numerics – Solve a transcendental equation with some parameter in Mathematica

I’m trying to solve a transcendental equation in Mathematica,

NSolve(a (1/
  2 (a Sqrt(1 - a^4/b^4 - a^2/b^2) + (
    b^2 ArcSin((a Sqrt(a^2 + b^2))/b^2))/Sqrt(a^2 + b^2)) + 
 1/2 (Y Sqrt(1 - (a^2 Y^2)/b^4 - Y^2/b^2) + (
    b^2 ArcSin((Sqrt(a^2 + b^2) Y)/b^2))/Sqrt(a^2 + b^2))) == mg/(2 B), Y)

But it didn’t work, NSolve::nsmet: This system cannot be solved with the methods available to NSolve.

Any suggestion for me to get a numerical solution or even an approximation?