When does the line between day and night become vertical? By which I mean, when does it run from the North Pole straight down to the South Pole?
During an ‘equinox’. The two equinoxes are times of year when day and night are as close to equal all over the globe as they ever get. The word ‘equinox’ itself derives from the words for ‘equal’ and ‘night’.
See the video below for a graphic demonstration of how this line changes over the course of the year. It is this angle that gives us our seasons and helps explain why the direction of sunrise and sunset changes so dramatically with the seasons.
With thanks to Leon Winnert and Astronomy Picture of the Day.