[amsat-bb] calculation of eclipse

Hi all!! How does one calculate the period and time of entry and exit of eclipse? I'm going to be helping a friend do an Arissat presentation for her class. Due to the fact of the battery failing, this gives a 15 minute hold before the power management allows transmission. So, for any chance of receiving the bird, it has to be in sunlight for 15 minutes (or so). I'd prefer a mathematical formula. Echo goes through some of the same trials. Thanks, Norm BTW: I am an engineering student, just throw the math my way.