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In addition to precession, the time-varying gravitational effects of the Sun and Moon on the non-spherical Earth drive nutation. Nutation is a small wobble of the Earth's rotational axis with an amplitude of about 9 arcsec and period of up to 18.6 years. Traditionally, nutation is represented by with variations in ecliptic longitude and obliquity (the angle between the ecliptic and the equator). Current models represent the nutation quantities with well-defined series. For efficiency, arc reads a previously generated nutation table for these values. Currently, the nutabl. file is generated yearly by MIT from the 1980 IAU nutation model (Seidelmann, P.K., "1980 IAU Theory of Nutation: The Final Report of the IAU Working Group on Nutation", Celestial Mechanics, Vol. 27, pp. 79-106, 1982, and Kaplan, G.H., "The IAU Resolutions of the Astronomical Constants, Time Scales, and teh Fundamental Reference Frame", United States Naval Observatory Circular No. 163, U.S. Naval Observatory, Washington, D.C., Dec. 10, 1981). This file is made available via anonymous FTP.
The nutabl. file has a two line header which describes the contents of the file followed by the table of values. The format of this file should be considered fixed. The first line of the header is purely descriptive and is ignored. The second describes the ephemeris and contains the following information:
Julian date + 0.5
modified Julian date + 2400001.0
.
In the current version of arc, only the start and stop time are read and used. The format for reading these values is: (35x, f7.0, 1x, f7.0). The other parameters are "hardwired" to:
Each line contains the PEP date - 2400000, and the components of the nutation values for that epoch. The first entry is at zero hours of the given start date; subsequent entries are every twelve hours thereafter.
Note: All nutation parameters are in units of 1.0E-4 arcsec. These values are no longer read using the format and scaling factor given in the header and entries must be consistent with this format.
Example
Nutation ephemeris for Nov 96 - Feb 98 rwk 18 Dec 96 (1x,i5,8i8,8x,i2) 2450361 2450920 4 -1 1.E-04 50361 18089 -87411 18101 -87401 18075 -87363 17998 -87303 50363 17864 -87225 17663 -87134 17392 -87035 17050 -86935 50365 16635 -86839 16152 -86754 15604 -86686 15002 -86640 50367 14355 -86622 13677 -86636 12983 -86685 12293 -86771 50369 11624 -86894 10997 -87052 10429 -87243 9939 -87461 50371 9540 -87700 9242 -87950 9052 -88204 8969 -88451 50373 8990 -88681 9101 -88885 9287 -89056 9527 -89186 50375 9795 -89272 10063 -89312 10304 -89307 10490 -89263 50377 10597 -89185 10607 -89085 10505 -88972 10288 -88860 50379 9956 -88762 9521 -88690 9000 -88653 8417 -88661 50381 7799 -88717 7177 -88824 6580 -88980 6035 -89181 50383 5566 -89418 5190 -89682 4914 -89962 4744 -90248 50385 4673 -90528 4694 -90793 4791 -91035 4948 -91247 50387 5146 -91425 5365 -91568 5587 -91674 5795 -91746 50389 5974 -91786 6110 -91798 6193 -91787 6214 -91758 50391 6168 -91716 6052 -91669 5865 -91621 5608 -91578 50393 5285 -91547 4904 -91533 4473 -91542 4003 -91578 50395 3509 -91646 3007 -91748 2514 -91886 2050 -92061
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