Indoor growing journal
Noticing how light moves through a room
Observe at different hours
Look at the same windowsill in the morning, around midday and later in the afternoon. Sketch where direct sunlight falls and where the room stays shaded.
Keep a seasonal note
Repeat the observation when the season changes. Nearby buildings, trees and the position of the sun can change the light a room receives.
Use the record
Keep these notes with information about the particular plants you grow. Observe their response when you change a position, and avoid drawing conclusions from a single afternoon.