A claim circulating online states that the earth is slowing and a day is now 24 seconds longer than it was only two years ago. Is this true or false?
Let’s first take a look at what is being claimed, such as this tweet by an internet fact page.
Scientists have discovered that Earth’s rotation is slowing, and in the past 2 years, there is about 24 more seconds in a day.
— Facts In Your Face (@FactsInYourFace) February 6, 2015
The claim above by the popular Twitter “fact” page is but one example of this assertion. Casual readers may find the thought of the earth slowing by such a significant amount in such a short period of time alarming.
Fortunately, it’s completely wrong.
The “fact” above appears to have misinterpreted the meaning of a “leap second” and made certain assumptions based on this misunderstanding. Leap seconds are periodic one-second adjustments to Coordinated Universal Time so that clocks more closely match mean solar time. As the U.S. Naval Observatory points out in the quote below, the insertion of leap seconds is more about coordinating clocks which use different methods to measure time rather than an indication that the earth is slowing by several seconds per year.
The first leap second occurred on June 30, 1972. The 27th leap second is due to occur on July 1, 2015. See a full list of leap seconds here.
The claim that the earth is slowing is correct, although the claim that it has slowed by 24 seconds in two years is far off base, and this has been observed by scientists for decades. NASA, in its explanation of the “leap second” in 2012 wrote that the mean solar day has increased by only 2.5 milliseconds since 1820. Using an average of about 1 millisecond every 76.8 years, this means it will take the earth over 70,000 years to slow by 1 full second, and about 1.8 million years to slow by the 24 seconds claimed above.
According to the Time Service Dept., U.S. Naval Observatory, the use of the leap second has led some to believe that the earth is slowing at an alarming rate.
Confusion sometimes arises over the misconception that the regular insertion of leap seconds every few years indicates that the Earth should stop rotating within a few millennia. The confusion arises because some mistake leap seconds for a measure of the rate at which the Earth is slowing. The 1 second increments are, however, indications of the accumulated difference in time between the two systems.
The claim that the earth has slowed by 24 seconds over the past two years is false. There have been 26 “leap seconds” since 1972, but that does not mean the earth has slowed by 26 seconds – only that two different systems of measurement have needed to be aligned periodically over the years.
It will take the earth nearly 2 million years to slow by 24 seconds.