From: "Georgi Dobrovolski Solar Observatory" To: "Denis Howe" Subject: Re: Coordinated Universal Time's Abbreviation Date: Sat, 4 Aug 2001 14:32:10 +1200 Dear Mr Howe, In answer to your question about the Earth's irregular rotation, I'll say "many things", some of which are: 1. Torque caused by the Moon. 2. Atmospheric movement, drag etc. 3. Oceanic movement. This is somewhat related to... 4. The weather, including El Niņo. (The Pacific Ocean is USUALLY on a tilt with the western edge about 20 centimetres higher than its eastern edge [caused by wind flow]. When El Niņo 'kicks in', the Pacific levels off thus reducing its drag on the Earth below. This allows the planet to speed up.) 5. A seasonal effect (not fully understood, by any one) which allows the Earth to spin fast around August and slow around April or May each year. 6. Core-mantle 'slippage' and other crustal movements, etc. 7. Polar motion, connected partially to No. 1 above. The North Pole is never stationary, but travels an erratic spiral path roughly down 80 degrees West longitude. The tighter the spiral, the faster the rotation (the balleria syndrome). This spiral is referred to as the 'Chandler Wobble' which has 2 components, one being that of No. 5 above resulting in a near-circular motion with a diameter of about 6 metres in 12 months and another (No. 6 above) with a diameter of 3 to 15 metres with a period of about 428 days. Consequently, these 2 components can either work together or against each other. There is a third cyclic motion that has a period of some 600 years (not fully observed) which not much is known about. The following are annual mean lengths of day in milliseconds in excess of 24 (atomic) hours, since 1971: 1971, 2.90; 1972, 3.13; 1973, 3.05; 1974, 2.72; 1975, 2.69; 1976, 2.91; 1977, 2.77; 1978, 2.88; 1979, 2.61; 1980, 2.30; 1981, 2.16; 1982, 2.16; 1983, 2.28; 1984, 1.52; 1985 , 1.45; 1986, 1.23; 1987, 1.36; 1988, 1.32; 1989, 1.53; 1990, 1.94; 1991, 2.04 ; 1992, 2.22; 1993, 2.37; 1994, 2.17; 1995, 2.31; 1996, 1.83; 1997, 1.84; 1998, 1.37; 1999, 0.99; 2000 (provisional), 0.72; 12 months ending 02/08/2001 (provisional), 0.61. The value to remember is 2.74 = the loss of 1 second a year. Over the past month, the Earth has GAINED 0.17 milliseconds per day on average; about 5.3 milliseconds in the month. >> The line "allowing for variations in the Earth's orbit, on which >> UT1 is based." should be "allowing for variations in the Earth's >> orbit and the Earth's irregular rotation, on which UT1 is based." Perhaps an even better wording would be: "allowing for the eccentricity of the Earth's orbit, the rotational axis tilt (23.5 degrees), but still showing the Earth's irregular rotation, on which UT1 is based." I trust that this answers the question. Regards, Howard Barnes