Why Planets Move Differently From the Background Stars

The sky is not one fixed layer. Stars and solar-system bodies have very different apparent motions.

Planets are nearby moving targets

Planets orbit the Sun while Earth is moving too, so their apparent positions against the background stars change noticeably over days, weeks, and months. Accurate charts use an ephemeris tied to the requested time.

Sources: NASA/JPL — Horizons System Manual

Observer location can matter for nearby bodies

Topocentric calculations refer positions to an observer on the surface of a rotating body. JPL Horizons can generate observer-based quantities, including azimuth and elevation, for specified locations.

Sources: NASA/JPL — Horizons System Manual · NASA/JPL — Horizons Tutorial

Stars also move, just on different scales

Stellar catalogs include position, parallax, and proper motion. Gaia DR3 provides astrometric solutions for roughly 1.46 billion sources, which illustrates how “fixed stars” are actually measured as moving objects with their own uncertainties.

Sources: ESA Gaia — Data Release 3