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:[email protected]] On Behalf Of Steve Lelievre
Sent: Sunday, 11 May 2014 12:22 AM
To: [email protected]
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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