VLBI, or Very Long Baseline Interferometry, is a technique that uses multiple radio telescopes to very precisely measure the Earth's orientation. It was originally invented back in the 1960s to take better pictures of quasars, but scientists soon found out that if you threw the process in reverse, you could measure how the ground beneath the telescopes moves around, how long days really are, and how the Earth wobbles on its axis as it revolves around the sun! Learn more about VLBI here!
For more information about NASA's Space Geodesy Project, visit http://space-geodesy.nasa.gov !
This video is presented in both stereoscopic 3D and standard 2D versions.
This video is public domain and can be downloaded at: http://svs.gsfc.nasa.gov/goto?10964
How will the leap second affect computers?
The leap year rule used in the Gregorian calendar — the most widely used international calendar — adds an extra day every four years. Addition of an extra day compensates for the extra time of 5 hours, 48 minutes and 46 seconds that the earth takes over 365 days to complete one revolution around the sun. This permanent rule is typically considered in all computer programmes and hence there is no problem. The leap second, on the other hand, cannot be predicted as the time taken by the earth to rotate around its axis depends on many factors like dynamics of the earth's core, variation in the atmosphere and oceans, ground water, ice storage and so on and can change from year to year. The irregularity of the leap second could cause problems in many computing systems. Some major web-based services suffered because of the addition of a leap second three years ago.
How many seconds ?
Strictly speaking, a day lasts 86,400 seconds. That is the case, according to the time standard that people use in their daily lives – Coordinated Universal Time, or UTC. UTC is “atomic time” – the duration of one second is based on extremely predictable electromagnetic transitions in atoms of cesium. These transitions are so reliable that the cesium clock is accurate to one second in 1,400,000 years.
UTC is "atomic time" - the duration of one second is based on extremely predictable electromagnetic transitions in atoms of cesium.
These transitions are so reliable that the cesium clock is accurate to one second in 1,400,000 years.
However, the mean solar day - the average length of a day, based on how long it takes the Earth to rotate - is about 86,400.002 seconds long.
"That is because the Earth's rotation is gradually slowing down a bit owing to a kind of braking force caused by the gravitational tug of war between the Earth, the Moon and the Sun," the US space agency said in a statement.
Scientists estimate that the mean solar day has not been 86,400 seconds long since the year 1820 or so.
This difference of two milliseconds, or two thousandths of a second - far less than the blink of an eye - hardly seems noticeable at first.
But if this small discrepancy were repeated every day for an entire year, it would add up to almost a second.
Typically, a leap second is inserted either on June 30 or December 31.
Normally, the clock would move from 23:59:59 to 00:00:00 the next day. But with the leap second on June 30, UTC will move from 23:59:59 to 23:59:60, and then to 00:00:00 on July 1.
In practice, many systems are instead turned off for one second.
Previous leap seconds have created challenges for some computer systems and generated some calls to abandon them altogether