Abrams SC-1 User manual

T
RDEPARTMENT TECHNICAL MANUAL
u
COMPASS, SUN
UNIVERSAL TYPE,
MODEL SC-1
This is a reprint of TM 5-9422, Compass, Sun, Universal
Type, Abrams, Model SC-1. No distribution will be made
to personnel possessing the original publication.
.. DEPARTMENT ,1NOVEMBER 1943

WAR DEPARTMENT TECHNICAL MANUAL
TM 5-9422
COMPASS, SUN
UNIVERSAL TYPE,
ABRAMS MODEL SC-1
WAR DEPARTMENT •1NOVEMBER 1943

WAR DEPARTMENT,
Washington 25, D. C, 1November 1943
TM 5-9422, Compass, Sun, Universal Type, Abrams Model SC-1,
is published for the information and guidance of all concerned.
/A.G. 300.7 (1 Nov. 43)./
By order of the Secretary of War:
G. C. MARSHALL,
Chief of Staff.
OFFICIAL:
J. A. ULIO,
Major General,
The Adjutant General.
II

TABLE OF CONTENTS
Title Page
SECTION I. Introduction 1
II. Description 1
III. Installation 5
IV. Operation 8
\. Inspection and Maintenance 21
VI. Disassembly, Inspection, Repair, Reassembly .21
Group Assembly Parts List 24
M558501

FIGURE 1.—The Universal Sun Compass.
Gimbal mounting
Level vial
Protractor plate
Date bar
Solar plate
G. Shadow bar
7. Pivot bolt
8. Gnomon rod
9. Sighting rod
IV

INTRODUCTION AND DESCRIPTION
SECTION I
INTRODUCTION
1. This manual is aHandbook of Instructions with aparts
list for the Universal Sun Compass Model SC-1, adirection
instrument for land navigation.
a. This Handbook contains descriptive data and instructions
for the installation, operation, maintenance, and disassembly
and assembly of the Sun Compass.
6. The Parts List contains agroup assembly parts list ar
ranged and intended to show the relationship of single parts
to sub-assemblies and of the sub-assemblies to the main as
sembly. Column 1contains alist of key numbers by which
each part is designated on the accompanying exploded view of
the instrument. Column 2is the manufacturers part number
by which each part and sub-assemblies may be procured. Col
umn Sis the part name with descriptive information on stand
ard commercial parts. Column 4indicates the number of parts
in asub-assembly, and the number of sub-assemblies in the
main assembly. When. apart number is preceded by an aster
isk ('*), this indicates that the part is procurable in the as^
sembly only.
SECTION II
DESCRIPTION
1. GENERAL DESCRIPTION.
a. The Universal Sun Compass is amechanical device which
utilizes the azimuth of the sun to obtain true direction. The
instrument is ruggedly built and can be used on any type of
vehicle on which it can be mounted properly.
6. The sun compass has many definite advantages over the
magnetic compass. It is more rugged than the magnetic com
pass. The sun compass is not affected by any local magnetic
attraction such as electrical circuits or metal as is the case
with the magnetic compass. -It is avery accurate direction
instrument and has long been used to correct errors due to
deviation of magnetic compasses. The sun compass can only
be used when the sun is shining, or by sighting on the north

MAINTENANCE MANUAL
star at night when it is not cloudy. So it is the complement of,
and does not replace the magnetic compass. The land navi
gator must be equipped with both, using the sun compass
whenever possible, and reserving the magnetic compass for
cloudy periods.
c. The sun compass is attached to the vehicle by means of a
universal gimbal mounting (figure 1, item 1) which allows tip
ping the instrument in any direction to make it level. Alevel
vial (item 2) attached to the extended arm of the gimbal
mounting indicates when the instrument is level. Acircular
protractor plate (item 3) is secured to the gimbal mounting.
Adate bar (item 4), an oval shaped solar plate (item 5), and
ashadow bar (item 6) are mounted successively on top of the
protractor plate. Apivot bolt (item 7) acts as apivot for
these parts. The date bar can be rotated afull 360 degrees to
any desired setting on the protractor. The solar plate can be
slid along the date bar to the proper date mark. The shadow
bar can be rotated afull 360 degrees over the surface of the
solar plate to the proper time-latitude setting. The solar plate
and the date bar are clamped at the desired settings by astar-
shaped clamping nut on the lower end of the pivot bolt. A
vertical gnomon rod (item 8) is mounted in the center of the
pivot bolt. It casts ashadow which falls on the shadow bar
when the proper adjustments have been made. This gnomon
rod is used in conjunction with asighting rod (item 9) on the
gimbal mounting arm, to obtain aforesight or to sight on astar.
2. DETAILED DESCRIPTION.
a. Oimbal Mounting. —The gimbal mounting (figure 1, item
1) frictionally prevents the compass from tilting, but allows it
to be readily leveled. The holes provided for the mounting
bolts in the bottom support of the gimbal mounting are made
slightly oversize to provide an adjustment in aligning the com
pass parallel to the centerline of the vehicle. Wing nuts on
the pivot bolts of the support can be tightened to rigidly secure
mounting after it has been leveled.
6. Level Vial.—The level vial (item 2) is around, heat re
sistant glass vial, enclosed in ametal cap and mounted on the
Gimbal arm. It provides aready means for leveling the com-

