A top-down view of the ecliptic plane that moves. Every position is solved live from Keplerian orbital elements, and the scale is genuinely linear — the inner planets are not enlarged to make the picture nicer. Pull back to watch Neptune complete a lap, or zoom in to see how visibly off-centre Mercury's ellipse really is.
What is an orrery?
An orrery is a scale model of the planets' motion around the Sun. This one computes the positions of the eight planets in real time from JPL's Keplerian orbital elements, keeps distances to a linear scale and looks down on the ecliptic plane from above; you can zoom, change the playback speed and jump to any retrograde episode.
An orrery shows planets orbiting; turn the coordinates with them and five fixed locations appear. The Lagrange-points lesson unfolds that rotating map.
Why do the outer planets orbit so slowly?
Because the farther a planet is from the Sun, the longer its orbit and the slower it moves along it — Kepler's third law, which says the square of the orbital period is proportional to the cube of the orbit's semi-major axis. Mercury goes round in 88 days and Neptune takes about 165 years, so when you play a few months the inner planets swing round most of a lap while Neptune barely moves.
Whenever an inner planet overtakes an outer one, the planet being overtaken slides backwards against the stars as seen from Earth. Why planets go retrograde — the retrograde lesson projects that reversal onto the background stars.
When Neptune fits on screen, Earth is almost a dot
Put all eight planets back at roughly honest distances and speeds, and the tidy row of textbook worlds immediately falls apart.
Textbooks squeeze the Solar System together
A neat row of eight planets is useful for learning names but badly distorts the gaps. At proportional orbital distances, everything inside Mars occupies a tiny central patch, while most of the displayed space lies beyond Jupiter.
The inner and outer views do not use different data. They are close and wide camera views of the same system: zoom in to separate Earth from Mars, then zoom out far enough to include Uranus and Neptune.
A Neptune year outlasts a human lifetime
Mercury circles the Sun in 88 days; Neptune takes about 165 years. Outer planets travel much longer paths and move more slowly, so a few simulated months can send inner planets far around while Neptune barely shifts.
Retrograde motion, opposition, and elongation all depend on these changing relative positions. The top-down view turns the vocabulary back into a practical question of which body is inside, which is faster, and how the directions line up.
It all comes down to how the planets trade places
Pick a day and pause, then see on which side of the Sun Earth, Mars and Jupiter sit. Play a few months forward: Earth soon overtakes Mars while Jupiter has crept only a short way ahead. That difference in speed directly changes the direction in which they appear from Earth.
Retrograde motion, opposition and greatest elongation are all about these relative positions. Back on this top-down view, the terms often reduce to who is on the inside track, who moves faster, and what angle the three bodies make.
About this diagram
Mercury's orbit is visibly not a circle, and the Sun is not at its centre. That is neither a drawing error nor deliberate exaggeration: Mercury's eccentricity is 0.206, and this is simply what solving Kepler's equation correctly produces. Detail that appears because the maths is right is worth more than any ornament.
In the outer view, the scatter of dots between Mars and Jupiter is the asteroid belt. The boundary between the two views is placed there because 1.52 AU to 5.20 AU is the largest relative gap anywhere in the solar system.
Time for one lap
At the current speed. Bold means it falls in the 8-40 second range that is comfortable to watch.
Retrograde timeline
Click any band to jump to that retrograde episode; the view switches to whichever one can actually resolve that planet.
Why some planets are only a ring
At this scale the entire orbit of Mercury · Venus · Earth · Mars falls inside the Sun's own disc — Earth's whole orbit is a few pixels across, so a dot there could not say where the planet is. Those planets are shown in the inner-system inset at the bottom right instead.
Read the scale bar before comparing speeds. The inner-system view separates Mercury through Mars so their relative motion is easy to follow; the outer-system view preserves the real distance to Neptune, compressing the four inner planets beside the Sun. Changing playback speed only advances the date faster. It does not alter an orbit or a planet's period.
Time for one lap
Planet
a (AU)
Time for one lap
Mercury
0.387 AU
0.24 yr
Venus
0.723 AU
0.62 yr
Earth
1 AU
1 yr
Mars
1.524 AU
1.88 yr
Jupiter
5.203 AU
11.86 yr
Saturn
9.537 AU
29.45 yr
Uranus
19.189 AU
84.02 yr
Neptune
30.07 AU
164.79 yr
FAQ
Why do the outer planets move so slowly in the orrery?
No single speed makes the whole solar system watchable at once. At 3 days per second Mercury laps in 29 seconds, while Neptune takes 5.6 hours. Getting Neptune into the comfortable range means running fast enough that Mercury sweeps most of a turn every frame. That conflict is not a failure of interface design — it is Kepler's third law, period growing as the 3/2 power of orbital radius, spread across a factor of about 680.
Is a backward jump at high playback speed true retrograde motion?
Which makes the speed control part of the content rather than a convenience. Once a planet moves more than 90° per frame it is redrawn as a ring. The rule is computed from the refresh rate actually measured on your display, not from an assumed 60 Hz — on a 30 Hz phone more planets become rings than on a 120 Hz screen.
What date range does this orrery cover?
The date range stops at 1800 and 2050 because that is the validity window of the element table itself, not a limit of the program. Going further needs a second set of elements carrying periodic correction terms — which the precession lesson will use.
Orbital elements from NASA/JPL, "Keplerian Elements for Approximate Positions of the Major Planets" (Standish & Williams, 1992), valid 1800–2050. Positions are geometric — no light-time or aberration — because this is a diagram of the ecliptic plane, not a simulation of a telescope view.
Interactive retrograde teaching tools have a long history: Nebraska (NAAP), Foothill AstroSims, the University of New Mexico, SimuFísica, NoA at the University of Fukui, Marble Cafe, and jsOrrery. None of their code is used here; the calculation and drawing are our own.