"These buried ice sheets that extend from the poles all the way down to 45 degrees or so (north and southlatitude) don't quite cover half of the planet, but (they) come close," he said.
Rosat's operating orbit meant it could have come down anywhere between 53 degrees north and southlatitude - a zone that encompasses the UK in the north and the tip of south America in the south.
The sun must be very high in the sky and it can not be seen at a latitude above 55degrees north (which includes Scotland and much of Canada) or below 55 degrees south.