 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 smearing of the image of Venus by turbulence in the Earth 's atmosphere or imperfections in the viewing apparatus . 
