system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How was Kepler 452b spotted with a telescope when it's 1400 lightyears away and travelling would take +-20million years?
gpt
It passed in front of its star.  That dimmed the light from the star a tiny bit.  By measuring how long it took to pass in front of the star, we know how fast (angular velocity) it was going.  By measuring how much it dimmed the star, we know how big it is relative to its star.  Those two parameters are combined to know its size and distance from the star.