How fast are you moving through space?

Start with the ground under your feet. Earth turns once on its axis in a little less than a day. At the equator, that carries you around at 0.4651 kilometres per second. There, the spin alone has carried you 0 kilometres since you opened this page. Farther from the equator you ride a smaller circle, so you move more slowly. Halfway to a pole, the speed is about 0.329 kilometres per second. At the poles you would simply turn on the spot. You don't feel any of it. The motion is smooth and steady, and the air, the oceans and the room around you all move with you. We notice speed only when it changes.

The spin is the slowest of your motions. Earth also circles the Sun, at an average of 29.78 kilometres per second. That is fast enough to carry you about 2.6 million kilometres along the orbit every day. Earth's path is not quite a circle, so the speed changes through the year. It stays between 29 and 30 kilometres per second, fastest in early January when we are closest to the Sun. The orbit is so wide that a day's travel barely bends away from a straight line. One lap takes a year, so every birthday you have had marks another full circuit. And all the while, the Sun itself is on the move.

The Sun is one of the many stars circling the centre of the Milky Way. It carries Earth and every other planet with it, at about 247 kilometres per second. Since you opened this page, it has carried you 0 kilometres along that path. Astronomers measured this from the positions and motions of young star-forming regions across the galaxy. Part of it is the galaxy's own rotation at our distance, about 236 kilometres per second. The rest is the Sun's own drift. Even at that speed, the galaxy is so large that one lap takes about 203 million years. Since the Sun last stood where it is now, all of human history, and far more, has passed.

Is there a way to say how fast we move through the universe itself? There is a good stand-in: the cosmic microwave background. It is the faint glow left over from the young universe, and it fills the whole sky. Because we move through it, that glow looks a little warmer ahead of us and a little cooler behind. The Planck spacecraft measured the difference very precisely. It shows the Sun moving at 369.82 kilometres per second, toward the faint constellation Crater, near Leo. Earth's yearly orbit nudges that speed up and down a little, depending on which way we are heading at the time.

It is tempting to add these speeds together. Don't. Each one is measured against something different, and they point in different directions. Here is the clearest example. The centre of our galaxy moves against the microwave background at about 565 kilometres per second. The Sun moves at only 369.82. That is because our orbit around the galaxy currently runs partly against the galaxy's own motion. Part of the speed cancels out. The same is true closer to home. Earth's orbit turns us through every direction in a year, and the spin does the same in a day. To know how fast you are moving, first ask: compared with what?

Not everything is moving away. The Andromeda galaxy, our large neighbour, is heading toward the Milky Way at about 109.3 kilometres per second. Gravity is pulling the two galaxies together. That speed is measured between the galaxies themselves, not from the Sun, which is busy circling our own. Recent measurements of Andromeda's sideways motion have made a direct collision less certain than it once seemed. Speeds like these are hard to picture, because the night sky hardly seems to change. Whatever happens, you have never been still. Every walk, every flight and every trip you take rides on top of all of these motions at once.