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Why clocks will get one extra second on June 30 | The day will be a little longer than usual on Saturday, June 30, 2012, and NASA
has the explanation for this change. An extra second, or “leap” second, will be
added at midnight to account for the fact that it is taking Earth longer and longer
to complete one full turn -- a day -- or, technically, a solar day. “The solar
day is gradually getting longer because Earth’s rotation is slowing down ever
so slightly,” said Daniel MacMillan of NASA’s Goddard Space Flight Center in Greenbelt
, Md. Scientists know exactly how long it takes Earth to rotate because they have
been making that measurement for decades using an extremely precise technique
called Very Long Baseline Interferometry (VLBI). VLBI measurements are made daily
by an international network of stations that team up to conduct observations at
the same time and correlate the results. NASA Goddard provides essential coordination
of these measurements, as well as processing and archiving the data collected.
And NASA is helping to lead the development of the next generation of VLBI system
through the agency’s Space Geodesy Project, led by Goddard. From VLBI, scientists
have learned that Earth is not the most reliable timekeeper. The planet’s rotation
is slowing down overall because of tidal forces between Earth and the Moon. Roughly
every 100 years, the day gets about 1.4 milliseconds, or 1.4 thousandths of a
second, longer. Granted, that’s about 100 or 200 times faster than the blink of
an eye. But if you add up that small discrepancy every day for years and years,
it can make a very big difference indeed. “At the time of the dinosaurs, Earth
completed one rotation in about 23 hours,” explained MacMillan, who is a member
of the VLBI team at NASA Goddard. “In the year 1820, a rotation took exactly 24
hours, or 86,400 standard seconds. Since 1820, the mean solar day has increased
by about 2.5 milliseconds,” he said. By the 1950s, scientists had already realized
that some scientific measurements and technologies demanded more precise timekeeping
than Earth’s rotation could provide. So, in 1967, they officially changed the
definition of a second. No longer was it based on the length of a day but on an
extremely predictable measurement made of electromagnetic transitions in atoms
of cesium. These “atomic clocks” based on cesium are accurate to one second in
1,400,000 years. Most people around the world rely on the time standard based
on the cesium atom: Coordinated Universal Time (UTC). Another time standard, called
Universal Time 1 (UT1), is based on the rotation of Earth on its axis with respect
to the Sun. UT1 is officially computed from VLBI measurements, which rely on astronomical
reference points and have a typical precision of 5 microseconds, or 5 millionths
of a second, or better. “These reference points are very distant astronomical
objects called quasars, which are essentially motionless when viewed from Earth
because they are located several billion light-years away,” said Goddard’s Stephen
Merkowitz, the Space Geodesy Project manager. For VLBI observations, several stations
around the world observe a selected quasar at the same time, with each station
recording the arrival of the signal from the quasar; this is done for a series
of quasars during a typical 24-hour session. These measurements are made with
such exquisite accuracy that it’s actually possible to determine that the signal
does not arrive at every station at exactly the same time. From the minuscule
differences in arrival times, scientists can figure out the positions of the stations
and Earth’s orientation in space, as well as calculating Earth’s rotation speed
relative to the quasar positions. Originally, leap seconds were added to provide
a UTC time signal that could be used for navigation at sea. This motivation has
become obsolete with the development of GPS (Global Positioning System) and other
satellite navigation systems. These days, a leap second is inserted in UTC to
keep it within 0.9 second of UT1. Normally, the clock would move from 23:59:59
to 00:00:00 the next day. Instead, at 23:59:59 on June 30, UTC will move to 23:59:60,
and then to 00:00:00 on July 1. In practice, this means that clocks in many systems
will be turned off for one second. Proposals have been made to abolish the leap
second and let the two time standards drift apart. This is because of the cost
of planning for leap seconds and the potential impact of adjusting or turning
important systems on and off in synch. No decision will made about that, however,
until 2015 at the earliest by the International Telecommunication Union, a specialized
agency of the United Nations that addresses issues in information and communication
technologies. If the two standards are allowed to go further and further out of
synch, they will differ by about 25 minutes in 500 years. In the meantime, leap
seconds will continue to be added to the official UTC timekeeping. The 2012 leap
second is the 35th leap second to be added and the first since 2008.
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