parallax

/ˌpɛrəˈlæks/

Other forms: parallaxes

Parallax is how an object's position seems to shift, the way a person appears to move when you alternate looking through a camera viewfinder and using just your eyes.

You can get an idea of what parallax is by closing first one eye and then the other; the object you're focused on seems to move slightly. Parallax also describes the way that objects appear different depending on where an observer is located. It's a concept that helps astronomers measure the distances between planets, stars, and moons. The root word is Greek, parallaxis, meaning "a change."

Definitions of parallax

1. noun

   the apparent displacement of an object as seen from two different points that are not on a line with the object
   types:

   annual parallax, heliocentric parallax

   the parallax of a celestial body using two points in the earth's orbit around the sun as the baseline

   diurnal parallax, geocentric parallax

   the parallax of a celestial body using two points on the surface of the earth as the earth rotates

   stellar parallax

   the heliocentric parallax of a star

   horizontal parallax

   the maximum parallax observed when the celestial body is at the horizon

   solar parallax

   the angle subtended by the mean equatorial radius of the Earth at a distance of one astronomical unit

   type of:

   optical phenomenon

   a physical phenomenon related to or involving light