Earth’s Slowing Rotation Caused Addition of Leap Second

Leap second

Tuesday, June 30, 2015, was officially longer than usual. There was an extra second, also being called a “leap second,” added to make up for a slight slowing of the Earth’s rotation. These leap seconds are a way for scientists to keep track of the changes, according to Daniel MacMillan of NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

One day is 86,400 seconds long, according to the time standard used on Earth and referred to as Coordinated Universal Time (UTC), or “atomic time.” The length of one second is based on extremely precise electromagnetic movements of electrons in a cesium atom. The cesium clock is accurate to a single second in 1,400,000 years.

The length of an average day is based on how long it takes the Earth to rotate, which is an average 86,400.002 seconds long, after accounting for the slowing of the Earth’s rotation, which is occurring because of a kind of force caused by the gravitational tug of war between the sun, the Earth, and the moon. Scientists have believed since 1820 that a solar day is 86,400 seconds long.

Leap SecondAt first, losing two milliseconds, which is less time than it takes a person to blink, is not really noticeable. However, if the same small discrepancy was to happen every day for 365 days, it would add up to an extra second. In reality, this is not exactly what happens. Even though the rotation of the Earth is slowing down, the length of each day varies unpredictably. There are several factors that influence the length of a day over periods of less than a year, including the atmosphere, and variations in seasonal and daily weather patterns have the capability to change the length of a day by a few milliseconds over a year’s time. Other contributors are the dynamics of the Earth’s inner core over long periods of time, groundwater flow, ocean tides, and ice storage, all of which occur over periods of anywhere from months to decades. Atmospheric variations caused by El Niño has also contributed to the slowing of the Earth’s rotation, increasing the length of a day by up to a millisecond.

Scientists have been monitoring how long it takes the Earth to complete one rotation using an extremely precise technique called Very Long Baseline Interferometry (VLBI). Measurements are done by a worldwide network of stations. Goddard Space Center is providing the essential coordination of VLBI and will analyze and archive the collected data.

Standard Time is referred to as Universal Time 1, or UT1, and is based on VLBI measurements of the Earth’s rotation. UT1 is not as exact as the cesium clock, so UT1 and UTC can drift apart. Therefore, leap seconds are added to keep the two standards of time within 0.9 seconds of each other. The decision to add leap seconds is made by a group within the International Earth Rotation and Reference Systems Service.

A leap second is generally added either on June 30 or December 31. The clock is moved from 23:59:59 to 00:00:00 the next day, however, because a leap second was added June 30, UTC moved from 23:59:59 to 23:59:60, and then to 00:00:00 on July 1. Often, many systems are turned off for that second.

Leap seconds in the past have created challenges for computer systems, and there has been the thought of abandoning leap seconds altogether. The need to add a leap second cannot be anticipated very far in advance. Leap seconds are not predictable, according to Chopo Ma, a geophysicist at Goddard, and a member of the board of directors of the International Earth Rotation and Reference Systems Service. More leap seconds will be added in the long term, but it cannot be said for sure that one will be added annually.

The addition of leap seconds began in 1972. From 1972 to 1999, leap seconds were added at an average rate of one per year. Since 1999, the addition of leap seconds has been less frequent. June 30th’s leap second was only the fourth to be added since 2000.

Scientists are unsure why fewer Leap seconds have been needed. Sometimes, geological events like earthquakes and volcanic eruptions can affect the Earth’s rotation in the short-term, but it gets more complex.

VLBI tracks these short-term and long-term variations with global networks of stations that observe quasars. Quasars are used as reference points because they are billions of light years away and therefore seem motionless. The observing stations are all over the globe because a signal from one quasar will take longer to hit some stations than others. Scientists can use these differences in the arrival time to gather detailed information using the exact positions of the observing stations, Earth’s orientation in space, and the rotation rate of the planet.

Leap secondThe current VLBI measurements are accurate down to three microseconds. NASA’s Space Geodesy Project has been developing a new system with international partners. Because of advancements in hardware, a greater number of observation stations, and a different distribution of stations around the world, future measurements with VLBI are expected to be even more precise, and as accurate as 0.5 microseconds.

According to Goddard’s Space Geodesy Project Manager, Stephen Merkowitz, the next system is being created to meet the needs of extremely demanding scientific applications both now and in the future. NASA manages a lot of the activities which support the International VLBI Service for Geodesy and Astrometry, including the day-to-day and long-term operations and the coordination and performance of the global network of VLBI antennas. The agency also coordinates data analysis and directly supports the operation of six global VLBI stations.

VLBI determines inertial reference as it is defined by the quasars, and at the same time, the exact positions of the antennas all over the world. The time difference measurements are exact to a few picoseconds. The positions of the antennas are exact within a few millimeters, and the positions of the quasars are down to fractions of a milliarcsecond. The antennas are fixed, so their locations are used to track the instantaneous orientation of the Earth in the frame of inertial reference. Antennas can be moved if indicated by the movement of tectonic plates, regional deformation, or a local uplift or subsidence.

The origin of VLBI occurred 40 years ago and is NASA-led technology. It includes the Crystal Dynamics Project that was extremely successful. There have been proposals made to get rid of the leap second. A decision will not be made regarding the use of leap seconds before the end of the year. Any change would be made by the International Telecommunication Union, which is a specialized agency of the United Nations that deals with issues in information and communication technologies.

VLBI observations that are made now are coordinated by the International VLBI Service for Geodesy and Astronomy (IVS) for analysis and development. This, however, involves 80 components in 20 countries, sponsored by 40 organizations. The Goddard Space Flight Center in Maryland houses the IVS Coordinating Center.

VLBI accurately determines the International Celestial Reference Frame that regards the quasar positions in the sky, the terrestrial reference frame that keeps track of where all the antenna are located on the Earth, and the Earth’s location in space. The future development of VLBI will continue to conduct research on the neutral atmosphere, perform measurement systems technology, and be able to integrate with different space geodetic techniques.

A valuable asset to NASA for scientific research and technology is VLBI. NASA’s scientific technology requires knowledge of the exact orientation of the Earth, which is found using VLBI data joined with a stable and accurate space reference frame.

By Jeanette Smith

Sources:

NASA: NASA Explains Why June 30 Will Get an Extra Second
Space Geodesy Project: SGP Techniques: VLBI

Top Image Courtesy of NASA’s Marshall Space Flight Center’s Flickr Page – Creative Commons License
Second Image Courtesy of NASA Goddard Space Flight Center’s Flickr Page – Creative Commons License
Third Image Courtesy of Erelster’s Flickr Page – Creative Commons License
Featured Image Courtesy of t-mizo’s Flickr Page – Creative Commons License

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