About the Scale Orrery
An interactive solar system with visible and true scale, the Moon, Jupiter's four large moons, dated eclipses, and a view from Earth toward the Sun. Move through time, choose a body, or compare the apparent and physical sizes.
Scale and time
Distance and radius share one unit: 15 scene units per astronomical unit. True scale leaves bodies at their physical proportions and adds fixed-size markers so small planets remain findable. Visible scale enlarges planets 1,000 times and the Sun 20 times. The date control and playback drive the scene through Julian Day.
The Earth-to-Sun view changes to true scale with a one-degree field of view. It opens on the 1882 transit of Venus; eclipse presets set the corresponding date and geometry.
How positions are computed
Planetary positions use the JPL Table 1 orbital elements and a Kepler solve. The Moon and Sun use the main terms described by Jean Meeus, with a delta-T correction. Io, Europa, Ganymede, and Callisto use orbits fitted against JPL Horizons reference dates. These are approximations, not spacecraft navigation products.
The planetary element set is intended for 1800–2050. Dates outside that interval are shown as illustrations with an in-app warning. Eclipse geometry is geocentric; it does not map a local path of totality.
Dated checks
The source repository includes an independent check.mjs harness and Horizons fixtures. Selected angular separations and distances from its published event checks:
| Event (UT) | Reference | Orrery |
|---|---|---|
| Venus transit, 1874-12-09 04:07 | 0.230° from Sun center | 0.231° |
| Venus transit, 1882-12-06 17:06 | 0.177° | 0.176° |
| Venus transit, 2012-06-06 01:29 | 0.154° | 0.154° |
| Mercury transit, 2019-11-11 15:20 | 0.021° | 0.023° |
| Jupiter–Saturn, 2020-12-21 18:20 | 0.10° apart | 0.11° |
| Mars closest, 2003-08-27 09:51 | 0.37272 AU | 0.37300 AU |
Sources and limits
The source code, technical README, and check harness document the calculations and fixtures. Planetary elements come from JPL Solar System Dynamics; the eclipse catalog is from NASA Eclipse Web Site. Earth and Moon textures are credited in the texture catalog.
Most planet surfaces are generated color maps, not photographic planetary maps. Jupiter's moons follow circular fitted orbits, so their transits and eclipses are not predicted here. Lunar eclipses appear in the readout without a rendered Earth shadow. Missions and the Earth-to-map transition are separate work and are not present in this instrument.