P r i m a L u c e L a b i S r l
S p i d e r 2 3 0 u s e r m a n u a l
Recall that the needle of a compass points the magnetic North Pole (the Earth's magnetic field) and not the geo-
graphic North (taking as a reference the North Pole). Magnetic North is currently in the Canadian Arctic Archipelago,
it’s not still and it changes position over time. So the compass needle, depending on where we are on Earth, will be
moved eastward or westward with respect to the geographic North. The angle between the direction of magnetic
North and the true North is called
magnetic declination
. You can check the current magnetic declination from the
world map of magnetic declination updated to 2010 found in the web page:
http://www.ngdc.noaa.gov/geomag/WMM/data/WMM2010/WMM2010_D_MERC.pdf
To correctly point true north with a compass, you have to
subtract to 360° the magnetic declination of your place
of observation: this will be the direction of the North. For
example, in Italy we have a magnetic declination East (ie
positive) by about 2 degrees. The compass will indicate
true north when the compass needle will be pointing not
at 360° but at 358° (360-2 = 358).
So take the compass, position yourself just behind the
mount and rotate to make it point to the correct direction
of true North (the red line in the photo at right). In this
way, having also set the latitude as we have seen before,
the polar axis of the mount will be pointed towards the
North Star with sufficient accuracy. We will read how to
make a precision polar alignment (during the night, th-
rough the polar scope that came with the mount itself)
on page 32 of this manual.
!
!
!
p a g e
!
1 2