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that is, 224°.
Magnetic Bearings
One problem is that a compass does
not point towards True North, but
Magnetic North, since the Earth
generates its own magnetism – and
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.
142 JAR Private Pilot Studies
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
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.
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
doing calculations, revert everything
to UTC first, and don't forget the
date! The International Date Line is
where a change of date is officially
made, being mainly the 180°
meridian which bends to
accommodate certain islands in the
South Sea and parts of Siberia.
Deviation
We saw above that the magnetism
from the Earth will vary the
direction displayed by a compass.
The aircraft's magnetism, created
from large amounts of metal mixed
with electric currents, will do the
same thing, called deviation, which is
applied to the magnetic heading to
get Compass North.
The phrase here is Deviation West,
Compass Best, Deviation East, Compass
Least. This means that if the
deviation is to the left of the
magnetic North, the difference
should be added to the course to get
the correct magnetic heading.
Navigation 143
Deviations will be displayed on a
small correction card next to the
compass, and are obtained after a
compass swing, a complex procedure
normally done by an engineer. There
will be an area on every aerodrome
　
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