Shell balance in spherical coordinates

[natanreal]

I want to derive equation of continuity in spherical coordiantes based on shell balance,can anybody tell me where the hell this sin(theta) comes from?
i don't want to transform from cartesian,

 

[Astronuc]

It comes from the fact that one is projecting onto the x,y-plane from some angle (zenith angle) from the z-axis, as opposed to the azimuthal angle, which sweeps around the x,y plane.

http://en.wikipedia.org/wiki/Spherical_coordinates#Cartesian_coordinate_system

http://en.wikipedia.org/wiki/Image:Spherical_Coordinates.png


This is even better - http://hyperphysics.phy-astr.gsu.edu/hbase/sphc.html