The celestial sphere can be described as a

The celestial sphere is a fictional sphere to which all celestial objects can be projected.

Explanation:

At any time an observer can see only the half of celestial sphere because the other half lies under the horizon. The celestial sphere is the medium used for sphere astronomy. The celestial sphere does not have a defined magnitude. It can be infinite or at least really big.

d. i., ii., iii., and iv.

the value of the 4 in the hundreds place is 400 while the value of th e 4 in the tens place is 40

400 is way bigger than 40.