The Collaborative International Dictionary of English v.0.48:

Sothiac \So"thi*ac\, Sothic \Soth"ic\, a.
   Of or pertaining to Sothis, the Egyptian name for the Dog
   Star; taking its name from the Dog Star; canicular.
   [1913 Webster]

   Sothiac year, or Sothic year (Chronol.), the Egyptian
      year of 365 days and 6 hours, as distinguished from the
      Egyptian vague year, which contained 365 days. The Sothic
      period consists of 1,460 Sothic years, being equal to
      1,461 vague years. One of these periods ended in July, a.
      d. 139.
      [1913 Webster]