Abrams SC-1, Technical Manual

Page: 18 / 39

MAINTENANCE MANUAL
(or
this irregularity.
So,
in
order
to
get days
of
equal length,
a
fictitious or
mean
sun is assumed
to
revolve around the earth
at
a
uniform
rate.
Mean
solar
time
is
time
based on
this
mean
sun.
The
real sun
is
either ahead
of
or behind the
mean
sun
by a
few minutes.
For
this reason, mean solar
time (sometimes
called
civil
time) cannot
be
directly
determined by observation
on
the sun,
but is
a
measure
of
time
based on
the
fictitious
mean sun.
It
is
noon
for
a
time
zone
according
to
the stand
ard time
system, when
the
fictitious
mean
sun
is directly
over
a
time
meridian
(see
figure 5).
(2)
The
section
of
the earth's surface
in
figure
5
is
divided
into five standard time
zones,
represented by
the
dotted lines
near
the
ground. Each of
these zones
uses
as
its
standard time,
the menn
solar time
of
the meridian which passes through
its
center (such as point A, figure 5). Thus,
for
example, the
clocks
all
through the zone
(a) to (b)
indicate
12
o'clock,
when
it
is
precisely noon
by mean solar time at point
A, nl:
though the
mean
solar
time
at
points
(a)
and
(b)
may range
from
11:30 o'clock
to
12:30 o'clock.
e.
Changing
from
Standard,
to
Mean
Solar
Time.
—
The next
step
in
changing the time piece
from
local watch time to local
apparent time (sun
time),
is
to
change the time
from
the
standard time
of
the zone to
the
mean
solar
time
for
the lo
cality.
If
the
locality is
on
the time meridian, the two times
are the
same.
If
it
is
not, the time must
be
corrected
for
the
number
of
degrees
in
longitude that the
locality
lies
east
or
west
of
the nearest time meridian.
As
a
time
zone
is
15
de
grees
wide and represents
one
hour of time,
each degree
of
longitude represents
4
minutes of time correction
to be
applied
to
standard time.
If
the
locality
lies
east
of
the time meridian
add the correction,
if
west, subtract it.
/.
Equation of
Time.
—
As
was stated before, the real sun
in
its
irregular
apparent travel around the earth
is
either
a
few
minutes ahead or behind the
fictitious
mean
sun which has
a
uniform
rate
of
travel.
Four
times
a
year the two
suns
co
incide. The
angular
difference
between
the positions
of
the two
suns
at
any time
is
referred to
as
the "equation of time", and
is
expressed in minutes
of
time. The equation
of
time
is
a
measure of how
ranch
the real sun
is
ahead or behind the
mean
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