Why is distance in Space measured in lightyears?
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Space is big, so we need a big unit of measurement.
The distance light travels in a year is 9.46×10^15 m
If something is 100000 light-years across, would you want to hear that as 10^21 m? It just helps keep the numbers manageable and easier to measure.
We also have parsecs, which is 3.262 light years. A parsec is a shortened form of “parallax second of arc”
Parallax is when you determine the distance from something by looking at it from two angles. Hold out your thumb and close one eye. Take note of where your thumb is, and then switch eyes, and your thumb will have appeared to move. That’s parallax. The “baseline” in that case is the distance between your eyes.
For measuring in parsecs, we take two observations of the night sky 6 months apart. This gives us a baseline of the diameter of Earth’s orbit, 2 AU, or 300 million kilometers.
We divide the whole sky up into 360°, 1° is 60 minutes, and 1 minute is 60 seconds. (Minutes of arc and seconds of arc it you want to avoid confusion with time). This means 1 second is 1/3600 of a degree
If the object we are looking at between the two observations moves 1 second of arc, that means it’s 1 parsec away. If it moves .1 seconds of arc, that means it’s 10 parsecs away. At a certain point, we can’t use this to measure distance anymore, but it’s a large unit that helps us convey distance.
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