Question

why do we call moon as maternal uncle

Answer 1

​​
​As we know in Indian tradition any grown up out side the family is always referred as uncle (Mama) and that's what most of the mothers teach their kids to do with respect to Moon. So it's normal that most of the Indians refer to moon as uncle.
​As we know in Indian tradition any grown up out side the family is always referred as uncle (Mama) and that's what most of the mothers teach their kids to do with respect to Moon. So it's normal that most of the Indians refer to moon as uncle.

Answer 2

It's all because of mythological stories. We call our earth as the mother earth. Moon is earth's natural satellite so as it is always with the earth like a good protector like as it's brother. So we call the moon chanda mama ( maternal uncle) as it being like the brother of our mother earth.
Thanks! :-)