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A Day Is Not Exactly 24 Hours

Exact Day Length* — Mon, May 17, 2021

Today's prediction: 23 hours, 59 minutes, 59.9998162 seconds (-0.1838 milliseconds)

Yesterday's prediction: 23 hours, 59 minutes, 59.9997364 seconds (-0.2636 milliseconds)

At the start of today, UT1 was 0.1822512 seconds behind UTC.

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

Star trails over an observatory.

The Earth's rotation slows down over time.

©iStockphoto.com/nukleerkedi

Earth's Rotation Defines Length of Day

Modern timekeeping defines a day as the sum of 24 hours—but that is not entirely correct. The Earth's rotation is not constant, so in terms of solar time, most days are a little longer or shorter than that.

The Moon is—very gradually—slowing the Earth's rotation because of friction produced by tides. Over the course of a century, the length of a day increases by a couple of milliseconds (where 1 millisecond equals 0.001 seconds).

Within this general trend, however, there is fluctuation: sometimes the Earth spins a bit faster, sometimes a bit slower. Recently, our planet has been speeding up a little, making for slightly shorter days.

How Long Is Today?

Today is predicted to be 0.1838 ms (milliseconds) or 0.0001838 seconds shorter than 24 hours. This is the time it takes Earth to rotate 8.55 cm (3.37 in), as measured at the equator.

This means that today lasts:

  • 23.9999999489 hours or
  • 24 hours minus 0.18 ms

On average, a mean solar day in the last 365 days was -0.19 ms under 24 hours, so today's day length is above average. Over this period, 183 days have been longer than today, while 183 have been shorter than today.

If every day were as long as today, a negative leap second would have to be added every 5440.70 days.

Today's Day Length* in Context
 Day lengthDate
Yesterday24 hours -0.26 msSun, May 16, 2021
Today24 hours -0.18 msMon, May 17, 2021
Tomorrow24 hours -0.03 msTue, May 18, 2021
Shortest 202124 hours -2.35 msSat, Aug 14, 2021
Longest 202124 hours +0.99 msMon, Apr 26, 2021
Last Year Average24 hours -0.00 msYear 2020
* Yesterday's, today's, and future day lengths are predictions.

Average Day Lengths & Leap Seconds

Overall, the Earth is a good timekeeper: the length of a day is consistently within a few milliseconds of 86,400 seconds, which is equivalent to 24 hours. However, over the course of months and years, these small differences can add up and put our clocks out of sync with the Earth's spin. When this happens, a leap second is used to bring them back into alignment.

Leap seconds can be positive or negative. A positive leap second adds a second to our clocks, while a negative leap second subtracts a second.

The system of leap seconds was introduced in 1972. So far, there have been 27 leap seconds, and they have all been positive. The table below shows the yearly average day lengths since 1973.

