LATEST NEWS:NASA explains Clocks will get an extra second on June 30

If the day seems a little longer than usual on Saturday, June 30,
2012, that's because it will be. 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," says 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," says 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."

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," says 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 miniscule 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 seconds 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.