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Answer:
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.
Answer:
The answer is Tm= 2.pi.r / Vm and Tm is greater than Te.
Explanation:
V = Sqrt (GM/r)
Vm = sqrt (Gme/r)
The time taken by satellite,
Te = 2. pi. r / Ve
Tm = 2 . pi. r / Vm.
Hence, Tm is greater.
What you're looking for in here is the hardness: a material can scratch another material if it's harder than it.
So the desired material cannot be harder than amphibole, which has a hardness of 6.
But it has to be harder than fluorite, which has a hardness of 4.
So we're looking for a hardness of between 4 and 6 and from the table we see it's magnesite (we'll need a hard magesite) - this is the correct answer!
