View Full Version here: : Declination
18-07-2012, 12:04 PM
I thought I had this all worked out, but I can not get my alignments right. Magnetic declination in Melb is 11.3 deg EAST.
Latitude: 37° 48' 50.4" S
Longitude: 144° 57' 48" E
Magnetic declination: 11° 34' EAST
Declination is POSITIVE
Inclination: -68° 44'
Magnetic field strength: 60024.9 nT
So I set up my mount facing south - 11.3 deg east of south.
NO wait, if true north is 11.3 deg EAST, then true south has to be 11.3 deg WEST. So point the mount 191.3 deg not 169.7 deg, yes?
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.
Am I just confusing myself, the more I think about it the more confused I get. Grey matter working too slow.
18-07-2012, 12:19 PM
I think you a right, 11.3 degrees west of south. Thanks for the website, I didn't realize that the inclination was so high in the east. Perth has less than half a degree:P In some places it was 0, which would make an astronomers life much easier:lol:
18-07-2012, 12:32 PM
When in doubt I use the Solar Noon method.
Print off a solar noon calendar for your long/lat.
Note the time of solar noon on the day you are going to do this.
Go outside and then use a vertical pole or a plumb-bob on a string and watch the shadow cast on the ground by it at solar noon.
The shadow will run true North/South at the exact time of Solar Noon (about 12:26 in Melbourne today)
I use it to check my other methods as my brain can often get in the way. The solar noon method is basically infallible as long as you have the time of day right, and your shadow casting implement (pole, post or plumbob and string) is perfectly vertical.
Reading a shadow on the ground is easier than applying offsets and such.
Or approximate times for well known cities around the world:
18-07-2012, 12:44 PM
I also find this converter useful:
I've attached a solar noon calendar using the long/lat from your first post.
So Solar Noon tomorrow (July 21st) would be at 12:26:39 for your location.
Simply look at the shadow at that exact moment, and that is your North-South line.
18-07-2012, 03:19 PM
You forget this is Melb - no sun - no shadows!
Thanks Sean, Peter I will check it out
Clear skies (except Melb)
18-07-2012, 09:07 PM
Throw the WEST theory out - alignment way out couldn't find stars in the finder at all.
Going back to true south 11.3 deg EAST of mag south. Alignment stars now at least in finder.
I am confused. :shrug:
18-07-2012, 09:18 PM
Something I was taught when I was an apprentice at sea; with Noah, just in case anyone asks :lol::
Compass (bearing) to True (bearing) ADd East
Therefore if the Magnetic variation is 11.3°E
C AD E T
180° 11.3°E 191.3°
C AD E T
168.7° 11.3°E 180°
so if you want to point your mount due south, your magnetic compass should read 168.7°
Hope this helps and I haven't added to the confusion
18-07-2012, 10:24 PM
In case anyone else wants a calendar like the one Poita supplied, go to http://www.solar-noon.com/
http://www.esrl.noaa.gov/gmd/grad/solcalc/ has a point-and-click interface to get the values for one day.
Rod, you have an obs on the corner of Bourke and Elizabeth Streets?
19-07-2012, 07:52 PM
The one and only clear and practical post on this matter.
From another mariner, who cannot figure why landsmen can make such a problem out of this!
19-07-2012, 08:24 PM
Nah, that's just the co-ordinates for Melb as selected by the web site. I'm actually a wee bit N/W of that in suburbia.
Thanks Rob and John, my feeble brain has now worked it out. Positive declination is EAST. So set up the mount pointing 11.3 deg EAST of 180 deg. It was what I was first doing, but not getting my scope/star, alignment very close. I obviously need to work harder on my polar alignment first. Practice and more practice - I'll actually enjoy that. Fancy having to spend more time out with the scope just to learn how to use it, well that's what I'll tell the wife anyway. :P
20-07-2012, 01:20 PM
By the time you have set up the compass and worked out were true north/south is the clouds will have set in for the duration.
Here is a novel way to find true north/south that should work for many people. Open Google Earth and locate the exact spot where you have your mount. Look along the north south axis that passes through your position and see if you can locate an obvious landmark on this axis that you can see (street view may help). Gunsight align your mount to this point this should get you within a few minutes of true north or south.
21-07-2012, 11:07 AM
Even if you just stand up straight at solar noon and note the direction of your own shadow you have a pretty good north-south line.
Enough to work out which way to add or subtract your deviation at any rate.
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