"Juxtapositions" of moon,stars and planets in the sky, What are "Juxtapositions" of the Moon, stars and planets called? When the Moon, stars and planets take exactly the same position after few or many years, how to describe the positions and what is called to describe the position?
There are several words to describe two celestial bodies being in the same place on the celestial sphere.

A conjunction is when two bodies seem to pass close to one another, as seen from the Earth. 

A transit is when a celestial  body passes between the observer (us) and a bigger body. For instance, Venus can transit in front of the Sun, when it passes between the Earth and the Sun.

An occultation is when a celestial body in the foreground hides a celestial body in the background. For instance, the Moon occults the stars it passes in front of. 

An eclipse designates what happens when one body passes in front of the other for an observer, as a result of a transit or an occultation. 

A syzygy is simply when three celestial bodies are roughly aligned. One of those three bodies can be the Earth, in which case, the two other bodies will either be in the same place on the celestial sphere (a conjunction) or at the antipodes.