Final answer:
Stellar parallax is an apparent shift in the position of stars caused by Earth's movement in space, as observed from different positions in its orbit around the Sun.
Step-by-step explanation:
The stellar parallax appears because D) Earth moves in space.
Stellar parallax is the apparent shift in the direction of a star when viewed from two different positions in Earth's orbit around the Sun. As Earth revolves around the Sun, the angular position of a nearby star changes compared to more distant stars, which effectively serve as a backdrop. This parallax shift happens because of the change in the observer's viewpoint, which in this case is the movement of Earth along its orbit, providing a baseline of 1 Astronomical Unit (AU) between observations taken six months apart.
Astronomers have historically struggled to measure stellar parallax due to the vast distances to stars and the small angles of shift that occur. It wasn't until the 18th century, with the advent of telescopes capable of detecting minute shifts, that parallax could be measured. The first successful measurements were made in 1838, and technology such as the Hipparcos satellite and the Gaia mission have since greatly refined our ability to measure these tiny shifts and thus calculate the distances to stars using parallax. The units of measure for stellar distance include the light-year and the parsec, with 1 parsec being the distance to a star with a parallax of 1 arcsecond.