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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 0.3512 ms (milliseconds) or 0.0003512 seconds shorter than 24 hours. This is the time it takes Earth to rotate 16.33 cm (6.43 in), as measured at the equator.

This means that today lasts:

23.9999999024 hours or

24 hours minus 0.35 ms

On average, a mean solar day in the last 365 days was -0.25 ms under 24 hours, so today's day length is below average. Over this period, 212 days have been longer than today, while 154 have been shorter than today. If every day were as long as today, a negative leap second would have to be added every 2847.38 days. Today's Day Length* in Context Day length Date Yesterday 24 hours -0.25 ms Tue, Jan 17, 2023 Today 24 hours -0.35 ms Wed, Jan 18, 2023 Tomorrow 24 hours -0.52 ms Thu, Jan 19, 2023 Shortest 2023 24 hours -1.61 ms Sun, Jul 16, 2023 Longest 2023 24 hours +3.45 ms Tue, Apr 18, 2023 Last Year Average 24 hours -0.25 ms Year 2022 * 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* Year Average day Total yearly difference Shortest day Longest day Leap second added 2023 -0.28 ms -100.84 ms Jul 16 -1.61 ms Apr 18 +3.45 ms - 2022 -0.25 ms -90.79 ms Jun 30 -1.59 ms Nov 5 +1.01 ms - 2021 -0.18 ms -65.15 ms Jul 9 -1.46 ms Apr 26 +1.00 ms - 2020 -0.00 ms -1.30 ms Jul 19 -1.47 ms Apr 8 +1.62 ms - 2019 +0.39 ms +141.25 ms Jul 16 -0.95 ms Mar 22 +1.68 ms - 2018 +0.69 ms +252.47 ms Jun 30 -0.64 ms Feb 4 +1.69 ms - 2017 +1.03 ms +375.01 ms Aug 4 +0.06 ms Apr 25 +2.20 ms - 2016 +1.34 ms +490.76 ms Jul 18 -0.03 ms Mar 10 +2.49 ms Dec 31 2015 +1.25 ms +458.03 ms Jun 17 +0.19 ms Oct 26 +2.31 ms Jun 30 2014 +0.99 ms +362.96 ms Jul 24 +0.02 ms Apr 26 +2.02 ms - 2013 +1.02 ms +373.99 ms Jul 6 -0.35 ms Mar 28 +1.97 ms - 2012 +0.83 ms +304.11 ms Jul 16 -0.35 ms Apr 5 +1.87 ms Jun 30 2011 +0.76 ms +277.94 ms Jul 27 -0.34 ms May 14 +1.85 ms - 2010 +0.70 ms +254.74 ms Jul 23 -0.76 ms Mar 1 +2.09 ms - 2009 +0.80 ms +293.37 ms Jul 6 -0.43 ms Apr 22 +1.81 ms - 2008 +0.87 ms +319.49 ms Jul 16 -0.41 ms Apr 5 +1.91 ms Dec 31 2007 +0.85 ms +310.81 ms Jul 27 -0.63 ms Apr 16 +2.31 ms - 2006 +0.82 ms +300.88 ms Jun 12 -0.40 ms Oct 7 +2.26 ms - 2005 +0.43 ms +157.76 ms Jul 5 -1.05 ms Feb 27 +1.73 ms Dec 31 2004 +0.31 ms +114.01 ms Jul 15 -1.05 ms Apr 5 +1.56 ms - 2003 +0.27 ms +100.16 ms Jul 13 -0.96 ms Mar 19 +1.55 ms - 2002 +0.48 ms +173.79 ms Aug 6 -0.74 ms Mar 2 +1.66 ms - 2001 +0.57 ms +208.94 ms Aug 2 -0.71 ms Mar 11 +1.64 ms - 2000 +0.72 ms +262.42 ms Aug 11 -0.25 ms Oct 26 +1.58 ms - 1999 +0.99 ms +361.19 ms Jun 30 -0.13 ms Apr 15 +1.93 ms - 1998 +1.37 ms +501.72 ms Jul 9 +0.01 ms Mar 1 +2.66 ms Dec 31 1997 +1.84 ms +671.08 ms Jul 4 +0.52 ms Apr 6 +2.98 ms Jun 30 1996 +1.82 ms +666.37 ms Aug 10 +0.67 ms May 12 +2.68 ms - 1995 +2.31 ms +843.66 ms Jul 25 +0.81 ms Mar 17 +3.29 ms Dec 31 1994 +2.19 ms +800.86 ms Jul 6 +0.86 ms Feb 27 +3.36 ms Jun 30 1993 +2.36 ms +862.66 ms Jul 17 +1.25 ms May 2 +3.49 ms Jun 30 1992 +2.22 ms +812.25 ms Jul 12 +0.84 ms Mar 18 +3.59 ms Jun 30 1991 +2.04 ms +743.88 ms Jun 27 +0.79 ms Mar 1 +3.00 ms - 1990 +1.95 ms +710.04 ms Jul 20 +0.63 ms Mar 26 +3.28 ms Dec 31 1989 +1.52 ms +555.00 ms Jul 2 +0.25 ms Nov 10 +2.82 ms Dec 31 1988 +1.31 ms +480.30 ms Jul 12 -0.09 ms Feb 20 +2.76 ms - 1987 +1.36 ms +497.35 ms Jul 23 -0.06 ms Mar 1 +2.67 ms Dec 31 1986 +1.24 ms +451.06 ms Aug 2 -0.04 ms Apr 23 +2.30 ms - 1985 +1.45 ms +528.83 ms Jul 16 +0.11 ms Mar 9 +2.64 ms Jun 30 1984 +1.51 ms +554.42 ms Jul 12 +0.16 ms Mar 18 +2.77 ms - 1983 +2.28 ms +832.08 ms Jul 23 +1.01 ms Feb 1 +3.57 ms Jun 30 1982 +2.16 ms +789.64 ms Aug 2 +0.84 ms Apr 23 +3.14 ms Jun 30 1981 +2.15 ms +786.03 ms Jul 16 +0.82 ms Mar 8 +3.42 ms Jun 30 1980 +2.30 ms +842.04 ms Aug 8 +1.34 ms Oct 23 +3.24 ms - 1979 +2.61 ms +953.02 ms Jul 23 +1.46 ms Mar 27 +3.65 ms Dec 31 1978 +2.88 ms +1051.83 ms Jul 31 +1.49 ms Mar 9 +3.83 ms Dec 31 1977 +2.77 ms +1012.60 ms Jul 14 +1.46 ms Apr 4 +3.72 ms Dec 31 1976 +2.91 ms +1064.67 ms Jun 26 +1.87 ms Oct 21 +3.90 ms Dec 31 1975 +2.69 ms +980.87 ms Jul 20 +1.54 ms Nov 1 +3.72 ms Dec 31 1974 +2.72 ms +991.99 ms Jul 30 +1.57 ms Apr 5 +3.79 ms Dec 31 1973 +3.04 ms +1106.21 ms Jan 2 +0.00 ms Apr 2 +4.03 ms Dec 31 * Current year's average day length and total yearly difference are predicted.

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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

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. This chart, produced using data from the IERS, shows the length of day going back to 1830. It indicates that Earth was spinning particularly fast around the year 1870, and particularly slow around the start of the 1900s. ©timeanddate.com

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.

Eclipse database for years 1900-2199

timeanddate.com - How Long Is a Day on Earth? - Time and Date

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