Average Solar Day Length*
YearAverage dayTotal yearly differenceShortest dayLongest dayLeap second added
2021-0.25 ms-90.24 msAug 14 -2.35 msApr 26 +0.99 ms-
2020-0.00 ms-1.30 msJul 19 -1.47 msApr 8 +1.62 ms-
2019+0.39 ms+141.25 msJul 16 -0.95 msMar 22 +1.68 ms-
2018+0.69 ms+252.47 msJun 30 -0.64 msFeb 4 +1.69 ms-
2017+1.03 ms+375.01 msAug 4 +0.06 msApr 25 +2.20 ms-
2016+1.34 ms+490.76 msJul 18 -0.03 msMar 10 +2.49 msDec 31
2015+1.25 ms+458.03 msJun 17 +0.19 msOct 26 +2.31 msJun 30
2014+0.99 ms+362.96 msJul 24 +0.02 msApr 26 +2.02 ms-
2013+1.02 ms+373.99 msJul 6 -0.35 msMar 28 +1.97 ms-
2012+0.83 ms+304.11 msJul 16 -0.35 msApr 5 +1.87 msJun 30
2011+0.76 ms+277.94 msJul 27 -0.34 msMay 14 +1.85 ms-
2010+0.70 ms+254.74 msJul 23 -0.76 msMar 1 +2.09 ms-
2009+0.80 ms+293.37 msJul 6 -0.43 msApr 22 +1.81 ms-
2008+0.87 ms+319.49 msJul 16 -0.41 msApr 5 +1.91 msDec 31
2007+0.85 ms+310.81 msJul 27 -0.63 msApr 16 +2.31 ms-
2006+0.82 ms+300.88 msJun 12 -0.40 msOct 7 +2.26 ms-
2005+0.43 ms+157.76 msJul 5 -1.05 msFeb 27 +1.73 msDec 31
2004+0.31 ms+114.01 msJul 15 -1.05 msApr 5 +1.56 ms-
2003+0.27 ms+100.16 msJul 13 -0.96 msMar 19 +1.55 ms-
2002+0.48 ms+173.79 msAug 6 -0.74 msMar 2 +1.66 ms-
2001+0.57 ms+208.94 msAug 2 -0.71 msMar 11 +1.64 ms-
2000+0.72 ms+262.42 msAug 11 -0.25 msOct 26 +1.58 ms-
1999+0.99 ms+361.19 msJun 30 -0.13 msApr 15 +1.93 ms-
1998+1.37 ms+501.72 msJul 9 +0.01 msMar 1 +2.66 msDec 31
1997+1.84 ms+671.08 msJul 4 +0.52 msApr 6 +2.98 msJun 30
1996+1.82 ms+666.37 msAug 10 +0.67 msMay 12 +2.68 ms-
1995+2.31 ms+843.66 msJul 25 +0.81 msMar 17 +3.29 msDec 31
1994+2.19 ms+800.86 msJul 6 +0.86 msFeb 27 +3.36 msJun 30
1993+2.36 ms+862.66 msJul 17 +1.25 msMay 2 +3.49 msJun 30
1992+2.22 ms+812.25 msJul 12 +0.84 msMar 18 +3.59 msJun 30
1991+2.04 ms+743.88 msJun 27 +0.79 msMar 1 +3.00 ms-
1990+1.95 ms+710.04 msJul 20 +0.63 msMar 26 +3.28 msDec 31
1989+1.52 ms+555.00 msJul 2 +0.25 msNov 10 +2.82 msDec 31
1988+1.31 ms+480.30 msJul 12 -0.09 msFeb 20 +2.76 ms-
1987+1.36 ms+497.35 msJul 23 -0.06 msMar 1 +2.67 msDec 31
1986+1.24 ms+451.06 msAug 2 -0.04 msApr 23 +2.30 ms-
1985+1.45 ms+528.83 msJul 16 +0.11 msMar 9 +2.64 msJun 30
1984+1.51 ms+554.42 msJul 12 +0.16 msMar 18 +2.77 ms-
1983+2.28 ms+832.08 msJul 23 +1.01 msFeb 1 +3.57 msJun 30
1982+2.16 ms+789.64 msAug 2 +0.84 msApr 23 +3.14 msJun 30
1981+2.15 ms+786.03 msJul 16 +0.82 msMar 8 +3.42 msJun 30
1980+2.30 ms+842.04 msAug 8 +1.34 msOct 23 +3.24 ms-
1979+2.61 ms+953.02 msJul 23 +1.46 msMar 27 +3.65 msDec 31
1978+2.88 ms+1051.83 msJul 31 +1.49 msMar 9 +3.83 msDec 31
1977+2.77 ms+1012.60 msJul 14 +1.46 msApr 4 +3.72 msDec 31
1976+2.91 ms+1064.67 msJun 26 +1.87 msOct 21 +3.90 msDec 31
1975+2.69 ms+980.87 msJul 20 +1.54 msNov 1 +3.72 msDec 31
1974+2.72 ms+991.99 msJul 30 +1.57 msApr 5 +3.79 msDec 31
1973+3.04 ms+1106.21 msJan 2 +0.00 msApr 2 +4.03 msDec 31
* Current year's average day length and total yearly difference are predicted.

How Is True Day Length Measured?

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.

The actual length of a day is expressed as the deviation of UT1 from TAI over 24 hours.

Why Isn't Earth's Rotation Constant?

The speed of the Earth's rotation 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 axis.

Find Day Length for Any Date

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How Far Back Does the Data Go?

Super-accurate atomic clocks were first developed in the 1950s and 1960s. So measurements of the Earth's rotation using atomic clocks only go back as far as then.

However, telescopic timings of stellar occultations by the Moon provide information about the Earth's rotation going back to the 17th century. An occultation is when the Moon, as seen from the Earth, passes in front of a star.

Ancient Records Give Away Earth's Speed

Going back even further, records of solar and lunar eclipses provide information from the 8th century BCE onwards.

For example, a Babylonian clay tablet tells us that a total solar eclipse was observable in the ancient city of Babylon on April 15, 136 BCE.

Modern computer models can calculate the path of totality for this eclipse with a high degree of accuracy. From this, we can work out the Earth's spin. For instance, if the Earth had been spinning a bit faster at that time, the path of totality would have passed to the west of Babylon—not directly over the city.

Topics: Earth, Timekeeping, Astronomy