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Major Topics - 14221_148
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Quartermaster 3 & 2 - Military manual for the Quartermaster rate
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Apparent and Mean Solar Time, Continued - 14221_150
Apparent Time and Mean Solar Time Background Information In this section of the course, we will discuss time in more abstract terms. We  will  look  at  how  time  is  measured,  some  basic  terms  and  definitions associated with time, time zones and time zone conversions, and how we convert time to arc and arc to time. The  instrument  for  measuring  time  is  a  timepiece.  Earth  itself  may  be considered  as  our  celestial  timepiece.  Each  complete  rotation  of  Earth  on its  axis  provides  a  unit  of  time  that  we  know  as  a  day.  Time  is  important to  you  because  of  its  relationship  to  longitude.  As  a  Quartermaster,  you will have to understand this relationship to do your job. The  Solar  Day The two types of time we will discuss here are: 1.  Apparent  solar  time. 2.  Mean  solar  time. You probably already know that the motion of the Sun and the stars around Earth is only apparent--an illusion created by the rotation of the Earth  itself.  Solar  time  is  based  upon  the  rotation  of  the  Earth  with respect to the Sun. The solar day is equal to one rotation of Earth relative to the Sun. Apparent  Time Apparent  solar  time  is  measured  upon  the  basis  of  the  apparent  motion  of the  real  Sun  (the  one  you  see  rise  and  set  every  day).  This  is  why  we  use the  term  apparent  when  we  measure  time  using  the  apparent  Sun.  When the  Sun  is  directly  over  our  local  meridian  (directly  overhead),  we  say  that it  is  noon,  local  apparent  time.  When  it  is  directly  over  the  meridian  that is 180° (on the opposite side of Earth) away from ours, it is midnight local apparent time. If Earth remained stationary in space, all the days reckoned by apparent time would be of the same length.    But Earth travels in an elliptical orbit around the Sun, and its speed relative to the Sun varies with its position in its  orbit.  Consequently,  the  time  required  for  a  complete  revolution  of Earth  on  its  axis,  although  constant  as  applied  to  points  on  Earth,  varies regarding  Earth  relative  to  the  Sun.  The  length  of  a  day  measured  by  a complete  revolution  of  Earth  with  regard  to  the  Sun,  also  varies.  For  this reason it is impractical for man-made timepieces to keep apparent time; another  solution  had  to  be  figured  to  account  for  these  unequal  lengths  of time. 5-3

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