A Day Is Not Exactly 24 Hours
Exact Day Length* — Thu, Jan 18, 2018
Today's prediction: 24 hours, 0 minutes, 0.0002322 seconds (0.2322 milliseconds)
Yesterday: 24 hours, 0 minutes, 0.0000820 seconds (0.0820 milliseconds)
At the start of today, UT1 was 0.2078127 seconds ahead of 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.
How Long Is Today?
Today is predicted to be 0.2322 ms (milliseconds) or 0.0002322 seconds longer than 24 hours. This is the time it takes Earth to rotate 10.80 cm (4.25 in), as measured at the equator.
This means that today lasts:
- 24.0000000645 hours or
- 24 hours and 0.23 ms
On average, a mean solar day in the last 365 days was 0.99 ms over 24 hours, so today's day length is below average. Over this period, 339 days have been longer than today, while 27 have been shorter than today.
If every day was as long as today, a leap second would have to be added every 4306.63 days.
|Today's Day Length* in Context|
|Yesterday||24 hours +0.08 ms||Wed, Jan 17, 2018|
|Today||24 hours +0.23 ms||Thu, Jan 18, 2018|
|Tomorrow||24 hours +0.36 ms||Fri, Jan 19, 2018|
|Shortest 2018||24 hours -0.77 ms||Thu, Jul 26, 2018|
|Longest 2018||24 hours +2.10 ms||Sun, Nov 4, 2018|
|Last Year Average||24 hours +1.03 ms||Year 2017|
|* 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 Mon, Jul 18, 2016 (day was 0.03 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).
Coordinated Universal Time (UTC)
Friday, January 19, 2018
UTC is the common time standard across the worldMore about UTC
- UTC – Coordinated Universal Time
- International Atomic Time (TAI)
- Universal Time (UT)
- Local Mean Time (LMT)
- Solar Noon
- A Day Is Not Exactly 24 Hours
- AM and PM: What Do They Mean?
- Atomic Clocks