This is the Astronomy Pic of the Day today and they explain it better than I could:
http://antwrp.gsfc.nasa.gov/apod/
I've seen this floating around in other places but haven't put it up here yet. But this one's special because we already made such a big deal out of the beautiful "earth at night" shot last year. This is a similar thing, done by the same people, only during the day. They only shot on cloud-free days...
It's interesting because even the most densely-populated bits are green.
The little dot just leaving Rome - is that guy from yesterday's IotD.