If #\delta# is the obliquity of the axial tilt then on the day of northern solstice the Sun will pass through the zenith at the time of meridian crossing at geographical points whose latitude are #\delta#. All the points in the northern hemisphere lying at this latitude mark the northern tropical boundary. Similarly on the day of southern solstice the Sun will pass through the zenith at the time of meridian crossing at a geographical points whose latitudes are #-\delta#. All the points in the southern hemisphere lying at this latitude mark the southern tropical boundary.
On Earth which has an axial tilt of #23.44^o# the northern tropical boundary lies at #+23.44^o (23.44^oN)# and the southern tropical bondary lies at #-23.44^o (23.44^oS)#.
