The Baltimore Astronomical Society was founded on November 30, 1881. The Society decided to join the Maryland Academy of Sciences in May, 1900, as the Academy's Department of Astronomy so that, among other things, Society members could qualify for a reduced railroad fare to view the total eclipse of the sun on May 28, 1900, at Rocky Mount, North Carolina.
A more detailed history is available.
Currently, membership in the BAS is a free benefit of membership in the Maryland Science Center. Information about Membership rates is available on the Maryland Science Center Membership page. Be sure to mention that you wish to join the Baltimore Astronomical Society. Other benefits include free admission to the Science Center and Davis Planetarium, a special entrance to eliminate or reduce waiting in line, several free IMAX movie admissions, reciprocal free admission to more than 200 other paarticipating science museums around the country, and others too numerous to mention here.