Galileo Draft Letter 1609 — Although draft of an August letter, in the late 1970s was it determined that the "doodles" on the lower half depict the positions of the Galilean moons on the nights in January when Galileo first observed them, thus proving that this document contains the original notes he took on the nights he made his observations.
Photo Credit: By Galileo Galilei - http://www.lib.umich.edu/special-collections-library/galileo-manuscript, Public Domain, https://commons.wikimedia.org/w/index.php?curid=30255933