1999

Earth’s Spin Slows Over Time

Our home planet spins on its axis once per day. For most of the history of astronomy, it has been sufficient to know that the rotation rate of our planet relative to the distant “fixed” stars is about 23 hours and 56 minutes. The fact that the Earth spins at this rate about 365 and a quarter times during every trip around the Sun has led to a variety of creative ways to add leap years to the calendar, culminating in the modern leap-year method developed during the Gregorian Calendar reform of 1582.

In our modern era of digital computers, global positioning system satellites, and interplanetary space probes, it has become much more important to be able to record time, including the Earth’s rotational rate, to a much higher degree of precision. Atomic clocks began to be used in the 1950s and 1960s to precisely reckon the passing of time, using the frequency of stable atomic-energy-level transitions in elements such as cesium. An internationally agreed-upon timekeeping system called Coordinated Universal Time (“UTC time”) was developed based on these atomic clocks. Using modern technology, it is now possible to measure the length of the day to almost one part in 10 billion.

The problem for astronomers, navigators, and timekeepers, however, is that the Earth’s rotation is not constant. Tidal friction with the Moon and Sun is slowing down our planet’s spin ever so slightly each year. In addition, very slight changes in the distribution of mass on the Earth’s surface (like glacial melting) and in its interior can also have tiny effects on our planet’s spin rate. Thus, since 1972, to keep UTC time precisely aligned with the passage of time as reckoned by the motion of the Sun in the sky, an organization known as the International Earth Rotation and Reference Systems Service has had to occasionally add extra leap seconds to UTC time.

In the 26 years from 1972 to 1998, 22 leap seconds were added to keep UTC time in sync with the Earth’s slowing spin. In the 20 years since 1999, however, the rate of slowing of Earth’s spin has decreased, and only five leap seconds have had to be added.

The astronomical clock in the town square of Prague, Czech Republic. Analog clocks such as this one, tracking hours and minutes and the motions of the Sun and Moon, have been replaced in scientific research by precise digital clocks and an internationally regulated system of timekeeping.