system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do we know instruments used in measuring the properties of distant objects (e.g. the mass of a star) are calibrated correctly?
gpt
Calibrating any measurement instrument is a combination of trial-and-error and testing agreement with mathematical models. If an instrument is measuring the thing it is designed to measure, doing this consistently, and getting results that make sense according to calculations about what they could/should measure based on how they work, then we would generally take them to be reasonable instruments. As our understanding grows, and as we encounter, analyze, and correct errors based on unexpected influences (e.g. human error, poor construction, degradation of the instrument, etc.), the idea of an acceptable instrument may change a bit, but the general idea is there.