JPL Horizons, Ephemerides, and the Observer in the Sky

An ephemeris answers a deceptively simple question: where is this object, as a function of time and reference point?

Horizons is built around target, time, and center

JPL Horizons provides ephemerides for planets, moons, small bodies, spacecraft, and other targets. Its output can be defined relative to an observer or coordinate center, including topocentric points on Earth.

Sources: NASA/JPL — Horizons System Manual

Observer tables are directly relevant to personal charts

Horizons can provide quantities useful to an observer, including right ascension, declination, azimuth, elevation, lighting, and apparent motion, depending on the requested table settings.

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

Reference frames should not be hand-waved

“Where is Mars?” is incomplete without a time, reference frame, and observer definition. Good chart systems make those choices explicit internally even when the user interface stays simple.