A frame of reference consists of an abstract coordinate system and the set of physical reference points that uniquely fix the coordinate system and standardize measurements. Speed or velocity is relative to a frame of reference. It is possible that two speeds/velocities are measured relative to a different frame of reference and thus cannot be directly compared.

Flat-Earthers would often compare two different speed measurements like an airplane moving at 900 km/h (560 mph) and Earth’s surface near the equator at 1674 km/h (1040 mph). They would conclude the airplane should not be able to catch up with Earth’s surface and would not be able to land if the Earth is rotating. They are wrong. The speeds are measured relative to the different frame of reference, and therefore, cannot be compared directly.

Likewise, on the airplane moving at 900 km/h (560 mph), we don’t have to run at the speed of 905 km/h (563 mph) to reach the lavatory on the front of the plane. At rest, we are already moving at the same speed as the airplane. We only have to walk and change our speed relative to the aircraft itself. Our speed is measured relative to the plane.

The spacecraft Apollo traveled at the maximum speed of 39600 km/h (24600 mph) while Earth is orbiting the Sun at 107000 km/h (66500 mph). Flat-Earthers would conclude that the spacecraft should not be able to fly back to Earth. They are wrong. The two speeds are measured to a different frame of reference. Earth orbiting the Sun at 107000 km/h is measured relative to the Sun. While the Apollo spacecraft moving at 39600 km/h is measured relative to the Earth. Before launch, the spacecraft already had the same speed as the Earth itself, or 107000 km/h orbiting the Sun.

## References

- The frame of reference – Wikipedia
- Earth’s orbit – Wikipedia
- Apollo 10 – Wikipedia