# what are various ways cultures calculated/estimate days (24 hour periods)

I have heard of sunset to sunset calculations of a description of a day..
then of course, the universe midnight to midnight..

are there are more variations of how days are calculated/measured/tracked? (start and end of full 24 hr period)
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Commented:
Julian day numbers are counted from noon GMT
Then there's sidereal time.
Commented:
Noon to noon is often used for navigation. It is more constant than sunset to sunset
Commented:
(assuming that constancy is desireable.  Some pre-industrial cultures had variable hour lengths)
Commented:
Your title mentions cultures and calculated. The period which we know as 24 hours was not invented last week. It's been in place for centuries as the Egyptians were responsible for the 24 hour day. They counted in base twelve so 12 hours for day and 12 for night. Sunset and sunrise depending on where you are alters the 12 hours for day and night. Where we live sunset is technically 7:10pm but we can still do outside activities for longer.
Sundials have been found that show the Egyptians used 12 hour periods.
We in the USA may divide the 24 hours a different way. An example would be 12 hours of darkness, 2 hours for the morning and late afternoon twilight periods, and 10 for darkness.
Hours did not have a fixed length until a much later period than the Egyptians when it became useful.
The hours changed in length with the seasons.
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Take Ireland as an example. Look here for the month of October. The length of the day shown by the day length column is 11hours and 36 minutes for October 1st. Then on October 31 the day length is down to only 9 hours and 34 minutes.
http://www.timeanddate.com/sun/ireland/dublin

In 1884 a conference at Greenwich reached agreement on global time measurement and adopted Greenwich Mean Time as the international standard. That is the standard today.
Here is the history of time measurement:
https://nrich.maths.org/6070

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