees.
Many regards
Hank
-----Original Message-----
From: sundial [mailto:sundial-boun...@uni-koeln.de
<mailto:sundial-boun...@uni-koeln.de>] On Behalf Of Steve Lelievre
Sent: Sunday, 11 May 2014 12:22 AM
To: sundial@uni-koeln.de <mailto:sundial@uni-koeln.de>
Subject: Using the moon to find south
Hi folks,
Only loosely related to my question just posted, I'm interested to
know more about a primative navigation method I've read of. The idea
is that if one projects an imaginary line through the cusps of a
crescent moon down to the horizon, that gives the approximate
position of South (or perhaps North depending on your hemisphere).
How accurate is this position compared to true south? I'm guessing
it depends on the time of year, phase of moon and latitude - can any
one supply formulae? Working it out from first principles is beyond
my math ability.
I'm thinking that if I can use the moon to find south, I can then
measure the azimuth of the sun and use that to get time of day...
Thanks,
Steve
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