## Direct and Indirect Measurements

Measurement can be direct, like measuring length directly with tape or a ruler. But it can also be indirect, like measuring distance by emitting a sound, listening for it, and measuring the delay the sound is received after being bounced back by an object.

Flat-Earthers like to discredit measurement results —like the distance to the Sun— by mentioning these are just results of calculation, not measured directly. In reality, many everyday instruments do measure indirectly. They measure a different value, then use calculations to get the desired calculation.

## Frame of Reference

A frame of reference consists of an abstract coordinate system and physical reference points that uniquely fix the coordinate system and standardize measurements. Speed or velocity is relative to a specific 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 like to 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 land if the Earth is rotating. In reality, both the speeds are measured relative to a different frame of reference, and therefore, cannot be compared directly.

## Human Perception of Speed and Acceleration

When aboard an airplane flying at cruising speed, we will not be able to feel that the plane is, in fact, moving at speed of more than 900 km/h. But if the plane changes its speed, turns or changes its altitude, we can easily feel it.

The same thing happens with the motion of the Earth. Due to Earth’s rotation, the surface of the Earth is moving at 1656 km/h near the equator. We never feel it because the speed is constant, or in other words, the acceleration is zero.

## Velocity is Measured From Specific Frame of Reference

A widespread misunderstanding within the flat-Earth victims is related to velocity/speed. They seem to think velocity is absolute, while in reality it is measured from a specific frame of reference. Example:

“If the Earth is in motion around the Sun at 30 km/s, and Apollo traveled at only 11 km/s, then how could Apollo astronauts possibly make it back to the Earth?”

We used to measure velocity/speed relative to the surface of the Earth. But it is not always like that.