What is the term for the apparent shift of a star's position due to Earth's orbit?

The apparent shift of a star’s position caused by Earth’s orbit is called stellar parallax. As Earth moves from one side of its orbit to the other, a nearby star appears to shift slightly against the much more distant background stars.

This effect is a direct consequence of Earth’s changing viewpoint. Astronomers observe the star at different points in the year and use the known diameter of Earth’s orbit as a baseline. The smaller the measured parallax angle, the farther away the star is. A star with an annual parallax of one arcsecond lies one parsec away, or about 3.26 light-years.

Stellar parallax is easy to confuse with related terms. Proper motion is a star’s actual movement across the sky over time, while radial velocity describes motion toward or away from Earth, measured through Doppler shifts. Aberration of light is another apparent positional effect caused by Earth’s motion and the finite speed of light, but it is not a distance measurement in the same way.

Because the shift is tiny, reliable stellar-parallax measurements were not achieved until the nineteenth century. The method remains a fundamental way to measure nearby stars and calibrate the wider cosmic distance ladder.