How many seconds are in a day?
There are 86,400 seconds in a day. That number comes from multiplying 24 hours by 60 minutes per hour and then by 60 seconds per minute.
The calculation is simple: 60 seconds in a minute times 60 minutes gives 3,600 seconds in an hour. Multiply 3,600 by 24 hours and you get 86,400 seconds. A week has 604,800 seconds, and a 365-day year has 31,536,000 seconds.
Earth's rotation is not perfectly steady. It changes slightly because of tides, earthquakes and movements in the core. To keep clocks matched with Earth's rotation, scientists added 27 leap seconds to official time between 1972 and 2016. In 2022, international scientists agreed to stop adding leap seconds by 2035.
Defining the second
The second was once defined as a fraction of a day. Since 1967, it has been defined using atomic clocks, as the time it takes for a cesium-133 atom to vibrate 9,192,631,770 times. Atomic clocks are so precise that they would lose less than a second in millions of years.
Did You Know?
A day on Earth is getting very slightly longer over long periods, by about 1.7 milliseconds per century, mainly because the Moon's tides slow our planet's spin.