Stand perfectly still and, by any honest accounting, you are moving at four very different speeds at once. The Earth Rotation Speedometer now reads all four, and the distance between them is one of the more humbling figures in everyday physics.
Begin with the speed in the name. The Earth turns at one angular rate, close to 15.04 degrees per hour, a single rotation per sidereal day, the same everywhere on the planet. The linear speed it produces depends on where you stand: the tangential velocity v = ω·R·cos(φ), set by your latitude φ, roughly 1,674 km/h at the equator and falling to zero at the poles. One rotation rate, a different speed through space at every latitude, and the gauge now carries a latitude control so you can watch it change.
- Rotation: up to about 1,674 km/h, the spin at your latitude.
- Orbit: about 107,000 km/h, Earth around the Sun.
- Galaxy: about 828,000 km/h, the Sun around the galactic centre.
- Great Attractor: about 2,160,000 km/h, the Local Group drifting relative to the cosmic microwave background.
Precision is the entire brand, so we hold our own instruments to it. A tool that calls itself a speedometer should be a real instrument: the correct equation underneath, the right caveats on top, and a needle that moves for an honest reason. The same standard runs through the research and the rest of the tools.
Switch channels and the gauge re-scales while its centre reshapes to match the frame: a spinning Earth for rotation, an orbiting Sun for the journey around it, a spiral for the galaxy, and a slow infall toward an unseen mass for the Great Attractor. Pick the wrong reference frame and "how fast are you going" has no single answer. Pick the right one and it has four.
Check your speed →