astronomical day
noun
A day of 24 hours, especially (until 31 December 1924) one beginning at noon in the middle of the corresponding civil day; (after that date) one beginning at the previous midnight, as with the civil day.
- There were proposals in the 1880s to change the start of the astronomical day from noon to the preceding midnight, but these were not internationally agreed until 1924..
Origin
Mid 17th century; earliest use found in George Walker (d. 1690), Church of Ireland clergyman.