Solar System Simulator
Eight planets, real relative speeds, and today's actual arrangement. Wind time forward, watch conjunctions form, read the numbers behind every planet — then see how much stuck with the quiz.
Eight questions, one per planet. Can you get all of them?
What is the Solar System Simulator?
This is a working scale model of our solar system that runs in your browser. All eight planets circle the Sun at their true relative speeds — Mercury laps the Sun four times while Earth goes around once, and Neptune barely creeps — and each planet starts from its real position: the model seeds every orbit from the planet's J2000 mean longitude, so the arrangement you see when the page loads roughly matches where the planets are in the real sky right now.
A simulator beats a static diagram because the solar system is fundamentally about motion. Textbook pictures freeze the planets into a neat line that never actually happens; here you can watch the inner planets whirl while the outer ones drift, speed the clock up to a year per second, and see for yourself why Mars only comes close to Earth every couple of years. It is the difference between reading about the machine and getting to turn its crank.
How to Use It
The Eight Planets
The numbers the simulator runs on. Distance is the semi-major axis in astronomical units (1 AU = the Earth–Sun distance, about 150 million km); the year is the orbital period; moon counts are the known totals, and they keep rising as surveys improve.
| Planet | Distance | Year | Day (rotation) | Diameter | Known moons |
|---|---|---|---|---|---|
| Mercury | 0.39 AU | 88 days | 58.6 days | 4,879 km | 0 |
| Venus | 0.72 AU | 225 days | 243 days (retrograde) | 12,104 km | 0 |
| Earth | 1.00 AU | 365 days | 23.9 hours | 12,756 km | 1 |
| Mars | 1.52 AU | 687 days | 24.6 hours | 6,792 km | 2 |
| Jupiter | 5.20 AU | 11.9 years | 9.9 hours | 142,984 km | 95 |
| Saturn | 9.54 AU | 29.5 years | 10.7 hours | 120,536 km | 274 |
| Uranus | 19.19 AU | 84 years | 17.2 hours (retrograde) | 51,118 km | 28 |
| Neptune | 30.07 AU | 165 years | 16.1 hours | 49,528 km | 16 |
Things to Try
A model you can steer rewards a little curiosity. Four experiments that each teach something real:
Watch Mercury lap everyone
Set the speed to a month per second and just watch the inner system. Mercury completes about four orbits for every one of Earth's — which is why it is so hard to catch in the real sky, and why its year is shorter than its own day. One Mercury rotation takes 58.6 Earth days; its year is only 88.
Hunt a conjunction
Since the model starts from today's real arrangement, wind time forward at a year per second and watch for moments when Jupiter and Saturn line up on the same side of the Sun. These great conjunctions happen roughly every 20 years — the 2020 one was the closest since 1623, and you can watch the next one form.
Feel the emptiness of the outer system
At a year per second, Neptune still needs nearly three minutes to finish one lap. Everything from the Sun to Mars fits in the middle of the model; the outer half is almost entirely nothing. And remember the distances here are compressed — at true scale, the gaps would push every planet off your screen.
Explain the Mars launch window
Watch Earth and Mars at a month per second. Earth catches up to Mars and overtakes it roughly every 26 months — and that overtaking moment is exactly when space agencies launch Mars missions, because the crossing distance is shortest. The launch window is not scheduling caution; it is orbital geometry, and you can see it.
Scale, Accuracy and Honest Limits
Every on-screen solar system lies about something, so here is exactly how this one lies. Orbital distances are compressed on a square-root scale and planet sizes on a logarithmic scale — without that, a screen wide enough to show Neptune's orbit would draw Earth smaller than a single pixel. Orbits are drawn as circles at the mean distance; real orbits are slightly elliptical, which matters a great deal to astronomers and very little to the shape of the dance.
The motion itself is the honest part. Each planet advances at its true mean angular speed, and the starting positions come from the J2000 epoch mean longitudes, so the arrangement tracks the real sky closely enough to watch real alignments form. What this model is not is an ephemeris: it ignores eccentricity, inclination and perturbations, so do not point a telescope with it. For a classroom, that trade — right rhythm, readable picture — is usually exactly the right one.
FAQ
Is the Solar System Simulator free?
Yes — the simulator is completely free and runs instantly in your browser. There is no download, no sign-up and no ads inside the model itself.
Are the planet positions real?
Approximately, yes. Each planet starts from its real J2000 mean longitude and advances at its true average orbital speed, so today's on-screen arrangement roughly matches where the planets really are. It is a mean-motion approximation — good enough to watch real conjunctions form, but not an ephemeris for telescopes or navigation.
Why are the distances and sizes not to scale?
A true-scale drawing would be almost entirely empty space — at the scale where Neptune fits on your screen, Earth would be smaller than a pixel. The simulator compresses orbital distances on a square-root scale and planet sizes on a logarithmic scale, which keeps the ordering and the sense of proportion while staying visible.
How long does each planet take to orbit the Sun?
Mercury 88 days, Venus 225 days, Earth 365 days, Mars 687 days, Jupiter just under 12 years, Saturn about 29 years, Uranus 84 years and Neptune 165 years. At the simulator's fastest speed a full Neptune orbit still takes almost three minutes of watching — which is itself a lesson in how big the outer solar system is.
Is my quiz score saved?
Yes. Your best planet-quiz score is stored in your browser's local storage on this device, so it is waiting for you next visit. No account is needed, and clearing your browser data resets it.