Mathematica plotting sometimes distorts the shapes of curves or surfaces by using different lengths for a single unit on different axes:

Plot3D[ Sqrt[ 9 - x^2 - y^2], {x,-5,5}, {y,-5,5}]

Use the “BoxRatios -> Automatic” option to force Mathematica to use consistent distances on all axes.

Plot3D[ Sqrt[ 9 - x^2 - y^2], {x,-5,5}, {y,-5,5}, BoxRatios->Automatic]

You can also specify your own aspect ratio with “BoxRatios.”

Plot3D[ Sqrt[ 9 - x^2 - y^2], {x,-5,5}, {y,-5,5}, BoxRatios->{1,2,3}]