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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
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 a GPS or
similar, the simplest way is to line up
with a known line feature and apply
the variation from the map (and the
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
Navigation 313
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). As it
goes round the Sun, the Earth spins
on its axis once every 24 hours, and
the Solar System itself is creeping
towards the star Vega, but that need
not concern us right now.
Johannes Kepler determined that
each planet moves in an ellipse, and
that the radius vector takes equal
time to sweep equal areas. The
perihelion (1-10 Jan) is where the
Earth is closest to the Sun, and the
aphelion (1-10 Jul) is where it is
furthest away. Because of Kepler's
second law (above), the Earth moves
faster round the Sun at the
perihelion, because it has further to
go in the same time:
The problem with the elliptical orbit
and the different speeds this is that
the length of the apparent solar day will
vary, so the mean solar day is used as
an average, at exactly 24 hours. This
means that a (non-existent) Mean Sun
actually transits any meridian at
noon, which is where the expression
local mean time comes from (the real
Sun is about 16 minutes ahead of the
mean Sun in November and 14
minutes behind in February).
The Earth, in particular, 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
　
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