what are the two reasons for variation of g with latitude?
(1) The earth is not a perfect sphere. It is flat at poles and bulges out at equator. Thus a point at poles is closer to the dense core of earth than the point on equator. Hence the free-fall acceleration at sea level would increase as one proceeds from equator to poles.
(2) Earth is rotating. An object which is undergoing circular motion experiences centripetal acceleration in the radial direction and this depends on the radius.
Hence at equator this centripetal force which is in opposite direction to gravitational acceleration is more and almost nill at poles. Hece due to this effect, acceleration due to gravity is less at equator than that of at poles.