The celestial sphere is
a. a spherical galaxy, centered on the sun.
b. the star-sphere discovered by the greeks and other ancient civilizations which shows the physical location in space of the nearby stars.
b. the orb of the sun; a normal star.
d. the central spherical region of the milky way galaxy, dense with stars.
e. a useful illusion used to map the stars and other objects in the sky.

This is all you, but i can give you some topics that you could try. if it rained when you want to do something outdoors, it 'limits' that human activity. maybe you want to do an experiment that goes throughout the whole year and you have to have above freezing the entire year. this 'limits' your activity. i thought of those just off of the top of my head. something that gets to me that might be getting to you is you are thinking too hard about what to do. think simply, and if it is too simple, then bump it up just a little bit.