How Long Until Andromeda Fills the Entire Night Sky?

Andromeda is already in our sky. On a dark night it shows as a faint oval smudge, and long-exposure photographs reveal that it spans an area several times wider than the full Moon. This calming space documentary follows what would have to happen for it to fill the whole night sky. We look at why the eye sees so much less than a photograph, why coming closer does not make a galaxy look brighter, how astronomers measure its approach along our line of sight and its slow sideways drift, and how gradually its size would grow over billions of years. Nothing in this story happens quickly, and recent measurements have made the long-term future of the two galaxies less certain than it once seemed. What stays constant is the patience of the sky itself. Let the slow numbers settle, and drift off beneath a sky that is still, for now, very quiet.