Where the planets actually are, right now
Not a diagram. Every position is worked out from the same orbital elements NASA publishes, solved fresh each frame, so the planets speed up as they swing past the Sun, and Pluto really does cut inside Neptune. Drag to turn it, scroll to zoom, click a planet. Both Voyagers and New Horizons are out there too, at today’s distances.
Click a planet to follow it; click empty space, press Esc, or tap its button again to let go. Keys 1–9 jump to a planet, space pauses, arrows scrub time. With reduced motion on it holds still, though you can still drag it about.
How it knows where the planets are
Every planet is described by six numbers: how big its orbit is, how squashed, how tilted, and three angles saying how that ellipse is oriented and where the planet sits along it. NASA publishes those six numbers for each planet, along with how fast each one drifts per century. That is the entire input.
The hard part is the last step. Knowing how far around the orbit a planet is on average is easy; knowing where it actually is means solving an equation that cannot be rearranged. You have to guess and correct, six times, every planet, every frame. That is what makes the motion honest: a planet on a squashed orbit genuinely races through perihelion and dawdles at the far end, because the maths says so and not because someone animated it that way.
The distances are a lie, and deliberately so
Neptune orbits seventy-eight times further out than Mercury. Draw that honestly and either the inner planets are a single pixel or Neptune is three streets away. So the radial distance is compressed, but only the distance, never the shape. Every ellipse keeps its true eccentricity and tilt, which is why Pluto still visibly crosses inside Neptune. Tick true distances and watch it stretch back out.
The probes are real too
Voyager 1, Voyager 2 and New Horizons ride a simpler model. Once a probe is past its last planet it travels in very nearly a straight line at very nearly constant speed, so each one is projected from a published mission milestone: Voyager 1 crossed 100 AU in 2006, Voyager 2 in 2012, and New Horizons passed Pluto at 32.9 AU in 2015. That lands them within about an AU of NASA's live trackers. Wind the clock back before those dates and they disappear rather than pretend: the model is not valid there, so nothing is drawn.
The bit worth clicking
Click a planet and it works out the closest that planet and Earth will come over the next forty years, by stepping both orbits forward three days at a time and keeping the smallest gap. Roughly five thousand steps, each solving that same equation twice. It is not looked up in a table. It is calculated when you click, which is why the date it gives you moves as the simulated date does.
Why build this?
The same reason as the clock and the liquid. Loading quickly, working on a phone and not falling over are the things that actually matter in a website and none of them photograph well. This does, and it is the same care. No libraries, no build step, about thirty kilobytes.