A leap second

According to ABC Science, a leap second will be added to clocks around the world at midnight on the 31st December. That means that the last minute of 2005 will contain 61 seconds rather than 60. The last time this happened was in 1998/1999.

These days a whole lot of professions - not just astronomers - rely on accurate timing and many make use of an international network of atomic clocks. Atomic clocks monitor the oscillations of ceasium atoms and are more reliable than other man-made clocks. The only trouble is that they gradually go out of sync with the day as measured by the position of the Sun. That isn't because they aren't accurate enough, but is actually due to the Earth changing its rate of rotation.

The rotation rate changes when anything happens to the distribution of matter on/in the planet. An example would be the recent tsunami which caused a large region of the Earth's crust to sink ever so slightly. When part of a rotating object moves inwards it causes the whole thing to speed up - this is called conservation of angular momentum. Admittedly, this is a tiny effect but it can gradually add up.

The International Earth Rotation Service (IERS), takes a decision every six month as to whether an extra second is required to keep Coordinated Universal Time (UTC) in line with GMT. If a leap seconds is needed it gets added to the end of June or December accordingly. One side effect is that the radio pips, heard every hour on some BBC radio stations, will gain an extra 'pip' for midnight on 31st December 2005.

Posted in astro blog by Stuart on Wednesday 27th Jul 2005 (15:52 UTC) | Permalink
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