"[M]: RE: Re: RE: Magnetic Declination web site." in the Meade Astronomy Archive

Subject: [M]: RE: Re: RE: Magnetic Declination web site.
From: Tom &/or Elizabeth Skinner
Reply To: mapug@shore.net
Date: Wed Jul 04 09:19:38 2001

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Hi all,

I think maybe the example given for an East declination is backward.

When you look at the expression for calculating true north from magnetic
north it is always written as: true = mag + dec, where true is where the
compass is really pointing, mag is what the compass is indicating, and dec
is the difference between true and magnetic at your particular location (by
convention, East is + and West is -).

Mostly, we want a variant of that expression which will yield the direction
we need to point at (true north) given our particular value of declination
and what the compass is indicating WHEN it is really pointing true north.
That variant is mag = true - dec. In other words, what should the compass
indicate (mag) when it is actually pointed at true north?

The example of an observer at a location where the magnetic declination is
15 East should go like this: mag = true - dec, or, mag = 360 (true North) -
12 (mag dec) = 348 (compass indication). So, with the compass needle
directly over the N, the example observer should turn his telescope
counter-clockwise in azimuth until its centerline is directly under 348
degrees on the compass.

The short version is this:

With the N on the compass lined up with the arrow:

If your declination is West, rotate the centerline of your telescope
clockwise to that value.

If your declination is East, rotate the centerline of your telescope
counter-clockwise to 360 minus that value.

I hope the above is correct, please point out the error if wrong.

Tom

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<HTML><HEAD>
<META http-equiv=3DContent-Type content=3D"text/html; =
charset=3Diso-8859-1">
<META content=3D"MSHTML 6.00.2462.0" name=3DGENERATOR></HEAD>
<BODY>
<DIV><SPAN class=3D660190016-04072001><FONT face=3DArial size=3D2>Hi=20
all,</FONT></SPAN></DIV>
<DIV><SPAN class=3D660190016-04072001><FONT face=3DArial=20
size=3D2></FONT></SPAN> </DIV>
<DIV><SPAN class=3D660190016-04072001><FONT face=3DArial size=3D2>I =
think maybe the=20
example given for an East declination is backward.</FONT></SPAN></DIV>
<DIV><SPAN class=3D660190016-04072001><FONT face=3DArial=20
size=3D2></FONT></SPAN> </DIV>
<DIV><SPAN class=3D660190016-04072001><FONT face=3DArial size=3D2>When =
you look at the=20
expression for calculating true north from magnetic north it is always =
written=20
as: true =3D mag + dec, where true is where the compass is really =
pointing, mag is=20
what the compass is indicating, and dec is the difference between true =
and=20
magnetic at your particular location (by convention, East is + and West =
is=20
-).</FONT></SPAN></DIV>
<DIV><SPAN class=3D660190016-04072001><FONT face=3DArial=20
size=3D2></FONT></SPAN> </DIV>
<DIV><SPAN class=3D660190016-04072001><FONT face=3DArial =
size=3D2>Mostly, we want a=20
variant of that expression which will yield the direction we need to =
point at=20
(true north) given our particular value of declination and what the =
compass is=20
indicating WHEN it is really pointing true north.  That =
variant is mag=20
=3D true - dec. </FONT></SPAN><SPAN class=3D660190016-04072001><FONT =
face=3DArial=20
size=3D2>In other words, what should the compass indicate (mag) when it =
is=20
actually pointed at true north?</FONT></SPAN></DIV>
<DIV><SPAN class=3D660190016-04072001><FONT face=3DArial=20
size=3D2></FONT></SPAN> </DIV>
<DIV><SPAN class=3D660190016-04072001><FONT face=3DArial =
size=3D2>The example of=20
an observer at a location where the magnetic declination is 15 East =
should go=20
like this:  mag =3D true - dec, or, mag =3D 360 (true North) - =
12 (mag=20
dec) =3D 348 (compass indication).  So, with the compass =
needle directly=20
over the N, the example observer should turn his telescope=20
counter-clockwise in azimuth until its centerline is directly=20
under 348 degrees on the compass.</FONT></SPAN></DIV>
<DIV><SPAN class=3D660190016-04072001><FONT face=3DArial=20
size=3D2></FONT></SPAN> </DIV>
<DIV><SPAN class=3D660190016-04072001><FONT face=3DArial size=3D2>The =
short version is=20
this:</FONT></SPAN></DIV>
<DIV><SPAN class=3D660190016-04072001><FONT face=3DArial=20
size=3D2></FONT></SPAN> </DIV>
<DIV><SPAN class=3D660190016-04072001><FONT face=3DArial size=3D2>With =
the N on the=20
compass lined up with the arrow:</FONT></SPAN></DIV>
<DIV><SPAN class=3D660190016-04072001><FONT face=3DArial=20
size=3D2></FONT></SPAN> </DIV>
<DIV><SPAN class=3D660190016-04072001><FONT face=3DArial size=3D2>If =
your declination=20
is West, rotate the centerline of your =
telescope clockwise=20
to that value.</FONT></SPAN></DIV>
<DIV><SPAN class=3D660190016-04072001><FONT face=3DArial=20
size=3D2></FONT></SPAN> </DIV>
<DIV><SPAN class=3D660190016-04072001><FONT face=3DArial size=3D2>If =
your declination=20
is East, rotate the centerline of your=20
telescope counter-clockwise to 360 minus that =
value.</FONT></SPAN></DIV>
<DIV><SPAN class=3D660190016-04072001><FONT face=3DArial=20
size=3D2></FONT></SPAN> </DIV>
<DIV><SPAN class=3D660190016-04072001><FONT face=3DArial size=3D2>I hope =
the above is=20
correct, please point out the error if wrong.</FONT></SPAN></DIV>
<DIV><SPAN class=3D660190016-04072001><FONT face=3DArial=20
size=3D2></FONT></SPAN> </DIV>
<DIV><SPAN class=3D660190016-04072001><FONT face=3DArial=20
size=3D2>Tom</FONT></SPAN></DIV>
<DIV><SPAN class=3D660190016-04072001><FONT face=3DArial=20
size=3D2></FONT></SPAN> </DIV>
<DIV><SPAN class=3D660190016-04072001><FONT face=3DArial=20
size=3D2></FONT></SPAN> </DIV>
<DIV><SPAN class=3D660190016-04072001><FONT face=3DArial=20
size=3D2></FONT></SPAN> </DIV></BODY></HTML>

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