What causes the systematic difference in visible stars between the winter sky and the summer sky as observed from Earth?

The Earth's changing direction of view as it travels around the Sun throughout the year.

The Earth is not stationary relative to the rest of the galaxy; it is constantly moving in its orbit around the Sun. This orbital motion dictates the direction the planet is facing into space at any given time. When the Earth is on one side of its orbit (e.g., during the winter months), our nighttime view is directed toward one section of the galaxy, revealing a specific set of stars. Six months later, when the Earth is on the opposite side of its orbit, the planet is looking in a nearly opposite direction, exposing an entirely different population of stars. This systematic change over the year ensures that only the stars in one hemisphere are visible on any single night.