Physical processPlanetary systemsUpdated 2026-08-15

Orbits, Gravity, and Ephemerides

The difference between measured tracking data, a dynamical model, orbital elements, and a predicted state.

Evidence status

Cites 2 sources, none of which has been read

The sources are named, but none has been retrieved and read as part of building this page. Nothing here has been matched to a passage, so the citations show where a reader might look rather than what was checked.

Rely on this page for

Orientation: how the topic is organised, which terms matter, and where to start reading.

Do not rely on it for

A claim you intend to act on or repeat. Follow the cited material yourself first.

Working definition

An orbit is a model of relative motion under gravity and other forces, fitted to observations. An ephemeris is a time-indexed prediction of position or state derived from that model; orbital elements are one parameterization and are not immutable properties independent of epoch, frame, perturbations, or fit uncertainty.

What is measured

  • Angles, ranges, range rates, imaging, and spacecraft tracking constrain states.
  • Observation times and station locations define geometry.
  • Residuals compare measurements with a dynamical solution.

What is inferred

  • State vectors and orbital parameters are estimated from observations.
  • Numerical integration propagates states through perturbations.
  • Future close approaches carry covariance and model uncertainty.

Fact-layer dependency

The explanatory layer cannot rewrite these fields.

Fact schema →
time.utcInstanttime.ephemerisTimeScaleobserver.positionsubject.identifiersreference.originreference.framereference.epochreference.correctionscoordinates.valuescoordinates.uncertaintyprovenance.provider

What is observed

Angles, ranges, range rates, imaging, and spacecraft tracking constrain states. Observation times and station locations define geometry. Residuals compare measurements with a dynamical solution.

Calibrated measurementRestates source[1]

JPL Horizons produces ephemerides only after the target, observing center, time, reference frame, corrections, and output quantities are specified.

Boundary: Osculating elements change with epoch and perturbations.

How inference enters

State vectors and orbital parameters are estimated from observations. Numerical integration propagates states through perturbations. Future close approaches carry covariance and model uncertainty.

Model-dependentCombines sources[1][2]

Orbital states and predictions are fitted and propagated under a declared dynamical model.

Boundary: Relevant gravitating bodies and force models are included.

Limits and unresolved questions

Osculating elements change with epoch and perturbations. A predicted position is not a direct observation. Long-term uncertainty can grow nonlinearly.

Open questionMaha inference[2]

Ephemeris accuracy varies by object, observation coverage, force model, and prediction interval.

Boundary: Long-term uncertainty can grow nonlinearly.

Sources

Each source states both what it establishes and where its authority ends. Access dates record the last public verification.

  1. [1]Horizons System Manual · NASA/JPL Solar System Dynamics · accessed 2026-08-15

    Establishes: Solar-system ephemerides, object and observing-center identifiers, time inputs, reference frames, corrections, and output quantities.

    Boundary: A Horizons result is reproducible only when its full query and response provenance are retained; it does not explain physical formation or meaning.

  2. [2]Standards of Fundamental Astronomy · International Astronomical Union SOFA Board · accessed 2026-08-15

    Establishes: IAU-approved algorithms for astronomical time scales, Earth rotation, astrometry, and transformations between celestial reference systems.

    Boundary: SOFA defines algorithms, not observations; results still depend on the release, input data products, observer, and correction choices.

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Direct answer

  • An orbit is a model of relative motion under gravity and other forces, fitted to observations. An ephemeris is a time-indexed prediction of position or state derived from that model; orbital elements are one parameterization and are not immutable properties independent of epoch, frame, perturbations, or fit uncertainty.

Mechanism and method

  • Angles, ranges, range rates, imaging, and spacecraft tracking constrain states.
  • Observation times and station locations define geometry.
  • Residuals compare measurements with a dynamical solution.
  • State vectors and orbital parameters are estimated from observations.
  • Numerical integration propagates states through perturbations.
  • Future close approaches carry covariance and model uncertainty.

What is measured

  • Angles, ranges, range rates, imaging, and spacecraft tracking constrain states.
  • Observation times and station locations define geometry.
  • Residuals compare measurements with a dynamical solution.

Limitations

  • Osculating elements change with epoch and perturbations.
  • A predicted position is not a direct observation.
  • Long-term uncertainty can grow nonlinearly.

Boundaries declared by the cited sources

  • A Horizons result is reproducible only when its full query and response provenance are retained; it does not explain physical formation or meaning. (boundary declared by Horizons System Manual)
  • SOFA defines algorithms, not observations; results still depend on the release, input data products, observer, and correction choices. (boundary declared by Standards of Fundamental Astronomy)

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