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www.timeanddate.com/time/earth-rotation.html

Modern timekeeping defines a day as the sum of 24 hours – but it's not that simple. Earth's rotation slows down over time. See the exact day length for today, yesterday, and tomorrow.

www.quora.com/If-one-day-is-not-exactly-24-hours-and-is-in-fact-23-hours-56...

The Earth rotates about its axis exactly once, or 360 degrees, in about 23 hours and 56 minutes. This is called a sidereal day. The 24-hour day we all know and love is called a solar day. It represents a rotation of approximately 361 degrees about...

www.answers.com/Q/How_many_hours_exactly_are_in_a_day

A day is defined by the scientific community as 86400 seconds (which also happens to be 24 hours). A solar day is not exactly 24 hours which is why leap seconds are added to certain days.

en.wikipedia.org/wiki/Day

24 hours (exactly) The full day covering both the dark and light periods, beginning from the start of the dark period or from a point near the middle of the dark period; A full dark and light period, sometimes called a nychthemeron in English, from the Greek for night-day; or more colloquially the term 24 hours. In other languages, 24 hours is ...

blog.keithw.org/2013/02/q-if-one-day-is-not-exactly-24-hours.html

You're right that a "sidereal" day is about 23 hours, 56 minutes, 4 seconds. But this is not a day in the everyday sense. A sidereal day is how long it takes the earth (on average) to make one rotation relative to the faraway stars and other galaxies in the sky.

medium.com/the-philipendium/a-day-is-not-24-hours-c36ee96078c6

Our system of telling time is based on the premise that every day is exactly 24 hours long — quite precisely, with no exceptions. This concept is fully ingrained into our culture, a core ...

www.universetoday.com/123218/how-long-is-a-day-on-earth-2

How Long is a Day on Earth? I’m going to ask you how long a day is on Earth, and you’re going to get the haunting suspicion that this is a trap. Your instincts are right, it’s a trap!

answers.yahoo.com/question/index?qid=20080429072401AA8SQWU

How many EXACT hours and minutes are in a regular day? ... a day was divided into exactly 24 hours, each hour divided into 60 minutes, which were divided into 60 seconds. Now, however, with SI standards, a second is defined "neutral" of Earth; it's defined in terms of the time it takes for a certain isomptope to decay. ... How many EXACT hours ...

www.grc.nasa.gov/.../Numbers/Math/Mathematical_Thinking/calendar_calculations.htm

Calendar Calculations . The tropical year is the period of time required by the sun to pass from vernal equinox to vernal equinox. It is equal to 365 days, 5 hours, 48 minutes, and 46 seconds, or 365.2422 days. The tropical year is used to keep track of seasons, planting, and harvesting.

pumas.gsfc.nasa.gov/files/04_21_97_1.pdf

under 1/4 of all years 366-day "leap" years. Exercise : Design a reasonable calendar for an imaginary planet. Your calendar will consist of a pattern of 366-day "leap" years and 365-day regular years that approximates your planet's average number of days per year.