March 25, 1998
"When you fire a .22 caliber rifle, the bullet gets about halfway down the barrel in 0.6 milliseconds, so we are talking about an extremely short period of time. It is a tribute to the accuracy of VLBI that we are able to notice such a small change in day length," said Gipson.
The sum of the extra day lengths over the entire El Niņo cycle to date is approximately 1/10 of a second, about the period of an eye blink. The peak in day length occurred around Feb. 5, 1998. The extra day length has decreased slowly as El Niņo has diminished in intensity; currently, it is at 0.4 milliseconds.
"The reason for the change in day length is that the Earth system, which includes its atmosphere and oceans, was spinning when it was formed. This rotational energy, called angular momentum, is fixed -- you can't get any more or take any away without applying a force. This is like living on a fixed income. Let's suppose you make 10,000 dollars per year, and spend 5,000 on rent and 5,000 on food. If your rent increases to 6,000 dollars, you only have 4,000 dollars to buy food. The same thing is happening with the Earth's spin. As the atmosphere speeds up during El Niņo, the Earth must slow down to conserve the combined angular momentum. Conversely, after El Niņo dissipates, the atmosphere will slow down, and the Earth will speed up again, making the day shorter," said Gipson.
The 1982 El Niņo had a peak effect of 0.9 milliseconds per day, concentrated in a much shorter time period. The extra day length was above 0.3 milliseconds per day for three months. The current El Niņo has been above 0.3 milliseconds per day for most of last year. Other atmospheric phenomena, such as hurricanes, can change the day length as well, although the effect is much smaller than that of El Niņo. The change in day length due to a typical hurricane is less than two microseconds (2/1,000,000 of a second) per day. The normal seasonal fluctuation is about one millisecond per day, peak to peak.
The VLBI network, coordinated at Goddard, determines the Earth's rotation speed from differences in the arrival time of radio signals from quasars, extremely bright objects at the edge of the observable universe. Because the radio telescopes are widely separated, the radio signal from a given quasar reaches some telescopes before others. As the Earth speeds up or slows down, this timing difference changes by a minuscule amount. The difference is measured in picoseconds, or trillionths of a second. These precise measurements also permit accurate determination of the distances between antenna within the network. Relative changes in the antenna locations from a series of measurements are used to indicate motion in the Earth's crust due to plate tectonics. Local deformations may reveal stress buildup as a precursor to earthquake or volcanic activity.
High precision VLBI measurements of the Earth's orientation began in 1980, and have continued about once per week since then, even in the current El Nino cycle.
Under the Continuous Observations of the Rotation of the Earth (CORE) program these measurements would be made continuously. Measurements of the Earth's orientation and rotation are essential for satellite navigation, and for communication with deep space satellites. The Global Positioning System requires Earth orientation measurements to provide precise measurements of longitude.
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