Earth is a good conductor. It can be given almost an unlimited charge. Its potential is zero.
Study 40% syllabus and score up to 100% marks in JEE
Considering Earth's potential as 0 is nothing but an assumption.We assume that Earth is a collection of infinte negative charge and it can even supply more charges to it.As earth has such a huge amount of negative charge with capacity to be supplied with even more any other body will be positive w.r.t Earth.Hence we assume its potential to be zero.
Aliter
As Earth can store unlimited charge its capacitance is infinity
Q=CV
=>C=Q/V
As C is infinity V has to be 0.
Hope this solved your query
Regards
admission open for distance and regular courses from one of the best board from Maharashtra .