Physics methods

Each world computes real quantities and documents their accuracy. This page summarises the methods; the authoritative detail is in the per-world docs/*_PHYSICS.md files, which state the inputs, the formulas, and the bounds.

Earth

• Day/night terminator: computed from the Sun’s subsolar point for the chosen time (solar geometry), so the boundary is physically correct and scrubs smoothly through time.
• Point forecasts: read directly from Open-Meteo for the clicked coordinate.
• Wind field: advected from the latest NOAA GFS analysis.

Solar System

• Orbits: planet and moon positions from JPL Horizons ephemerides, not stylised circles.
• Mars atmosphere: the seasonal CO₂ cycle uses Viking-measured values.

Beyond

• Hubble’s law (galaxies): distances from measured redshifts via cz/H0 with an adopted H0 = 70 km/s/Mpc. This is the correct low-redshift linear law and is explicitly not valid for high-redshift cosmology. The depth axis is redshift-space (fingers-of-God distortion is present).
• Black holes: Schwarzschild radius, photon sphere, and ISCO from mass, with a lensing shader for illustration (labelled as illustrative).
• Neutron stars: real rotation frequencies drive the pulse timing; the audio is synthesised at the true frequency and is opt-in.

The accuracy discipline

Every physics note leads with an honesty statement: what is real, what is illustrative, and where the limits are. A representative example, from the galaxies note:

The map shows a real galaxy distribution computed with a real but low-redshift cosmology. The positions are correct for the map’s scale and not valid for high-redshift cosmology. The whole depth scale moves if a different H0 is chosen.

That is the standard: state the bound, do not overclaim. See The honesty mandate for the principle behind it.