Find a location given approximate latitude and longitude near the location

I have a gps-related Oracle problem that I can use some help with.

Here is the scenario that I am trying address in Oracle 8.1.7.4:

  I have the following tables with latitude and longitude stored in decimal
  degrees:

    1. Bus stops along with their exact latitude and longitude.
    2. Bus riders along with the approximate latitude and longitude of where
       they boarded a bus.  I cannot assume that I will have the exact
       coordinates, only a reasonably close approximation.

Can anyone help me with a solution that, given the latitude and longitude
where a rider boarded, will return the bus stop where they boarded?  The
coordinates where the rider boarded will be guaranteed to be within a specified distance of the bus stop latitude and longitude.

This problem is outside the day-to-day programming issues that I normally deal with and I am woefully deficient in the math department.  I'm totally at a loss as to where to begin, so specific calculations, formulas, sql and/or PL/SQL code would be welcome. In any case, ANY help or advice on how to proceed is much appreciated.

TIA