[AJames]

Originally Posted by g__day

Isn't it funny how something authorative sound in print is can be treated like holy writ? I remember an Australian Astronomy website that had rough polar alignment advice saying true South was about 11 degrees West of magnetic South for Sydney. I asked my dad - a retired surveyor and he said no its the opposite - due East. A google again identified two points of view - which I alerted the website owner too - he checked and corrected his advice!

If you are after the definitive explanation of this, then go to;
http://www.ga.gov.au/oracle/geomag/agrfform.jsp

This explains that the magnetic variation "D" is;

"D, the magnetic declination (sometimes called the magnetic variation), is the angle between the horizontal component of the magnetic field and true north. It is positive when the compass points east of true north, and negative when the compass points west of true north. Declination is given in degrees and its annual change is in degrees per year.
The value of magnetic declination should be added to a magnetic compass bearing to yield the true north bearing."

The calculation example on this page says;

"Let us say the magnetic declination for Perth (31 57'S, 115 51'E) at 1 July 2002 is -1.6 degrees. A compass bearing of 72 degrees in Perth converts to a true bearing of 70.4 degrees [72 + (-1.6)].
Map and compass users often require the angle between grid north and magnetic north. Grid north differs from true north by the "grid convergence". The MGA94 grid convergence for the Perth location above is -0.6 degrees. A true bearing of 70.4 degrees in Perth converts to a grid bearing of 69.8 degrees [70.4 + (-0.6)].
Grid convergence and magnetic declination are shown in diagrammatic form on some topographic maps. The signs of these values can be deduced from the diagram."

You can also calculate the magnetic variations for any location in Australia (on this same webpage) using the provide calculator, just providing the latitude and longitude, and height above sea level.

However, importantly, the magnetic variation is significantly different from place to place - in Australia and throughout the world.

Ie. In Sydney, 18th Jan 2008, the variation is "D" is 12.598 deg, while in Perth this is "D" -1.492. If you were on Macquarie Island in the Southern Ocean, the variation would be about 32.5 degrees. At Casey Station in Antarctica, the variation is -91.7 - meaning geographic north points to magnetic west ! [The message here is very clear : Don't get lost in Antarctica!"]

A suitable map for the variations can be obtained from; http://www.ga.gov.au/image_cache/GA5824.jpg

The error is so very easy to make. In astronomical calculations it is more frequent to place longitudes that are east of Greenwich are given as negative Ie. -151 deg East.

For all geophysical data, the latitudes east of Greenwich are usually positive (as it is with the Australian Geophysics website!)

The error in the statement you quote is just a matter of the direction.

Magnetic variation "D" is either;

- It is positive when the compass points east of true north
- It is negative when the compass points west of true north
- It is positive when the compass points west of true south
- It is negative when the compass points east of true south

The error in the statement you provide is the reverse direction being referred from magnetic south. In truth the statement should be,
as some references state;

"...about -11 degrees West of magnetic South for Sydney"
(interpreted as)
"...about +11 degrees East of magnetic South for Sydney"

Anyway. I hope this helps.

Andrew James

NOTE: What really should be stated to avoid the confusion completely is that the azimuth should always be quoted - measured in degrees from north through east. This avoids the whole east-west dilemma.
For example, the azimuth of magnetic south in Sydney, is 180 - 12.598 or 168.402 degrees from north.

COMMENT: Using this method for alignment is only approximate anyway.

I do think it is better if you want to approximately find true south, then just walk a couple metres away from the telescope and just look where the mount is actually pointing!

[If you don't know where south is, then you can use the Southern Cross by drawing a line through Acrux and Gacrux, measure 4.5 times the distance towards the celestial pole, and drop this place to the horizon. This is south.]

Visually you should be able to get within two degrees without much trouble. Then return to the telescope, and move the mount to this direction. Repeat if necessary. THEN, if you must use a compass, adjust the direction by the difference of the magnetic variation to make a better refinement.

If your do require more accuracy, then just use stars (by the many methods] to refine the direction of the mount.