Day

A day (symbol d) is a unit of time equivalent to approximately 24 hours. It is not an SI unit but it is accepted for use with SI. The SI unit of time is the second. The word 'day' can also refer to the (roughly) half of the day that is not night, also known as 'daytime'. Both refer to a length of time. Within these meanings, several definitions can be distinguished. 'Day' may also refer to a day of the week or to a calendar date, as in answer to ... more

A day (symbol d) is a unit of time equivalent to approximately 24 hours. It is not an SI unit but it is accepted for use with SI. The SI unit of time is the second. The word 'day' can also refer to the (roughly) half of the day that is not night, also known as 'daytime'. Both refer to a length of time. Within these meanings, several definitions can be distinguished. 'Day' may also refer to a day of the week or to a calendar date, as in answer to the question "On which day?". The term comes from the Old English dæg, with similar terms common in all other Indo-European languages, such as Tag in German, dies in Latin, dydd in Welsh or dive in Sanskrit or even dag in Norwegian. A day is defined as 86,400 seconds. A day on the UTC time scale can include a negative or positive leap second, and can therefore have a length of 86,399 or 86,401 seconds. The International Bureau of Weights and Measures (BIPM) currently defines a second as … the duration of 9 192 631 770 periods of the radiation... less