■

知り合いに、物理シュミレーションの結果を簡単に描画する方法は無いかと相談されていたので、Ruby/Tkでちょっと遊んでみた。（←仕事しれ）
中央にある円のまわりを小さい円が周るだけのたわいのないスクリプト。万有引力の式とか入れればちっとは面白いかも。

#!/usr/bin/env ruby

require "tk"

SCREEN_SIZE_X = 500
SCREEN_SIZE_Y = 500

class Planet
  attr_reader :origin_x, :origin_y, :radius
  def initialize(canvas, origin_x, origin_y, radius, fillcolor="white", linecolor="white")
    @canvas    = canvas
    @origin_x  = origin_x
    @origin_y  = origin_y
    @radius    = radius
    @fillcolor = fillcolor
    @linecolor = linecolor
    @shape = TkcOval.new(canvas, origin_x - radius, origin_y - radius,  
      origin_x + 2 * radius, origin_y + 2 * radius, "fill" => fillcolor, "outline" => linecolor)
end
	
  def coords(x, y)
    @shape.coords(x + @origin_y - @radius, y + @origin_y - @radius, x +
      @origin_y + @radius, y + @origin_y + @radius)
  end
end

TkCanvas.new { |c|
  width  SCREEN_SIZE_X
  height SCREEN_SIZE_Y
  TkcRectangle.new(c, 0, 0, SCREEN_SIZE_X, SCREEN_SIZE_Y, "fill" => "navy", "outline" => "navy")
  center_x = SCREEN_SIZE_X / 2
  center_y = SCREEN_SIZE_Y / 2
  $earth = Planet.new(c, center_x, center_y, 10, "blue", "lightblue")
  $moon  = Planet.new(c, center_x, center_y,  5, "yellow", "white")
  pack
}

def time_pass(n)
  x = 230 * Math.sin(Math::PI * n * 0.02)
  y = 230 * Math.cos(Math::PI * n * 0.02)
  $moon.coords(x, y)
end

def timer(n)
  Tk.after(50, proc {
    time_pass(n)
    timer(n + 1)
  })
end

timer(0) 

Tk.mainloop