During a total solar eclipse, the Sun and the Moon give us the impression of being the same size and the size of the shadow around us (about 100 km diameter) all contribute to the impression of shrinking the space between the ground and the sun.
Earthlit space from orbit will also shrink the universe to our planet (almost half of everything around) , the sun and the moon while the earth would look like a giant globe.

But in the shadow, the Earth is very much like the rest of the universe and nothing is close enough (with enough light) to reassure the mind.

Perhaps that's what Franklin meant.