DESCRIPTION
pass. Aspare level is provided with each instrument in case
of breakage.
c. Protractor Plate. —The protractor plate (item 3) is a
round steel plate fastened to the gimbal mounting and marked
off in one degree intervals numbered every ten degrees, and so
arranged to be read in acounter-clockwise direction. Aline on
the 180 degree to 0degree diameter of the protractor plate
must be parallel to the center-line of the vehicle, with the
arrowhead pointing in the direction of travel. The protractor is
used in setting the azimuth of apre-determined course, or for
determining in whiqh direction the vehicle is traveling.
d. Date Bar.
(1) The date bar (item 4) is aflat steel bar mounted at its
center on the pivot bolt. The solar plate can be moved along
the date bar. The date bar has acenter line terminating in an
arrowhead by which the azimuth is set on the protractor. In
operation, the arrow points true north if all settings on the
instrument have been correctly made.
(2) There are aseries of date lines engraved on the date bar,
which represent the 10th, 20th, and last day of each month.
These date lines indicate the proper position for the outside
edges of the solar plate. Adjustment of the edge of the solar
plate in between the lines can be made for intermediate dates.
This setting compensates for the changing angle of the sun's
rays ip the earth, as the altitude of the sun changes from day
to day.
FIOUBE 2—The Date Bar.
(3) Along the outer edges of the date bar, there are aseries
of numbers. These numbers represent the equation of time or
the amount of time in minutes to be added or subtracted from
the mean solar time (the time at that instant on the time
3

MAINTENANCE MANUAL
meridian) according to the indicated sign, plus or minus, to
obtain apparent or true sun time. Each number is directly
opposite one end of aheavy line. The lines are opposite the
date lines which represent the date to which the correction
is applied. Intermediate values are obtained by adjustment
within the length of the line. For apart of the months of De
cember and January, the equation factors are given separately
opposite the appropriate date. The date bar is made for the
year 1943; however, the equation of time factors do not vary
more than afew seconds from year to year.
e. Solar Plate.—The solar plate (item 5) is an oval-shaped
plate mounted so that it will slide on the date bar. It has
marked on its surface aseries of elipses, one for each third
degree of latitude starting at the equator and running each way
to the north and south 45th latitude lines. These are crossed
by aseries of hyperbolas which represent local apparent time
(true sun time) from 6 a. m. to 6p. m. by quarter hour inter
vals. The order of the hour numbers is reversed in the north
and south latitudes. The solar plate indicates where to set
the shadow bar for any given time-latitude setting.
FIOUBE 3—The Solar Plate.
4

INSTALLATION
/. Shadow Bar.—The shadow bar (item 6) is astraight,
narrow bar pivoted on the shadow bar bearing on the pivot
bolt. It determines where the shadow cast by the gnomon rod
should fall when it is turned to the proper intersection of time
and latitude lines on the solar plate. Aspecial shadow bar
washer exerts apressure on the shadow bar, tending to hold it
in the position at which it is set.
g. Gnomon Rod. —The gnomon rod (item 8) is astraight
slender rod 6inches long. It is screwed into the head of the
pivot bolt. Its purpose is to cast anarrow shadow line on the
solar plate. It is equipped with aradium tip to obtain bear
ings at night.
ft. Sighting Rod. (Polaris) —The sighting rod used when
sighting on Polaris (north star) is aslender rod, 3inches long,
which screws into the end of the date bar opposite the arrow
head. It also has aradium tip for use at night. It is shorter
than the gnomon rod so that Polaris can be easily sighted in
the higher latitudes.
i. Sighting Rod. (foresights) —The sighting rod (item 9) is
used to obtain aforesight on ndistant object and is aduplicate
of the Gnomon rod. It is screwed into ahole in the gimbal
arm near the level vial.
/. Clamping Nut. —The clamping nut is alarge, star-shaped
nut which screws on the lower end of the pivot bolt. It is used
to clamp the solar plate and date bar in place after all settings
have been correctly made.
k. Carrying Case. —Acarrying case is provided in which to
store and transport the sun compass when it is not in use. Only
the gnomon and sighting rods need be disassembled from the
instrument to place it in the case.
SECTION III
INSTALLATION
1.MOUNTING.
a. The sun compass must be mounted away from shaded por
tions of the vehicle where the1 sunlight will fall on it at all
times and in such aposition that the driver or his assistant
can see the shadow cast on the solar plate by the gnomon rod.

