How it works

A short, honest method.

SpaceTimes is an editorial almanac. The clocks are accurate enough to be interesting; they are not authoritative time. We tell you when we are simplifying.

The method

Every clock is propagated forward from the J2000.0 epoch — Jan 1, 2000, 12:00 UTC. We take the seconds elapsed since then, modulo each body's rotation period, and divide that result into 24 equal “body-local” hours. It is the same convention NASA's Mars Mission Control uses for sols. Year position uses each body's orbital period the same way. Retrograde rotators (Venus, Uranus, Pluto) are counted in their natural direction; we mark them in the fact sheet.

The procedure is intentionally simple. It will drift roughly 1% per decade against true ephemerides because we do not include perturbations from other bodies, axial precession, or relativistic corrections. For a few minutes' precision, fine. For navigation, please use JPL Horizons.

Sources

• Rotation, orbit, gravity, radius and tilt: NASA Planetary Fact Sheets.
• Rotational elements convention: IAU 2015 working group.
• Higher-precision ephemerides for cross-checks: JPL Horizons.
• Mars sol convention: NASA Mars Mission Control sol-keeping handbook.

What we don't model

• Axial precession (Earth's 26,000-year cycle and equivalents elsewhere).
• Tidal evolution of moons (Moon's recession from Earth, Phobos's spiral-in).
• Relativistic time dilation between gravity wells.
• Atmospheric refraction at sunrise/sunset on the sol arc.
• Eccentricity of orbits (we use mean orbital periods, not Kepler's equation).

Image credits

All planetary and lunar imagery is public-domain or Creative Commons sourced. Specific credits:

• Mercury — NASA / MESSENGER (CC0)
• Venus — NASA / Mariner 10 (public domain)
• Earth — NASA / Apollo 17 (public domain)
• Moon — Gregory H. Revera (CC BY-SA 3.0)
• Mars — ESA / MPS / OSIRIS (CC BY-SA 3.0 IGO)
• Jupiter — NASA / Hubble (public domain)
• Io — NASA Hubble (CC BY 2.0)
• Europa — NASA / JPL-Caltech / SETI (public domain)
• Saturn — NASA / Cassini (public domain)
• Titan — NASA Cassini imagery via razielabulafia (CC0)
• Enceladus — NASA Goddard (CC BY 2.0)
• Uranus — NASA / Voyager 2 (public domain)
• Neptune — NASA / Voyager 2 (public domain)
• Ceres — NASA Goddard / Dawn (CC BY 2.0)
• Pluto — NASA / New Horizons (CC0)
• Eris — artist's impression by tonynetone (CC BY 2.0)

Adding a new body

A body is a single record. Provide an id, name, kind (planet / moon / dwarf), parent if applicable, hue, rotation period in seconds (negative for retrograde), orbital period in Earth days, axial tilt, surface gravity, mean radius, moon count, blurb, fact list, and a square hero image. Drop it into the BODIES array; every page will pick it up automatically. Edit the array in the page widget HTML, or extract it to a single source file when the engine moves to a custom theme.