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.
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|
|Yesterday||24 hours +0.43 ms||Sat, Mar 28, 2020|
|Today||24 hours +0.28 ms||Sun, Mar 29, 2020|
|Tomorrow||24 hours +0.20 ms||Mon, Mar 30, 2020|
|Shortest 2020||24 hours -1.14 ms||Sun, Jul 19, 2020|
|Longest 2020||24 hours +1.57 ms||Wed, Mar 11, 2020|
|Last Year Average||24 hours +0.39 ms||Year 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.
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.
Find Day Length for any Date
(*) Based on mean solar day. Numbers provided by the International Earth Rotation and Reference System Service (IERS).