MAINTENANCE MANUAL
On atank, it should be mounted outside the turret where the
tank commander can observe it when the hatch is open.
6. Abracket made of %in. x2in. strap iron, which can be
bolted to an appropriate place on the vehicle, provides asuit
able mount for the compass.
o. The compass must be mounted so that the 0-180 degree
line on the protractor is parallel to the center-line of the
vehicle, with the arrowhead at the 0degree mark pointing in
the direction of the forward travel of the vehicle. It is fas
tened securely to the mounting bracket, or directly to the
vehicle, by two bolts. If abracket is used, this furnishes a
means of permanently aligning the compass parallel to the
center line of the vehicle (see paragraph d). Unless the com
pass mount is permanently marked for alignment, it must be
realigned if it becomes necessary to remove the compass for
any reason.
d. Amethod of mounting and aligning the compass using a
mounting bracket follows:
(1) Select amounting position where the compass is readily
accessible to the operator and where sunlight will fall unob
structed on its surface.
(2) Install the strap iron mounting bracket perpendicular
to the center line of the vehicle.
(3) Drill two *%a in. diameter holes spaced 1% inches, cen
ter to center, in the mounting bracket, approximately perpen
dicular to the center line of the vehicle. There is only %z ad
justment due to enlarged holes in the compass lower support
member so that aline connecting the centers of these two
holes must be very nearly perpendicular to the center line
of the vehicle.
(4) Secure the compass lower support member to the mount
ing bracket with one bolt, so that the arrow on the protractor
points in the direction of forward travel.
(5) Measure the distance (C-B, figure 4) from the center of
the vehicle to the center of the compass.
(6) At some point (at least 6feet ahead of the vehicle)
again determine the center of the vehicle (point D).

INSTALLATION
(7) Mark off the distance C-B at point Dperpendiculur to
the center line of the vehicle (line C-D). This determines
point A.
(8) Stretch astring from point Ato the center of the com
pass (point B).
(9) Turn the compass until the string is directly over the
"0" degree mark on the protractor. CAUTIpN :Be sure the
string is tight and not thrown out of line by any object.
(10) Insert the second bolt and tighten them both securely.
Kecheck the alignment.
CENTtft-UNE-OE ,D
FIGURE 4—Mounting Diagram.
(11) After the compass is completely aligned, mark aline
on the mounting bracket, using one edge of the lower support
as aguide. This mark will be used as aguide to realign the
compass in reinstallation, if it should be removed.
(12) When not in use, the sun compass is stored in acarry
ing case provided. Only the gnomon and sighting rods need be
disassembled to place the compass in the case.

MAINTENANCE MANUAL
SECTION IV.
OPERATION
1. CONVERTING WATCH TIME TO SUN TIME (Apparent Time).
a. General. —Because the operating principle of the sun com
pass is based on determining direction from the shadow cast
by the sun, the time piece used must be set and maintained on
sun time. Sun time, called apparent time, is based on the move
ment of the sun which is found to be irregular throughout the
year while watch time is based on a standardized system of
regular time adopted by the countries of the world for con
venience's sake. To convert local watch time to local apparent
time (local sun time) it is necessary to perform aseries of time
changes and to fully understand the terms applied to them.
•6. Standard Time.
(1) Asystem of time called "standard time" has been
adopted by nearly all nations of the civilized world. The world
has been divided into 24 imaginary longitudinal sections called
time zones (see figure 6), each zone representing one hour of
the 24 hour day. Theoretically, these zones should be of the
same size and shape. However, referring to figure 6we see
that due to political division, some zones have very irregular
boundaries and have adopted times which are not standard for
the zone in which they theoretically should lie.
(2) The system of standard time was developed by taking
Greenwich, England as the prime meridian and establishing
24 different time meridians around the world, 15 degrees of
longitude apart, so that, standard times in zones would agree
with Greenwich in minutes and seconds but differ in hours by
whole numbers. That is, when it is 12:20 A.M. in Greenwich,
it is 1:20 A.M. in acity in the next time zone east. All the
clocks iif atime zone are set to agree with the time based on
the time meridian in the center of that zone. This is true if
the people within the zone have not adopted adifferent time for
economic reasons.
c. Changing from Fait to Standard Time.
(1) It happens that in many cases, for reasons of saving
daylight hours, many countries or zones within acountry have
adopted the standard time of the zone to the east of them.
8

