Just recall the basics of trigonometry, for a right-angled triangle
, where P denotes perpendicular, B denotes base and θ denotes angle between hypotenuse and base of triangle.
In your case, where you want to compute arctan(1/0), the length of base B of triangle becomes 0. As B tends to 0, the hypotenuse and perpendicular of triangle start becoming colinear, and θ tends to 90 degrees or 
(i.e 1.5708) radians.
(i.e 1.5708) radians.That's why you get 
