To humans, a day is generally thought of as a 24 hour period, encompassing a day and a night. This concept is ingrained in our psyche as well as our biology. Even when thinking of the length of a day on another planet, we must hear it in "Earth Days." Modern cultures define a day as the 24 hours starting at midnight. There are actually 2 ways of determining the length of a day: by looking at the sun and by looking at a "fixed" star.


  • Sidereal day-

Length of hours

Babylonians and Egyptians...

Why Does A New Day Start at Midnight?

  • 1. In the bible it describes a day as "night, then day." so the Jews start their day at sundown the day before. So, sundown Friday night is the start of Saturday, the Jewish Sabbath.

However that gives a variable day. Sun down is not the same time through the year. If you switch from one day to the next in the middle of the day, like at noon, it makes record keeping hard. By having the day switch at midnight when most people are asleep, it allows you to have most of your waking period in the same day and you do not have to worry about the day switch while you are awake. The development of the clock put the final nail on when the day starts and ends. Noon is 12 o'clock with the hands straight up, like the sun is straight overhead. So making midnight, 12 hours later the time to switch from one day to the next was easy.

FYI the astronomers do it the other way. Their days switch at noon so ever night period of observation is included in the same "work day" for them.

  • 2.At roughly midnight we go from getting farther away from the sun to getting closer to it.

If we agree that noon is the high point of the day -- that point where the sun is at its zenith, then midnight is the time farthest from that point. Until midnight you can always get farther from noon. After midnight you're getting closer to noon.

--Kript2 04:15, 5 March 2009 (UTC)