Why you need instrument for making accurate scientific measurements?

Answer 1

Before the last millennium, most distance measurements were based on the lengths of body parts. The foot was the length of a foot, the inch was the width of the thumb at the cuticle, a mile was 1000 paces, and so on. The obvious problem is that those lengths vary, often greatly, from person to person. So when someone wrote, for example, ten feet, others who didn't know exactly how big the writer's feet were could not know precisely how long that was (although some still use units called feet and miles, they have since been standardized).

Similar problems existed with other types of measurements, like weight, brighness, etc. Eventually mankind recognized the need for international measurement standards. Today, for example, a meter means exactly the same distance to matter where you go in the world. When recording scientific observations, unless measurements are made using instruments that have been calibrated to the international standards, they are as meaningless as those of the guy using his foot thousands of years ago.