NASA scientists have announced that on this June 30, everyone gets an extra second, making Tuesday a bit longer than any other day. Because Earth is slowing down its rotation in a very gradual way, scientists have found a way to account for that by introducing the concept of the “leap” second.
Daniel MacMillan from NASA’s Goddard Space Flight Centre explains that, in theory, all days should last 86,400 seconds, which is how we use the time in our daily lives, measured in Coordinated Universal Time, or UTC.
UTS is also dubbed the “atomic time” because of the exceptionally predictable electromagnetic evolutions that happen inside cesium atoms – which gives us the exact duration of a second.
Just so you can wrap your head around how reliable these transitions of the cesium clock are, try to grasp its accuracy to one second in 1,400,000 years – that’s its margin of error. However, an average day – a full rotation of the Earth – usually lasts about 86,400.002 seconds.
This happens because the Earth is in the middle of a gravitational struggle – a sort of braking force – with the Sun and the Moon, causing it to gradually slow down its rotation. According to scientific estimations, the average solar day has last measured 86,400 seconds around the year 1820.
One would argue 2 milliseconds – less than the time it takes you to blink – is not something that needs to be measured and kept account of. But when these two thousands of a second keep repeating every day for 356 days, you almost get an entire second.
Even though the fact that Earth’s rotation is slowing down was calculated on average, each specific day has its own unpredictable variation. Plenty of other factors are involved in the day’s length, but the atmosphere is one of the most important. Leap seconds are usually inserted at the middle (June 30) of the year or at the end of it (December 31).
Typically, the clock would change from 23:59:59 to 00:00:00. But June 30’s leap second was inserted as to move the UTC from 23:59:59 to 23:59:60, and then on July 1, it passes to 00:00:00 .
Unlike in theory, the way systems incorporate the leap second in practice is by turning them off for one second. Because previous leap seconds have created a great deal of hassle for some computer systems, there are some who decided to ignore them altogether.
Leap seconds first became a thing in 1972, and until 1999, they would usually come very close to one another, almost in each year. However, after 2000, only three other leap seconds were introduced. June 30 will mark the fourth to be added since 2000.
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