Thanks, Hank. Very helpful observations. The ecliptic north pole lies in the curve of Draco's (The Serpent) neck. 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. Is there a way to use this information (perhaps the time of year) to help refine finding south by the moon's crescent? -Bill
On Mon, May 12, 2014 at 3:07 AM, Hank de Wit <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] On Behalf Of Steve > Lelievre > Sent: Sunday, 11 May 2014 12:22 AM > To: 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 > > > > > > --------------------------------------------------- > https://lists.uni-koeln.de/mailman/listinfo/sundial > > --------------------------------------------------- > https://lists.uni-koeln.de/mailman/listinfo/sundial > >
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