Science Puzzle
How Fast Are You Moving Right Now?
You're sitting perfectly still, reading this. Yet Earth spins once a day, it travels round the Sun once a year, and the Sun, taking Earth with it, travels round the centre of our galaxy, the Milky Way, once every 230 million years or so.
Which of these three motions carries you along fastest?
The Answer
The trip round the galaxy, by a long way. It's the slowest lap, but by far the biggest.
Earth's spin carries a point on the equator about 40,000 km in a day: roughly 1,670 km/h. Further from the equator the circle is smaller, so in London you're spinning at about 1,040 km/h.
Earth's orbit round the Sun is a loop of about 940 million km, covered in a year. That's about 107,000 km/h, roughly a hundred times faster than your spin in London.
The Sun lies about 26,000 light years from the centre of the Milky Way. Its lap is so enormous that, even taking around 230 million years, it has to travel at about 828,000 km/h. Earth has made only about 20 of these laps since it formed.
None of it can be felt, because you only feel changes in motion: speeding up, slowing down or turning. Every motion here is very nearly steady, and everything around you moves with you. Galileo made the same point nearly 400 years ago, with a ship sailing smoothly on a calm sea, where nobody below deck could tell whether it was moving.
The principle: Relative motion. Speed is always measured compared with something else, and steady motion at any speed feels exactly like standing still.