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A Day Is Not Exactly 24 Hours

Exact Day Length* — Thu, Oct 1, 2020

Today's prediction: 23 hours, 59 minutes, 59.9998494 seconds (-0.1506 milliseconds)

Yesterday's prediction: 23 hours, 59 minutes, 59.9997990 seconds (-0.2010 milliseconds)

At the start of today, UT1 was 0.1735219 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 mean solar time as Universal Time (UT1), a time standard based on the average 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 0.1506 ms (milliseconds) or 0.0001506 seconds shorter than 24 hours. This is the time it takes Earth to rotate 7.00 cm (2.76 in), as measured at the equator.

This means that today lasts:

  • 23.9999999582 hours or
  • 24 hours minus 0.15 ms

On average, a mean solar day in the last 365 days was 0.05 ms over 24 hours, so today's day length is below average. Over this period, 244 days have been longer than today, while 122 have been shorter than today.

If every day was as long as today, a negative leap second would have to be added every 6640.11 days.

Today's Day Length* in Context
 Day lengthDate
Yesterday24 hours -0.20 msWed, Sep 30, 2020
Today24 hours -0.15 msThu, Oct 1, 2020
Tomorrow24 hours -0.14 msFri, Oct 2, 2020
Shortest 202024 hours -1.46 msSun, Jul 19, 2020
Longest 202024 hours +6.12 msWed, Dec 30, 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 Tue, Sep 29, 2020 (day was 0.37 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 excessShortest dayLongest dayLeap second added
2020+0.01 ms4.42 msJul 19 -1.46 msDec 30 +6.12 ms-
2019+0.39 ms141.25 msJul 16 -0.95 msMar 22 +1.68 ms-
2018+0.69 ms252.47 msJun 30 -0.64 msFeb 4 +1.69 ms-
2017+1.03 ms375.01 msAug 4 +0.06 msApr 25 +2.20 ms-
2016+1.34 ms490.76 msJul 18 -0.03 msMar 10 +2.49 msDec 31
2015+1.25 ms458.03 msJun 17 +0.19 msOct 26 +2.31 msJun 30
2014+0.99 ms362.96 msJul 24 +0.02 msApr 26 +2.02 ms-
2013+1.02 ms373.99 msJul 6 -0.35 msMar 28 +1.97 ms-
2012+0.83 ms304.11 msJul 16 -0.35 msApr 5 +1.87 msJun 30
2011+0.76 ms277.94 msJul 27 -0.34 msMay 14 +1.85 ms-
2010+0.70 ms254.74 msJul 23 -0.76 msMar 1 +2.09 ms-
2009+0.80 ms293.37 msJul 6 -0.43 msApr 22 +1.81 ms-
2008+0.87 ms319.49 msJul 16 -0.41 msApr 5 +1.91 msDec 31
2007+0.85 ms310.81 msJul 27 -0.63 msApr 16 +2.31 ms-
2006+0.82 ms300.88 msJun 12 -0.40 msOct 7 +2.26 ms-
2005+0.43 ms157.76 msJul 5 -1.05 msFeb 27 +1.73 msDec 31
2004+0.31 ms114.01 msJul 15 -1.05 msApr 5 +1.56 ms-
2003+0.27 ms100.16 msJul 13 -0.96 msMar 19 +1.55 ms-
2002+0.48 ms173.79 msAug 6 -0.74 msMar 2 +1.66 ms-
2001+0.57 ms208.94 msAug 2 -0.71 msMar 11 +1.64 ms-
2000+0.72 ms262.42 msAug 11 -0.25 msOct 26 +1.58 ms-
1999+0.99 ms361.19 msJun 30 -0.13 msApr 15 +1.93 ms-
1998+1.37 ms501.72 msJul 9 +0.01 msMar 1 +2.66 msDec 31
1997+1.84 ms671.08 msJul 4 +0.52 msApr 6 +2.98 msJun 30
1996+1.82 ms666.37 msAug 10 +0.67 msMay 12 +2.68 ms-
1995+2.31 ms843.66 msJul 25 +0.81 msMar 17 +3.29 msDec 31
1994+2.19 ms800.86 msJul 6 +0.86 msFeb 27 +3.36 msJun 30
1993+2.36 ms862.66 msJul 17 +1.25 msMay 2 +3.49 msJun 30
1992+2.22 ms812.25 msJul 12 +0.84 msMar 18 +3.59 msJun 30
1991+2.04 ms743.88 msJun 27 +0.79 msMar 1 +3.00 ms-
1990+1.95 ms710.04 msJul 20 +0.63 msMar 26 +3.28 msDec 31
1989+1.52 ms555.00 msJul 2 +0.25 msNov 10 +2.82 msDec 31
1988+1.31 ms480.30 msJul 12 -0.09 msFeb 20 +2.76 ms-
1987+1.36 ms497.35 msJul 23 -0.06 msMar 1 +2.67 msDec 31
1986+1.24 ms451.06 msAug 2 -0.04 msApr 23 +2.30 ms-
1985+1.45 ms528.83 msJul 16 +0.11 msMar 9 +2.64 msJun 30
1984+1.51 ms554.42 msJul 12 +0.16 msMar 18 +2.77 ms-
1983+2.28 ms832.08 msJul 23 +1.01 msFeb 1 +3.57 msJun 30
1982+2.16 ms789.64 msAug 2 +0.84 msApr 23 +3.14 msJun 30
1981+2.15 ms786.03 msJul 16 +0.82 msMar 8 +3.42 msJun 30
1980+2.30 ms842.04 msAug 8 +1.34 msOct 23 +3.24 ms-
1979+2.61 ms953.02 msJul 23 +1.46 msMar 27 +3.65 msDec 31
1978+2.88 ms1051.83 msJul 31 +1.49 msMar 9 +3.83 msDec 31
1977+2.77 ms1012.60 msJul 14 +1.46 msApr 4 +3.72 msDec 31
1976+2.91 ms1064.67 msJun 26 +1.87 msOct 21 +3.90 msDec 31
1975+2.69 ms980.87 msJul 20 +1.54 msNov 1 +3.72 msDec 31
1974+2.72 ms991.99 msJul 30 +1.57 msApr 5 +3.79 msDec 31
1973+3.04 ms1106.21 msJan 2 +0.00 msApr 2 +4.03 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