The problem with these relatively common visitors is that the orbits of different comets vary a great deal in size. Halley's Comet, for example, has an orbit of 76 years. The Hale-Bopp Comet, on the other hand, has an orbit of over 4,000 years. If you missed it when it last skirted the earth in 1995, you'll have to wait another 4,000 years.
問題是，彗星週期有長有短，哈雷彗星七十六年繞一圈，一九九七年繞道地球的海爾波普彗星，則花了四千多年，這回沒瞧見，就要再等四千年，看彗星可真要好好把握良機。