If a shadow was cast over the entire earth. We would die. How fast?
Well if the shadow was cast by something being directly in front of the
sun at its surface (that's a pretty darn big "something") then it would
probably take about 1 hour for everyone to die. 8 minutes for us to see
the sun get blocked out and for the light to stop hitting the earth.
Another 52 minutes for the entire atmosphere to cool... well freeze...
then everyone would die. The temperatures would plummet fast enough to
freeze a layer of ice over all the oceans in a few hours. People could
survive this if they had warning, although there would be no habitable
world to come back to.
You need to understand that, without any sunlight, the earth would
plummet to at least about -171º C. that's -279º F. What keeps us warm is
the fact that one side of the planet ALWAYS has sunlight. + we have
oceans and atmosphere to distribute heat. But if both sides of the
earths atmosphere had to be subjected to those temperatures, it would
simply freeze the more and more of the atmosphere until it got to the
level that you breath O2 at. The oceans would cool much too fast to
sustain any heat distribution and the ice caps would grow faster and
faster and faster until the earth is frozen over.
My estimate is about 1 hour for ground level temperatures to hit a critical life threatening level
