You will be aware that the earth casts a shadow. It is conical in shape and stretch way out into space, you will b see it when there is a lunar eclipse and the shadow passes over the surface of the moom. There are times when it is visible at dusk (perhaps also at dawn). Wait till the Sun goes down and look for a dark bandjust above the horizon. SEE it here at
http://www.atoptics.co.uk/atoptics/earshad.htm
click on the right hand arrow, top corner, for a panorama where you can see the the shadow is curved, of course it is! Can't wait for a clear evening when I'm away from the city to observe this.
Happy shadow hunting.