How to measure distances such as depth of sea level or diameter of Earth?
In a word, we measure things we cannot measure directly by indirect means. And there are many means for doing this. Here are just a few. We measure the distance to specific stars by comparing their relative magnitudes to their known absolute magnitudes for that kind of star. And when we know the stellar distances, we know the distances of the galaxies those stars reside in. We can also use trig to observe the so-called parallax of the closer stars and calculate their distances from those optical measurements. We measure the distance to reentry vehicles entering the Earth’s atmosphere by knowing the speed of a transmitted radar signal is always c; the speed of light, so the distance to the vehicle m is S = ct/2; where t is the measured time between transmission and reception of the signal back at the radar station. We measure the depth of water by knowing v, the speed of sound in the water, so the distance S = vt/2 where t is the time between a transmitted and received SONAR signal. (v
