I have tried a couple of techniques, but so far nothing works to center text in an absolutely centered sphere. The size of the sphere is known, but not the length of the text. Here is an example that is lacking vertical alignment:
css:
div
{
position: absolute;
border-radius: 50%;
top: 50px;
width: 100px;
height: 100px;
background: yellow;
overflow: hidden;
vertical-align: middle;
text-align: center;
}