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    Celestial navigation in the field artillery
    From: Gary LaPook
    Date: 2007 Aug 17, 02:41 -0700

    
    Here is another use of celestial techniques that you probably didn't
    know about.
    
    When firing long range cannons at targets many miles away that are
    not
    in sight you must calculate in exactly what direction you must point
    the
    guns to hit the target and at what elevation you have to launch the
    projectile for it to travel the exact distance to the target. You
    take
    into account many factors including powder temperature (warmer powder
    burns faster creating more pressure, raising muzzle velocity), bore
    erosion (each shot causes some of the bore to wear away allowing more
    powder gasses to escape around the projectile resulting in lower
    muzzle
    velocity),  the exact weight of the projectile, winds and air density
    at
    different altitude levels (some projectiles go up to 30,000 feet in
    altitude), the difference between the height of the guns and of the
    target above sea level, the fact that the trajectory is not rigid in
    space, that gyroscopic and aerodynamic forces working on the
    projectile
    cause it to drift to the right (with right hand rifling), and that
    the
    earth will turn an appreciable amount while the projectile is in
    flight
    which can be up to ninety seconds. It goes without saying that you
    must
    accurately know the location of the guns and the target for the
    computation.
    
    It is also necessary to determine a very accurate direction reference
    for the sighting equipment on the guns. The person in charge of
    positioning the guns uses an "aiming circle" which is an instrument
    much
    like a transit to do this. But first he must establish direction for
    the
    aiming circle and there are several ways to do this. The least
    accurate
    way is to use the built in compass but this is only accurate to about
    ten mils, about one half of a degree. A better way is for a survey
    team
    to come to the position which establishes the coordinates of the
    position and also a direction by the positioning of two stakes on a
    known azimuth as established by the survey party. This is good to one
    mil.
    
    There are also two methods which utilize celestial. One is called
    "simultaneous observation" in which personnel at battalion
    headquarters,
    at a prearranged time,  track the sun with an aiming circle while the
    three outlying firing batteries at three different locations (10 to
    15
    kilometers away) do the same while listening to battalion on the
    radio
    saying "tracking...tracking...tracking...tip" at which point you stop
    tracking the sun and deflect the line of sight down to ground level
    and
    direct an assistant to emplace a stake on that azimuth. The
    headquarters
    then informs you what that azimuth is that they have determined, most
    commonly by survey, or by celestial computation.
    
    Another method of establishing direction involving celestial involves
    using the aiming circle to observe polaris. You set up the aiming
    circle
    and set the recording scale to zero. You then sight on polaris using
    the
    non recording motion to move the line of sight right or left which
    allows the scale to remain set on zero. After you do this you use the
    recording motion to measure the azimuth of kochab in relationship to
    the
    zero point established with polaris. You then looked at a graph
    contained in Field Manual FM 6-50 which gave you the true azimuth of
    polaris based on the azimuth of kochab (actually the difference in
    the
    azimuth of polaris and kochab.) This allowed for the daily movement
    of
    polaris around the true pole. You then returned the aiming circle
    telescope to the zero point, deflected it downward, and had an
    assistant
    emplace a stake on that azimuth. The graph in FM 6-50 was calculated
    for
    the latitude of Germany (go figure) and for the 1970s. I don't
    remember
    the graph being updated to allow for the change in the coordinates of
    polaris.
    
    
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