What is the Earth's prenumbra?

Answer 1

There is no such thing as 'prenumbra'. I think you mean 'penumbra'.

When there is a solar eclipse, the Moon's shadow only falls on a narrow part of the Earth, its track. This is the point on Earth of totality, and is named the 'Umbra'.

Outside the track of of totality, there is a region on either side of the track, which is in partial shadow; a partial eclipse as seen from the Earth. This partial shadow is properly named the 'Penumbra'.

Answer 2

The "penumbra" of the earth's shadow consists of everywhere in space behind

the earth from which you could see part of the sun but not all of it, because the

earth itself covered part of the sun from your view.