Gravity and orbits, in 3D

What the simulation shows

• The Sun and the eight planets going round it at their real relative speeds (labels the Sun, the Earth, Neptune).
• Close on the inner planets with trails: the closer a planet, the faster it moves (Mercury 47 km/s).
• Gravity as a dent in spacetime: a grid sags around the Sun and the planets ride its slope.
• Newton's cannon on a mountain: balls fired faster and faster each land farther away.
• At 7.9 km/s the ball falls all the way round the Earth and never lands: an orbit.
• At 11.2 km/s the ball escapes the Earth for good.
• The Earth and the Moon: the oceans bulge towards the Moon and on the far side, so most places get two high tides a day.
• Arrows of the Moon's pull on the Earth: longest on the near side, shortest on the far side; the difference stretches the oceans into two bulges.
• The Sun replaced by a black hole: spacetime plunges into a deep well, a disk of glowing gas swirls in, the event horizon labelled.

The real numbers it runs on

The simulation is built on these facts, so the narration and the picture always agree:

• Every mass pulls every other mass; the Sun holds 99.8% of the solar system's mass.
• The closer a planet is to the Sun, the faster it moves: Mercury at 47 km/s takes 88 days a lap, the Earth 30 km/s and a year, Neptune 5.4 km/s and 165 years.
• Einstein showed gravity is the bending of space and time by mass; planets follow the curve.
• An orbit is falling while moving sideways so fast that the ground curves away as fast as you fall: near the Earth's surface that's about 7.9 km/s (28,000 km/h).
• To escape the Earth's gravity for good you need about 11.2 km/s (40,000 km/h).
• Astronauts in orbit feel weightless because they are falling all the time, not because there's no gravity: at the space station's height gravity is still about 90% as strong.
• The Moon's pull is strongest on the side of the Earth facing it and weakest on the far side, so the oceans bulge on both sides: about two high tides a day, every 12 hours 25 minutes.
• A black hole is mass squeezed so small that not even light can escape; its edge is the event horizon. If the Sun were squeezed into a ball about 6 km wide it would become one.
• On the Earth's surface gravity accelerates a falling object by 9.8 metres per second every second.

The script this video was made from

“Why the Moon never falls”: the narration of the video above. Each highlighted moment in the simulation happens on the word that names it.

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