OPERATION
That is, by setting clocks at 12 o'clock noon when it is 11 o'clock
by standard time. Sunrise and sunset can be made to come
one hour later by the clock. This time may be called "Daylight
Saving Time", "Wartime", or "Fast Time", and is usually
adopted during the summer months.
(2) The first step in changing from local watch time to local
apparent time (sun time) is to change the local watch time to
standard time. Determine from military or civil sources tin
time at which local clocks are set and adjust the watch used
with the sun compass to standard time.
d. Mean Solar Time.
(I) The standard system of time is based on mean solar time.
The real sun has an apparent motion around the earth which
due to the travel of the earth along its orbit, is non-uniform
from d:iy to day. Ordinary time pieces are not able to correct
MEAN SUN
.SLAR T/M£
S1ANDARD TIME
\2 00 I00
CROSS SECTION OF EARTH
FIUUBI 5—Comparison of Mean Solar Time and Standard Time.
9

MAINTENANCE MANUAL
(or this irregularity. So, in order to get days of equal length,
afictitious or mean sun is assumed to revolve around the earth
at auniform rate. Mean solar time is time based on this mean
sun. The real sun is either ahead of or behind the mean sun
by afew minutes. For this reason, mean solar time (sometimes
called civil time) cannot be directly determined by observation
on the sun, but is ameasure of time based on the fictitious
mean sun. It is noon for atime zone according to the stand
ard time system, when the fictitious mean sun is directly over
atime meridian (see figure 5).
(2) The section of the earth's surface in figure 5is 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 atime zone is 15 de
grees wide and represents one hour of time, each degree of
longitude represents 4minutes 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 afew
minutes ahead or behind the fictitious mean sun which has a
uniform rate of travel. Four times ayear 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
10

OPERATION
sun. This factor is expressed in minutes on the date bar of the
sun compass, opposite the appropriate date.
g. Changing Mean Solar Time to Local Apparent Time.
(1) When the equation of time is applied to the mean solar
time for our position, the result is the local apparent time (true
local sun time) for the position. The equation of time is ex
pressed in minutes as aplus or minus factor according to the
accompanying sign on the date bar. It should be added or sub
tracted to the mean solar time as the sign indicates. The time
piece, after the equation of time is applied is now set at local
apparent time and must be maintained at this time as long as
it is used with the sun compass in any one locality.
(2) Actually, the entire conversion process from local watch
time to local apparent time for alocality requires only afew
seconds when the required data is available. The data re
quired and an outline of the conversion process follows :
h. Outline of Time Conversion Process.
(1) Ascertain from military authorities that the timepiece
to be used with the sun compass is keeping accurate time in ac
cordance with the time adopted for use in this locality.
(a) Determine whether local time is the standard time for
the zone in which the locality lies, or whether afast or day
light saving time is used. If afast time is used, ascertain the
number of hours it is faster than the standard time, and cor
rect the time piece to standard time.
It is possible to determine standard time by time sig
nals sent out by many observatories either daily, hourly, or
sometimes, continuously every second or every other second ;
to various parts of the world for the purpose of giving accur
ate time. They are sent out by radio or over telegraph or tele
phone wires.
(2) Determine your position (your local meridian) to the
nearest degree from amap or from instruments.
(3) Locate the time meridian upon which local standard
time is based (see figure 6).
(4) If your local meridian (your position) is east of the
time meridian add four minutes for each degree to local stand
ard time. To each degree west of the time meridian subtract
11

The hour zoois are numbered at the top and bottom of the map
and in some of the irregular zones to clarify their position. The half
hour tones are numbered within their boundaries. At localities where
12