 Unfortunately , it was impossible to time the exact moment of the start and end of the transit because of the phenomenon known as the " black drop effect " . This effect was long thought to be due to Venus ' thick atmosphere , and initially it was held to be the first real evidence that Venus had an atmosphere . However , recent studies demonstrate that it is an optical effect caused by the <unk> of the image of Venus by turbulence in the Earth 's atmosphere or imperfections in the viewing apparatus . 
