How did aristotle first prove the earth was round

Aristotle noticed that there were star constellations that could be seen in what is today the south of Europe that couldn't be seen further north, or more generally speaking that the night sky changed as one moved further away from the equator. Since that wouldn't be possible if the earth had a flat surface, he concluded that the earth had to be round. 

Amath expression is a collection of one or more terms, which are separated by the signs ( + , - , x , / ).example :   8+b8+b  or  a−3a−3in algebra, an equation is a mathematical statement with two expressions where, each expression is separated by an equal (=) sign. variables can be present on both the sides of the equality (=) sign and we use letters to denote such variables.example 1:   x=4+2x=4+2  , [here, we can observe that the variable is present only on one side of the equation].example 2:   2x+3=x+42x+3=x+4  , [here, we can observe that the variables are present on both side of the equation].