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A Day Is Not Exactly 24 Hours

Exact Day Length* — Sun, Mar 29, 2020

Today's prediction: 24 hours, 0 minutes, 0.0002809 seconds (0.2809 milliseconds)

Yesterday: 24 hours, 0 minutes, 0.0004287 seconds (0.4287 milliseconds)

At the start of today, UT1 was 0.2244489 seconds behind UTC.

Star trails over an observatory.

The Earth's rotation slows down over time.

©iStockphoto.com/nukleerkedi

Modern timekeeping defines a day as the sum of 24 hours – but that is not quite correct. The Earth's rotation slows down over time. So in terms of solar time, most days are a little longer than 24 hours.

Astronomers and timekeepers express solar time as Universal Time (UT1), a time standard based on the speed of the Earth's rotation. UT1 is then compared to International Atomic Time (TAI), a super-precise time scale calculated by a network of atomic clocks, and the actual length of a day is expressed as the deviation of UT1 from TAI over 24 hours.

How Long Is Today?

Today is predicted to be 0.2809 ms (milliseconds) or 0.0002809 seconds longer than 24 hours. This is the time it takes Earth to rotate 13.06 cm (5.14 in), as measured at the equator.

This means that today lasts:

  • 24.0000000780 hours or
  • 24 hours and 0.28 ms

On average, a mean solar day in the last 365 days was 0.29 ms over 24 hours, so today's day length is below average. Over this period, 191 days have been longer than today, while 175 have been shorter than today.

If every day was as long as today, a leap second would have to be added every 3559.99 days.

Today's Day Length* in Context
 Day lengthDate
Yesterday24 hours +0.43 msSat, Mar 28, 2020
Today24 hours +0.28 msSun, Mar 29, 2020
Tomorrow24 hours +0.20 msMon, Mar 30, 2020
Shortest 202024 hours -1.14 msSun, Jul 19, 2020
Longest 202024 hours +1.57 msWed, Mar 11, 2020
Last Year Average24 hours +0.39 msYear 2019
* Yesterday's, today's and future day lengths are predictions.

Average Day Lengths & Leap Seconds

In rare cases, a day can also be shorter than 24 hours. The last time this happened was on Sat, Jan 11, 2020 (day was 0.19 ms short). However, the average day exceeds 24 hours. See the table below for yearly average day lengths.

To make up for the additional duration, leap seconds are added to our clocks from time to time.

Average Solar Day Length
YearAverage dayTotal yearly excessLeap second added
2020+0.17 ms62.04 ms-
2019+0.39 ms141.26 ms-
2018+0.69 ms252.47 ms-
2017+1.03 ms375.01 ms-
2016+1.34 ms490.76 msDec 31
2015+1.25 ms458.03 msJun 30
2014+0.99 ms362.96 ms-
2013+1.02 ms373.99 ms-
2012+0.83 ms304.11 msJun 30
2011+0.76 ms277.94 ms-
2010+0.70 ms254.74 ms-
2009+0.80 ms293.37 ms-
2008+0.87 ms319.49 msDec 31
2007+0.85 ms310.81 ms-
2006+0.82 ms300.88 ms-
2005+0.43 ms157.76 msDec 31
2004+0.31 ms114.01 ms-
2003+0.27 ms100.16 ms-
2002+0.48 ms173.79 ms-
2001+0.57 ms208.94 ms-
2000+0.72 ms262.42 ms-
1999+0.99 ms361.19 ms-
1998+1.37 ms501.72 msDec 31
1997+1.84 ms671.08 msJun 30
1996+1.82 ms666.37 ms-
1995+2.31 ms843.66 msDec 31
1994+2.19 ms800.86 msJun 30
1993+2.36 ms862.66 msJun 30
1992+2.22 ms812.25 msJun 30
1991+2.04 ms743.88 ms-
1990+1.95 ms710.04 msDec 31
1989+1.52 ms555.00 msDec 31
1988+1.31 ms480.30 ms-
1987+1.36 ms497.35 msDec 31
1986+1.24 ms451.06 ms-
1985+1.45 ms528.83 msJun 30
1984+1.51 ms554.42 ms-
1983+2.28 ms832.08 msJun 30
1982+2.16 ms789.64 msJun 30
1981+2.15 ms786.03 msJun 30
1980+2.30 ms842.04 ms-
1979+2.61 ms953.02 msDec 31
1978+2.88 ms1051.83 msDec 31
1977+2.77 ms1012.60 msDec 31
1976+2.91 ms1064.67 msDec 31
1975+2.69 ms980.87 msDec 31
1974+2.72 ms991.99 msDec 31
1973+3.04 ms1106.21 msDec 31
* Current year's average day length and total yearly excess are predicted.

Why Are the Days Getting Longer?

The speed of the Earth's rotation decreases over time, but it also varies from day to day. One of the main factors are the celestial bodies surrounding us. For example, the Moon's gravitational pull causes tides and changes the Earth's shape, ultimately resulting in a lower rotational speed. The distance between Earth and Moon changes constantly, which makes for daily variations in the speed our planet rotates around its own axis.

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(*) Based on mean solar day. Numbers provided by the International Earth Rotation and Reference System Service (IERS).

Topics: Earth, Timekeeping, Astronomy