(a) Equator : It is a great circle whose plane is perpendicular to axis of rotation of the earth. The equator lies in a east-west direction and divides the earth into northern and southern hemispheres.
(b) Earth Axis : It is a vertical line joining the two poles of the earth, lying perpendicular to the equator and passing through the centre of the earth.
(c) Meridian : A meridian is a half great circle joining the two poles. Every great circle joining the two poles is a meridian and its ante-meridian indicates the North-South direction. A meridian together with its ante meridian is a great circle.
