Final answer:
Parallax refers to the apparent shift in an object's position as the observer's viewpoint changes. In astronomy, stellar parallax is used to measure the distance to stars, based on the angular shift observed when viewing the star from different positions in Earth's orbit.
Step-by-step explanation:
Parallax is an effect where the apparent position of an object changes due to a shift in the observer's point of view. A well-known example of this phenomenon is stellar parallax, which is the shift in the apparent direction of a star caused by Earth's motion around the Sun. The ancient Greeks tried to observe parallax to measure distances to the stars, but the shifts were too subtle for them to detect because the stars were much farther away than they realized.
Astronomers measure parallax in terms of angles. Specifically, they define parallax as half the angle that a star appears to shift when observed from opposite sides of Earth's orbit. This definition helps to standardize measurements using the average distance between the Earth and the Sun, or 1 Astronomical Unit (AU) as a baseline instead of 2 AU.
Applying trigonometry, we can calculate distances in space. The smaller the observed parallax angle, the farther away the object is from us. This is why parallax is an invaluable tool in understanding the scale and structure of our universe.