On my way to work in jamestown ca i was driving around lake tulloch in copperopolis ca on obyrnes ferry rd headed to hwy 108/120. as i was crossing the bridge, i looked to my right and was surprised to see a large, reflective, silver sphere up in the sky hovering over the lake. the glare off the surface was so bright that it could not have been a balloon or a plane. there were no wings on the craft and it made no noise. as i approached the "hair pin" curve it moved across the sky and disappeared.