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    Why Sigma Octantis?
    From: Sean C
    Date: 2015 May 6, 09:14 -0700

    The United States Naval Observatory's "Multiyear Interactive Computer Almanac" (MICA) has the ability to use external catalogs of objects for its calculations. Several catalogs are included with the program, including a "Navigational Star catalog". The MICA help file says the following about this catalog:


    The MICA Navigational Star catalog contains the 59 bright stars commonly used for Celestial Navigation. The stars are the same ones as listed in The Nautical Almanac with the addition of Polaris and Sigma Octantis.


    The reason for the inclusion of Polaris is obvious to me, but why include Sigma Octantis, as well? It seems to be a rather inconspicuous star with an apparent magnitude of 5.5 (Stellarium gives 5.45). In terms of the navigational stars, Sigma Octantis does occupy a relatively empty area between Achernar, Atria and Miaplacidus, but is relatively close to Miaplacidus and Atria. Additionally, there is a similarly "empty" area between Alphard, Suhail and Gienah, in which no additional stars have been added. Nu Hydra is in this area and has an apparent magnitude of 3.1. Including this star would at least bring the number of stars in the catalog to an even 60. So, what's so special about Sigma Octantis?


    Thanks in advance for any insights.


    -Sean C.

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