Space Station Question

A student asked asked a wonderful question yesterday while I was mentioning the International Space Station (ISS) will be visible for 6 minutes Monday morning (1/16/12) at 6:29.  He asked why will it be visible at 4:55 am as well.  He, as well as most people, was not aware that it travels around the earth in approximately 1.5 hrs.  The ISS has a velocity of 27,743 km/hr or 17,239 mph.  I am sure he learned something that day.