A Day Is Not Exactly 24 Hours
Exact Day Length* — Sun, Jan 17, 2021
Today's prediction: 23 hours, 59 minutes, 59.9998124 seconds (-0.1876 milliseconds)
Yesterday's prediction: 23 hours, 59 minutes, 59.9997231 seconds (-0.2769 milliseconds)
At the start of today, UT1 was 0.1717624 seconds behind UTC.
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.
In past decades, the Earth's average rotational speed has consistently decreased, so most days were a little longer than 24 hours. Recently, however, our planet has been spinning faster again, making for slightly shorter days.
How Long Is Today?
Today is predicted to be 0.1876 ms (milliseconds) or 0.0001876 seconds shorter than 24 hours. This is the time it takes Earth to rotate 8.73 cm (3.44 in), as measured at the equator.
This means that today lasts:
- 23.9999999479 hours or
- 24 hours minus 0.19 ms
On average, a mean solar day in the last 365 days was -0.03 ms under 24 hours, so today's day length is below average. Over this period, 215 days have been longer than today, while 151 have been shorter than today.
If every day was as long as today, a negative leap second would have to be added every 5330.49 days.
| Today's Day Length* in Context | ||
|---|---|---|
| Day length | Date | |
| Yesterday | 24 hours -0.28 ms | Sat, Jan 16, 2021 |
| Today | 24 hours -0.19 ms | Sun, Jan 17, 2021 |
| Tomorrow | 24 hours -0.16 ms | Mon, Jan 18, 2021 |
| Shortest 2021 | 24 hours -1.72 ms | Thu, Jul 22, 2021 |
| Longest 2021 | 24 hours +1.29 ms | Wed, Nov 3, 2021 |
| Last Year Average | 24 hours -0.00 ms | Year 2020 |
| * 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 Fri, Jan 15, 2021 (day was 0.42 ms short). However, the average day exceeds 24 hours most years. 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.
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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
(*) Based on mean solar day. Numbers provided by the International Earth Rotation and Reference System Service (IERS).