oc·tant /ˈɑktənt/
 八分圓,八分儀,離角45度的位置


 Oc·tant n.
 1. Geom. The eighth part of a circle; an arc of 45 degrees.
 2. Astron. & Astrol. The position or aspect of a heavenly body, as the moon or a planet, when half way between conjunction, or opposition, and quadrature, or distant from another body 45 degrees.
 3. An instrument for measuring angles (generally called a quadrant), having an arc which measures up to 9O°, but being itself the eighth part of a circle.  Cf. Sextant.
 4. Math. & Crystallog. One of the eight parts into which a space is divided by three coordinate planes.

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 octant
      n : a measuring instrument for measuring angles to a celestial
          body; similar to a sextant but with 45 degree calibration