What am I looking at?

• Gravity. Every body pulls on every other body with Newton's law of gravitation, every step. Nothing is on rails: a planet goes round because the Sun pulls it, not because it was told to.
• Masses of the Sun, the planets, Pluto and the Moon.
• Axes. Every planet spins around its real axis, so Earth's northern summer falls in June and Saturn's rings are seen from Earth as they really are. The big moons orbit around their planet's equator.
• Starting positions. The simulation starts at today's date, with every body roughly where it really is now (from NASA JPL orbital elements).
• Distances between the planets. Orbits are drawn to scale relative to each other: Mars really is about 1.5 times as far from the Sun as Earth, Neptune about 30 times.
• Orbital periods and speeds. Earth takes a year, Mercury 88 days. The speeds in the info panel are real km/s.
• Collisions happen at the real sizes of the bodies. Mass and momentum are conserved when two bodies merge.

• Sizes of the bodies. Everything is drawn hundreds of times too big, and small bodies are blown up more than big ones. At real size Earth would be a tiny fraction of a pixel. The "Planet size" slider changes how much.
• Size ratios. In reality the Sun is 109 times as wide as Earth; here only about 8 times. Jupiter is really 10 times smaller than the Sun; here not even 3 times.
• Moons. They are drawn much further from their planet than they really are (our Moon about 30 times), otherwise they would disappear inside the oversized planet or Saturn's rings. Their order is kept. The physics uses the real distances.
• Things that seem to touch usually don't. Because everything is drawn so big, a planet can visually pass "through" another one or through the Sun without anything happening. Only a real collision, at real size, merges them.
• Time. Up to almost three years pass every second; set it with the Speed slider. "Backwards" (or R) runs it the other way: Newton's laws work just as well backwards, so the planets retrace their paths, very closely but not to the metre: after a century there and back the Moon is about two days off along its orbit. Only merged bodies stay merged.

Drawn versus real size, at the default planet size (11×):

• Pluto is a switch: a planet from 1930 to 2006, a dwarf planet since. Switched back on, it returns where it would be at that moment.
• Small moons, the asteroid belt, comets and the Kuiper belt. The big moons of Jupiter, Saturn, Uranus, Neptune and Pluto are optional (the "Moons" switch); where they start along their orbit is made up. Mars' two tiny moons are left out: Phobos goes round in under eight hours, which would slow the whole simulation down.
• Relativity, solar wind and radiation. Bodies are points with a mass and a radius; tides only count near a black hole.
• Collisions never shatter anything: two bodies simply merge into one.
• A "black hole" is a very heavy object drawn with a radius of about 450,000 km. The event horizon of a real one of ten solar masses would be about 60 km across. What it swallows follows from its tides: a body that comes within about a million km (Earth) to 1.5 million km (the Sun, Jupiter) is torn apart, and that is where it disappears.
• Starting positions are good to about a tenth of a degree, the Moon to half a degree: the small pulls of the planets on each other since 2000 are ignored when placing them.

• Solid trails show where a body has been during its last orbit. The oldest part fades out.
• Dashed lines with a see-through ghost show where a body would have been if you had not launched or changed anything. Switch them on with "Original orbits"; they appear once a body is visibly pushed off course.
• The orange dotted line while aiming predicts the path of what you are about to launch, about three years ahead.

• Press L or "Launch mode", then drag on the plane of the orbits. The arrow sets the direction, its length the speed.
• Click without dragging to put the new body in a perfectly round orbit around whatever it is closest to (usually the Sun). Untick that option to use the speed and direction you typed instead.
• Drag an existing body to move it. Time stands still while you hold it. It keeps its speed and direction relative to what it orbited, and its moons come along. They stay with their planet when you change its speed or mass too. A moon moves around its planet, at the real distance that belongs to where it is drawn. When you let go, a window lets you change its mass, speed and direction, shows the dotted path ahead, and offers the speed for a round orbit around whatever it orbits there, so it does not fly off or fall in. Too far from its planet, a moon gets pulled away by the Sun anyway: a planet only holds on to orbits within about half of its Hill sphere (for Earth some 750,000 km).
• Speed and direction are measured relative to that nearby body. Direction 0° means "along with the orbit", 90° straight outward, 180° against the orbit, 270° straight inward.

The scenarios, in the collapsible block at the top of the launch panel, start from today's solar system and add one disaster, from a passing star to a stopped Earth. They are thought experiments, but the physics that follows is the same as everywhere else in the app. Two of them are good questions on their own: a stopped Earth falls into the Sun in about 65 days, and if the Sun lost half its mass at once, a planet on a round orbit would have exactly enough speed to leave. One is not a disaster at all: the Sun as a close binary star, with the planets going round both stars as if nothing changed. That only works when the stars are much closer together than the innermost planet. Another looks mad but holds: twelve Earths sharing one orbit, Saturn and Uranus riding along with Jupiter at the corners of an equilateral triangle with the Sun, Venus as a double planet and Mars going backwards. Five more go further: Jupiter and Saturn dancing round each other, a crown of ten Jupiters, orbits tipped every which way, the giants and the rocky planets swapped, and three suns chasing each other along a figure eight. And then four, eight, sixteen or thirty-two suns, in pairs of pairs of pairs: that holds as long as every level is a few times wider than the one inside it.

When a body is flung out of the solar system or destroyed in a collision, a message pops up and the event goes into the Events list in the launch panel. Click "Replay" or the event to watch it again in slow motion, from a few seconds before it until a few seconds after, then time runs at its old speed again. That works however long ago it happened; Reset and a new scenario start a fresh list. "Back to where you were" takes you back to the point you left.

A body counts as ejected once it no longer has enough pull from everything else to ever come back, and has moved well away (at least 10 AU, and twice as far as where it broke loose). The event is dated to the moment it broke loose. A moon that leaves together with its planet is not counted separately, and a visitor that came from outside cannot be ejected.

Normally the picture is centred on the balance point of the whole system, which sits close to the Sun. Pick another body under "Centre" (or select one and press C) to see everything as it moves from that body's point of view.

With Earth in the centre, the Sun circles Earth once a year and Mars and Venus trace loops: they seem to move backwards for a while. That is what astronomers saw for thousands of years, and why the old Earth-centred models needed circles on circles. Nothing about the physics changes; only the point of view does.