每個小時都有相互垂直的時候。

以12點為起點，此時時針分針位於同一位置，用分做單位。

時針旋轉速度=360/（12×60）=6°/12=（1/2）°/分

分針的速度=360/60=6°/分

t（分）時刻，時針轉過的角度=（t/2）°

分針轉過的角度=（6t）°

兩者角度差

y=（6t）°-（t/2）°=5.5t（°）

相互垂直：5.5t=（2k-1）90°

t=k（90/5.5）=（2k-1）（180/11）

2k-1=11t/180

t1=180/11=16.36min=0:16.36

t2=3×180/11=540/11=49.09min=0:49.09

t3=5×180/11=81.82min=1:21.82

t4=7×180/11=114.55min=1:54.55

t5=9×180/11=147.27min=2:27.27

t6=11×180/11=180min=3:00

t7=13×180/11=212.73min=3:32.73

t8=15×180/11=245.45min=4:05.45

t9=17×180/11=278.18=4:38.18

t10=19×180/11=310.091min=5:10.91

t11=21×180/11=343.64min=5:43.64

t12=23×180/11=376.36min=6:16.36

t13=25×180/11=409.09min=6:49.09

t14=27×180/11=441.82min=7:21.82

t15=29×180/11=474.55min=7:54.55

t16=31×180/11=507.27min=8:27.27

t17=33×180/11=540min=9:00

t18=35×180/11=572.73min=9:32.73

t19=37×180/11=605.45min=10:05.45

t20=39×180/11=638.18min=10:38.18

t21=41×180/11=670.91min=11:10.91

t22=43×180/11=703.64min=11:43.64