How to play
It opens on the Solar System as it is today: the Sun, the eight planets, Pluto and ten big moons. Drag to move around, scroll or pinch to zoom, and tap anything to see its orbit, speed and distance. Follow keeps a body in the middle, so you can follow Earth and watch the Moon go round. The speed slider runs from an hour to two hundred years every second, Reverse runs time backwards, and Tilt turns the view so you can see orbits from the side.
To build, pick Add, choose a star, planet, moon, black hole or asteroid belt, and tap. A tap puts it on a round orbit around whatever holds that spot. Drag instead to throw it: the longer the drag, the faster it goes, and a dashed line shows where it will go before you let go. Move drags things to a new place, Erase removes them, and a selected body can be renamed, made heavier or lighter, or put on a round orbit. Your system is kept on this device, and Save keeps up to 12 named ones.
Things to try
Make a black hole. Drop one in the middle of the inner planets and watch them get flung around or swallowed.
Make the Sun heavier. Select the Sun and slide its mass up: the planets speed up and move inwards. Make it lighter and they drift outwards or fly off.
Throw a planet. A short drag gives a slow, stretched orbit that dips close in. A drag of about 80 pixels is round-orbit speed, and about 1.4 times that is escape speed: it leaves and never comes back.
Run time backwards. Gravity works the same both ways, so after a while in reverse and then forward again, everything is back where it started (unless something crashed).
Look at the scenes. Jupiter's moons Io, Europa and Ganymede go round in a 1 : 2 : 4 rhythm; the Trojan asteroids share Jupiter's orbit 60° ahead and behind; three equal stars can chase each other round one figure-eight path; and a passing star can throw Neptune out of the Solar System.
How accurate is it?
The planets start from NASA JPL's published orbits, worked out for today's date, so they are where they really are to within a fraction of a degree, and our Moon starts at its real place too (the full moons come out on the right nights). The masses and sizes of everything are the real ones. From then on nothing is scripted: every body pulls on every other body by Newton's law of gravity, in three dimensions, and the orbits come out of the sums. The method used (leapfrog) keeps energy steady, so orbits don't slowly spiral in or out; the Energy readout shows how well it is doing.
Some things are left out or simplified. Planets are treated as points, so there are no tides and no slow wobble from their flattened shapes, and there is no relativity, so Mercury's orbit doesn't turn the extra 43 arcseconds a century that Einstein explained. Moons other than ours start at made-up points along their real orbits. Mars' tiny moons and hundreds of smaller ones aren't included, because their fast orbits would make everything else slow down. Bodies that touch merge into one. Sizes are drawn much bigger than real so you can see things; turn on Real sizes to see how empty space really is. At high speeds a slower device can't keep up with the fast moons, and the speed label says how fast it is really going.
Is my system saved? Yes, on this device. Nothing is sent anywhere.