The Arcsine is the inverse of the sine.
Therefore, for the simple rightangled triangle below, the arcsine of the opposite side, o, divided by the hypotenuse, h, is equal to the angle θ.
I.e. for the triangle above,
The Excel ASIN function calculates the arcsine (i.e. the inverse sine) of a given number, and returns an angle, in radians, between π/2 and π/2.
The syntax of the function is:
Where the number argument is the value, between 1 and +1, for which you want to calculate the arcsine.
Converting from Radians to Degrees
If you want the angle returned by the Asin function to be expressed in degrees, you can convert it, using the Excel Degrees function:
The following spreadsheet shows the Excel Asin Function, used to calculate the arcsine of four different values.
Formulas:

Results:

Note that in cell A4, the Excel Degrees function is used to convert the calculated angle from radians to degrees.
For further information and examples of the Excel Asin function, see the Microsoft Office website.
If you get an error from the Excel Asin Function, this is likely to be one of the following:
#NUM!    Occurs if the supplied number argument is not in the range 1 to +1. 
#VALUE!    Occurs if the supplied number argument is nonnumeric. 