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.
• 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.

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, tides, solar wind and radiation. Bodies are points with a mass and a radius.
• Collisions never shatter anything: two bodies simply merge into one.
• A "black hole" is just a very heavy object with a swallowing radius of about 450,000 km. The event horizon of a real one of ten solar masses would be about 60 km across.
• Starting positions are good to roughly 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. They appear once a body is visibly pushed off course. Turn them off with "Original orbits".
• 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.
• 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 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.

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.