Why Are Longitudinal Lines Perpendicular to the Equator?

[prashant singh]

I think its a defination to have infinite longitudnal lines , if line are not perpendicular than the infinite point will not add up to give the circumferance of equator,
please help

 

[Svein]

Points on Earth are defined using a spherical coordinate system (https://en.wikipedia.org/wiki/Spherical_coordinate_system, look under In geography).