Animated scale model of the solar system with accurate orbital periods. Show planet positions for any date and track conjunctions.
This tool animates the Sun and its eight planets in a top-down view of the orbital plane, each moving at its real period: Mercury in 88 days, Mars in 687, Jupiter in 11.86 years. Set any date and the model places every planet at the position its orbit calls for, or run the animation and watch the inner planets overtake the outer ones. Conjunction tracking flags dates when two planets share nearly the same longitude. The surprise is that this maps the orbital plane, not the sky: it says nothing about what you would see from Earth, and sizes and distances cannot both be to scale.
A concrete input and expected output from the current implementation.
Input
2020-12-21
Expected output
Jupiter and Saturn sit almost on top of each other in the diagram. The tracker flags a conjunction: the Great Conjunction of December 2020, when the pair appeared about 0.1 degrees apart in the sky, their closest pairing since 1623. These two realign every 19.86 years, about 7,254 days.
Jupiter's 11.862-year and Saturn's 29.457-year sidereal periods give a synodic period of 1/(1/11.862 - 1/29.457) = 19.86 years, and December 21, 2020 was one such alignment. The 0.1 degrees is the separation as seen from Earth; in the top-down diagram the pair are nearly coincident.
Each planet moves at its mean sidereal period, the time for one full lap of the Sun: Mercury 87.97 days, Venus 224.7, Earth 365.25, Mars 686.98, Jupiter 4,333 days, Saturn 10,759, Uranus 30,687, Neptune 60,190. For the chosen date the tool counts the elapsed days and advances each planet's longitude by 360 degrees times the completed fraction of its period.
Two planets are in conjunction when they share the same heliocentric longitude, lining up with the Sun in the top-down view. The time between successive conjunctions, the synodic period, is 1/(1/P1 - 1/P2): about 116 days for Mercury against Earth, 780 for Mars, 399 for Jupiter, and 19.86 years for Jupiter and Saturn. The tool scans dates for such alignments and reports the pair and the date.
Not directly. The orrery shows orbital positions, while visibility from your yard depends on time of night, latitude, horizon, and glare from the Sun; a planet can sit near the Sun in the model and be unobservable for months. Use the diagram to see where a planet stands relative to the Sun, then check a sky chart for the observing details.
Because the periods are real and their spread is enormous: Mercury covers about 4.1 degrees of orbit per day, Neptune about 0.006 degrees. At any animation speed that makes Mercury's motion visible, the outer planets look frozen, which is accurate rather than a bug. Shift the date by years to see Neptune's slow drift.
The example and behavioral notes were checked against the browser implementation. Standards and primary references below define the relevant format, formula, or platform behavior.
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