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the two Norths (or Souths) do not
coincide at their respective Poles.
The next is that the magnetic force is
not constant over the globe – it may
be varied by local deposits of metals
under the ground, for example, and
bend the magnetic flux lines. The
way to Magnetic North will
therefore vary from place to place.
In addition, the lines of force will be
vertical near the poles:
The North Magnetic Pole was
discovered by Soviet explorers to be
the rim of a magnetic circle 1000
miles in circumference:
On a map, which is drawn initially
for True North, there is a dotted line
called an isogonal that represents the
local magnetic variation to be
applied to any direction you wish to
plan a flight on:
It is westerly where the variation is
to the left of the meridian, and
Easterly when to the right. It also
changes every year, since the
magnetic pole moves East, about
one degree every six years.
Isogonals are accurate worldwide to
±2°. Magnetic variation, therefore, is
the angle between True North and
Magnetic North. An agonic line exists
where magnetic variation is zero, or
where they are both the same.
There's one near Frankfurt, running
142 Canadian Private Pilot Studies
North/South. The line of zero dip
(the Equator) is the aclinic line.
The phrase to remember is Variation
East, Magnetic Least, Variation West,
Magnetic Best, that is if the variation
on your map is, say, 21° West, the
final result should be 21° more than
the true track found when you draw
your line. If you travel over many
variations, use an average about
every 200 miles.
By convention, the North Pole is
blue, and the South Pole is red.
Remember also that variation on a
VOR bearing is applied at the station,
and on an ADF at the aircraft.
Sun's True Bearing
In the North, where the compass
becomes unreliable, you need an
alternative means of aligning your
DGI. If you haven't got GPS, the
simplest way is to find a known line
feature, line up on it and apply the
variation from the map (and
compass correction – see below).
Another way is to use the fact that
the Sun's movements are known and
are tabulated as bearings relative to
True North. That is, it is in the same
position at a particular point and
time from year to year. Once you
have the bearing from the tables in
the Air Almanac, line up with the
Sun and set your DGI.
Unfortunately, aside from actually
needing to see the Sun, some
calculations are required, since the
tables are based on average figures,
and you need an accurate time
anyway (time zones are wide). Your
precise longitude produces the Local
Civil Time (LCT) – divide your
longitude by 15 to get hours and
minutes, and subtract from UTC
(Canada is West from Greenwich).
Use the figures to get the bearing in
the tables, which will have the date
as one axis, and the time as the
other. Interpolate as necessary.
Time & Time Zones
You know already that the Earth,
together with 8 other planets,
revolves round the Sun. 1 year is the
time it takes to go once round, in the
Earth's case 365¼ days (the odd
quarters are consolidated every four
years into one day in a leap year).
The Earth does not spin vertically,
like a top, but is inclined. When the
inclination points towards the Sun,
the Northern Hemisphere days are
long and the nights are short. The
day when this is at its maximum
value is the Summer Solstice on June
21 (Solstice is Latin for Sun Stand
Still). The Winter Solstice, when the
inclination is at its maximum away
from the Sun is December 21. Days
and nights are of equal length on the
Spring and Autumn Equinoxes,
March 21 and September 23
(Equinox means Equal Night),
because the spin axis is vertical to
the Earth's orbit.
The Prime Meridian is the standard
to which all local mean times are
referred. Local Mean Time there
used to be called Greenwich Mean Time
(GMT), but is now referred to as
Universal Coordinated Time, or UTC.
The Greenwich day starts when the
mean Sun transits the anti-meridian,
and transits the Easterly ones before
it reaches Greenwich. The local
mean time in those places will
therefore be ahead of UTC, and that
of those West will be behind. When
Navigation 143
doing calculations, revert everything
to UTC first, and don't forget the
　
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