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Lost In Space? Just Use Relativity
One of the hardest things for many people to conceptualize when talking about how fast something is going is that they must ask, "Compared to what?" All motion only makes sense from a frame of reference, and many spacecraft traveling in the depths of the void lack any regular reference from which to understand how fast they're going. There have been several different techniques to try to solve this problem, but one of the ones that have been in development the longest is StarNAV - a way to navigate in space using only the stars.
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