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Coordinate System

  1. May 3, 2016 #1
    MENTOR note: moved from General Math hence no template

    What would be the Y-Axis if:

    X-Axis: theta=266.4 phi=-28.94
    Z-Axis: theta=192.85 phi=27.13

    where:
    theta=atan(Y/X)
    phi=asin(Z/R)

    My thinking, theta is +90 from X-Axis and phi is -90 from the Z-Axis.
    Is the Y-Axis theta=356.4 phi=-62.87?
     
    Last edited: May 3, 2016
  2. jcsd
  3. May 3, 2016 #2

    Ray Vickson

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    Are these supposed to be angles in a spherical coordinate system? If so, please specify precisely which convention you are using. There are two common, but different conventions: (1) ##\theta = ## angle between the ##z##-axis and the vector ##(x,y,z)##, ##\phi = ## angle from the positive ##x##-axis, with counterclockwise angles being positive (so ##\phi## = longitude, measured west to east and ##\theta## = latitude, measured down from the North pole); and (2) the roles of ##\theta## and ##\phi## are swapped from the previous use. Convention (1) is most common in Physics, while (2) is used a lot (but not universally) in Math.
     
  4. May 3, 2016 #3
    I thought:

    theta=atan(Y/X)
    phi=asin(Z/R)

    explained it.

    In spherical coordinates:
    theta is measured on the x-y plane from the x-axis.
    phi is measured upward from the x-y plane to the z-axis.
     
  5. May 3, 2016 #4

    Ray Vickson

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    The usual convention for a ##\phi## like yours would be ##\phi = \arccos(z/r)##, so ##\phi## would be latitude as measured down from the north pole; see, eg., the second figure in https://en.wikipedia.org/wiki/Spherical_coordinate_system or the diagram in http://mathworld.wolfram.com/SphericalCoordinates.html or http://tutorial.math.lamar.edu/Classes/CalcIII/SphericalCoords.aspx .

    In any case, if you can determine (in ##(x,y,z)##-space) the vectors ##\vec{e_X}## and ##\vec{e_Z}##, which are the unit vectors along the ##X## and ##Z## axes, you can take ##\vec{e_Y} = \vec{e_Z} \times \vec{e_X}## as the unit vector along the ##Y## axis. (Here, ##\times## denotes the vector cross-product.)
     
    Last edited: May 3, 2016
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