球面上の大円 - DOSEIの日記法線が n := (θ, φ) = [sinθcosφ sinθsinφ cosθ]⊤ の大円をパラメタ形式で表すと, R(θ, φ) [cos(t) sin(t) 0]⊤ より, [x] [ cos(θ)cos(φ)cos(t)−sin(θ)sin(t) ] [y]=[ cos(θ)sin(φ)cos(t)+cos(φ)sin(t) ] [z] [−sin(θ)cos(t) ] Gnuplot で描画する例 set para th=pi/4 # 天頂角 ph=pi/9 # 方位角 set arrow to sin(th)*cos(ph), sin(th)*sin(ph), cos(th) # 法線ベクトル splot [0 to 2*pi] cos(th)*cos(ph)*cos(u)-sin(ph)*sin(u), \ cos(th)*sin(ph)*cos(u)+cos(ph)*si
