what is parallex and star trails?

Question

please give simple definitions

Answer 1

Parallax is the change in the apparent position of something if the observer's position changes. The easiest example is to close one eye & look at something on the wall. Now open that eye and close the other one -- see how the object "jumps"? THAT is parallax. As applied to "stars", it would be, for example the relative change in position of a star as seen from an observatory on one side of the earth, as opposed to one miles away.

Star trails are when you take pictures of the sky by leaving the shutter open for LONG periods of time. As the earth revolves, the stars seem to rotate through the sky above. If you take a photo of that with a long exposure, there will be enough light for that "movement" to show up as streaks from each of the stars. See the link below for some explanation & pictures.