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Re: True North - Sorry - 1 hour error in original calc.



Murray,

Sorry but you have a false assumption there.  Solar time is apparent time,
based on the apparent sun.  UTC is mean time, based on the mean sun a
mathematical construct which casts no shadow.  Your method may be off by
as much as 15 minutes (twice a year). It will be correct four times a year.
The difference between Apparent time (sundials) and Mean time (clocks)
is given by "The Equation of Time". (Cool name huh?)

I just did a search for "Equation of Time" and these came up
http://www.sundials.co.uk/equation.htm
http://www.uwrf.edu/sundial/Eqntime.html

For a description of Cooks voyage
http://pacific.vita.org/pacific/cook/cook1.htm

If I were going to find true south by looking at shadows, I might try;
Mark the location of the shadow at sun rise, either the azimuth from the
gnomon (tower) or the shadow's tip.  Repeat at sunset.  Bisect the angle,
or find the perpendicular bisector of the tip to tip length.  You should
be real close.  

N8NPA Brian


>Date: Thu, 8 Feb 2001 09:09:42 +1100
>From: "Murray Peterson  VK2KGM" <vk2kgm@ihug.com.au>
>Subject: [amsat-bb] Re: True North - Sorry - 1 hour error in original calc.
>
>Hi David,
    >I will try to explain this more clearly and correctly.
>
    >The thing I was looking for was solar noon (12:00).
>Solar noon at Greenwich (0 degrees of longitute) is at 12:00 UTC or as it
>used to be known GMT (Greenwich Mean Time). (Greenwich is a place in London
>where the British Navy, centuaries ago decided was the place that
>longititude should be measured from and so they went to considerable expense
>(British Board of Longitude) setting up observatories at may places arrouund
>the world which they used to determine the longitude of that location. Since
>clocks several centuaries ago did not keep time all that accurately, they
>used astronomical events to claibrate their measurements. Australia (the
>east coast) was discovered by Captain James Cook and his crew in 1770. They
>were sent on a mission by the British Board of Longitude to observe the
>transit of the planet Venus across the Sun from the islands of Taheti and
>the position of the sun in the sky in order to accurately determine the
>longitude of Taheti. The transit of Venus could be accurately calculated and
>so time in Greenwich and Taheti could be accurately calculated and therefor
>the longitude difference.
>
    >We now have accurate clocks and we can usually find out our longitude
>and so calulating solar noon is not so difficult.
>
    >As I previously said, the thing we are looking for is solar noon as this
>will directly give us the direction of North or South. We know solar noon at
>Greenwich is 12:00 UTC. For my longitude which is 151.06278 Degrees. The
>Earth rotates once (360 Degrees - actually almost 361 as the Earth moves
>almost 1 degree around the Sun each day but we are interested in angles with
>respect to the position of the Sun, not the rest of the Universe) in 24
>hours and so that is 15 degrees per hour (360/24=15). My home is
>151.06278/15 = 10.070852 Hours ahead of UTC. Therefore my solar noon will
>occur 10.070852 Hours before 12:00 UTC and so that is 12.000000 - 10.070852
>= 1.929148 or 01:55 and 45 seconds. [In my previous email there is a 1 hour
>error - I forgot that I have Daylight savings operating here - clocks put
>forward 1 hour during summer - dumb idea - I think it would be good for the
>whole world to set clocks to UTC!]
>
>Regards,
>Murray Peterson
>VK2KGM
>
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