system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 how did people in medieval times measure time at night?

In any medieval tv show/movie, the characters will be waking each other up at different points in the night for their watch shifts. On waking up, could someone tell if they've been woken up at the right time? How would they know when to wake up the next one?
gpt
Stars. Like the sun, the stars are "static" in the night sky while the earth rotates "under"them. Pick a star, track its position, and you have a relatively good timer.

Further, hourglasses and candle clocks existed too, so one could set an hourglass or have a candle specially designed to burn at a known rate such could be used to note the passage of time.