Coordinate system for space I am high school student, I  have doubt if I have to locate a planet or any star in space, how do I locate it, because we have no perfect coordinate system, I have read in some books, that time is define as the one of the coordinate, in coordinate system, can we define something only by time as coordinate.
For locating objects in the sky, the horizontal and equatorial coordinate systems are commonly used. These systems describe the position of some object in the sky very well, but do not explain the position of the object in space (if you know the distance you know "where" the object is, but this is relative to the equatorial/horizontal plane and is tricky to wrap your head around).
If you want to precisely locate the position of some object in space relative to the ecliptic, the ecliptic coordinate system exists for that, and for locating deep sky objects, the galactic coordinate system is normally used, as it nicely describes positions of objects relative to our galactic disc.
The space with the fourth coordinate is used for relativistic models (it fits well into special relativity), but not for describing positions of objects (to see more about that, look up Minkowski space).