Question

why the lines of longitude are also called meridian​

Answer 1

Answer:

Because in Geographical sense, meridians are great circles which are not parallel to each other but intersect each other at the North and the South Poles. Same stands true of the longitudes. All longitudes are great circles which meet at the Poles.

Answer 2

Longitudes are known as meridians because in Geographical sense, meridians are great circles which are not parallel to each other but intersect each other at the North and the South Poles. Same stands true of the longitudes. All longitudes are great circles which meet at the Poles.