Abrams SC-1, Technical Manual

Page: 24 / 39

MAINTENANCE MANUAL
(a) This information is sometimes given on the map and in
cluded in the figure representing direction, such as
that
found
in the "Compass rose."
It
can also
be
obtained
from
an iso-
gonic
chart
which includes
the
locality in
question. The iso-
gonic chart is
covered
with
continuous isogonic lines. Each
isogonic line represents
the
number
of
degrees declination,
east
or west, that the compass
needle
bears
from
true north
as
it
travels along an isogonic line.
That
is,
at any point
along an isogonic line,
the
compass
needle
will
have the
same
declination
from
true north.
(b) This declination must
be
added or subtracted
to the
magnetic compass reading
to determine
a
true north azimuth
reading.
(2) Turn
the
date bar on the sun compass
until
the
arrow
head points true north.
(3)
Slide
the
solar plate along
the
date bar
until
the
out
side edges coincide
with
the
date line
on
the date bar which
represents
the
date
of
the
day.
(4)
We can now find the apparent time (sun time)
for
the
locality
by
determining
the
time line indicated by the
inter
section
of
the shadow cast by the center gnomon rod and
the
latitude
line which represents the
latitude of
the
locality
to
the nearest
degree.
Follow
the
time line
to
the
edge
of
the
solar plate and read
the
time indicated. The time lines rep
resent
each
hour
of
the
day from
6
A.M.
to
6
P.M. o'clock,
broken down
into quarter
hours.
3.
OPERATION INSTRUCTIONS.
a.
General. The
driver
of
the
vehicle or his assistant may
use
the sun
compass.
If
the
assistant reads
the
compass and
passes
instructions
on to
the driver,
a
great deal of
benefit
will
be
derived
from
the teamwork
between
the two.
This
allows
the
driver
to
concentrate
on
maintaining
the
course.
The
azimuth
to
be
followed
on a
predetermined course is
obtained
from
a map.
NOTE
:
Magnetic or grid azimuths must
be
converted to true azimuths.
6.
Operation Procedure.
(1)
Orienting the Vehicle
to a
Predetermined
Azimuth.
(a) Set the timepiece
to
be
used
with
the sun compass
to
local apparent time
(see
paragraph
1,
section
IV).
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