I adovocate here a new spheric triangle on the celestial observation which is denoted by Opposite Meridian Zenith Distance (S), Zenith Distance (z) and Diagonal of a Square consisted of double Declinations and Hour Angle (H) on the Equator. Basing on this idea, the following cosine rule is produced : [numerical formula] By the formula, the author has gained a logarithmic nautical table with which nomogram is also drawn. I believe precisely that Zenith Distance at the moment is in proportion to Hour Angle corresponding to Meridian Zenith Distance (D) which is constant multiple, and that resolution into factors on the rule is very useful for the calculations on the sea, yet theoretical investigation is needed hereafter.