Hi John,
I would be reluctant to consider a cell phone as an "accurate clock". A few
years back I was showing a sundial to a friend and after accounting for the
equation of time, I told her what time is was. She was amazed that the time
was only two minutes different from her cell phone. I told her that her
cell phone was wrong. She insisted that the time signal from her provider
had to be accurate. A little later we got to my computer and went to the
naval time signal. Her cell phone was 1.5 minutes wrong. That is not
much, but was detected by a good sundial.
Chuck Nafziger
48 deg 36 N
122 deg 22 W
----Original Message Follows----
From: "John Carmichael" <[EMAIL PROTECTED]>
To: "Brooke Clarke" <[EMAIL PROTECTED]>
CC: Sundial List <[email protected]>
Subject: Re: Google Earth accuracy/finding North
Date: Mon, 24 Jul 2006 08:37:37 -0700
When my sundial customers ask me if they can use their GPS devices to
determine True North for setting their sundials, I tell them no. I just
don't trust GPS for that. I tell them to use this method instead:
FINDING TRUE NORTH
Using The Noon Shadow Method
1. Finding True North
Before sundial installation, you need to determine True North (not magnetic
North). Do NOT use a magnetic Compass.
To find True North you will need the following:
A piece of clothes line rope or think string about 8 ft. long.
A plumbob or heavy weight
A free standing ladder 5 or 6 feet tall
A broom handle or stiff board
Duct tape or clamps
A flat board like plywood (about 3' x 3')
An accurate time source (cell phone, clock or watch)
The exact time of High Noon for your location on the date you perform the
measurement.
A sunny day
Finding True North using this method is easy and extremely accurate. You
will use sun to give you true North by looking at the shadow cast from a
vertical rope at HIGH NOON (local apparent solar noon). At High Noon, the
sun is exactly due south and will cast a shadow on the ground directly
towards the north.
1. Determine the time of High Noon:
High noon does not occur at the same watch time every day and is different
at different locations! This website will tell you the exact time of High
Noon (Apparent Solar Noon) at your location and date:
http://www.solar-noon.com/ If you have questions, contact John Carmichael.
2. Setup plumbob: First attach a broomhandle or stiff board horizontally to
the top of the ladder with ductape or clamps. Tie the end of the string or
rope to the end of the broomhandle, then tie the pumbbob to the other end so
that it hangs just above ground level. Place the ladder off to the side so
that it is not in the way of the construction zone and so that it won't
shade the plywood board. If it's a windy day, you can keep the plumbob from
swinging if you place it in a glass of water to dampen the movement.
3. Setup plywood projection screen: Place a leveled horizontal plywood board
on the ground to the north of the plumbob. So that the strings shadow is
visible on the board. The shadow cast onto the board by the rope will point
due North at High Noon (Solar Noon).
4. Mark the North/South line: Looking at your accurate clock, at the moment
of Solar Noon, mark on the board with a pencil the position of the string's
shadow. This line is a true North/South line.
5. Rebar Placement: Tie a string between two pieces of rebar and position
the string directly above the North/South line on the plywood board. Pound
rebar into ground so that you will know where the North/South line is after
you remove the board. When positioning the sundial to true North, use this
string. You can measure off a line that's parallel to the rebar string.
----- Original Message -----
From: Brooke Clarke
To: [email protected]
Sent: Sunday, July 23, 2006 8:22 AM
Subject: Re: Google Earth accuracy
Hi Roger:
I have a web page listing the U.S. LORAN-C stations. And with the Google
imaging you can easily see the transmitter antennas which are rather large
since the frequency is 100 kHz. The transmitters are in precisely known
locations (WGS-84), but there is still a small amount of error. see:
http://www.pacificsites.com/~brooke/Loran-c.shtml
click on the station name in the table for the Google image.
Have Fun,
Brooke Clarke
Roger W. Sinnott wrote:
List:
I'm sure Mike meant that the Greenwich Meridian Line is about 5" west,
not 5' west, of where it is shown on Google Earth. (I just looked to confirm
this!)
This discrepancy is simply due to Google Earth's adopted geodetic
datum, WGS84. GPS receivers can often be set to show coordinates for other
datums (data?) as well (for example, GB36, the Ordnance Survey of Great
Britain 1936, on which the Greenwich meridian line is *much* closer to 0
longitude).
The datum choice probably explains the equator discrepancy in Ecuador
as well. It's simply that each country, historically, conducted geodetic
surveys that are internally consistent to high accuracy. But they don't join
up perfectly with those done in other countries. The WGS84 datum is intended
to apply to the entire world. We had a brief discussion of this issue in Sky
& Telescope for August 2005, page 110.
I learned this lesson the hard way. When I went to find the gravestone
of telescope maker Russell W. Porter in Port Clyde, Maine, I had GPS
coordinates from a friend. After an hour and a half, no luck -- so I gave
up. Only later did I learn that his receiver was set to WGS84 while mine was
set to NAD27.
-- Roger
At 08:27 AM 7/23/2006 +0100, Mike Shaw wrote:
>>>>
Bill G. said: <<There is a large monument in Ecuador that marks the
exact location of the equator, so they claim. Google Earth puts it about 780
feet south of the equator. Any thoughts on who is more accurate: Google
Earth or the Ecuadorian surveyors? >> I have also noticed that, according
the Google Earth, the meridian line at the Greenwich Observatory (surely
correct) is at 5.26 minutes West. Mike Shaw 53.37N
3.02W www.wiz.to/sundials
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