# See end of file for documentation YYYY MM DD Lon Source/comments 2017 5 1 194 (extrapolation; see comments) 2007 5 1 114 (Jul 2007 S&T) 2006 6 1 109 (Aug 2006 S&T) 2006 3 1 104 (May 2006 S&T) 2005 3 1 98 (May 2005 S&T) 2005 1 1 98 (Mar 2005 S&T) 2004 12 1 94 (Feb 2005 S&T) 2004 11 1 94 (Jan 2005 S&T) 2004 10 1 94 (Dec 2004 S&T) 2003 11 1 84 (Jan 2004) 2003 9 1 84 (Nov 2003) 2003 1 1 80 (Mar 2003) 2002 3 1 80 (May 2002) 2002 2 1 80 (Apr 2002) 2002 1 1 77 (Mar 2002) 2001 12 1 77 (Feb 2002) 2001 11 1 77 (Jan 2002) 2001 10 1 77 (Dec 2001 S&T) 2001 9 1 76 (Nov 2001 S&T) 2001 8 1 76 (Oct 2001 S&T) 2001 5 1 76 (Aug 2001 S&T) 2001 3 1 76 (May 2001 S&T) 2001 2 1 76 (Apr 2001 S&T) 2001 1 1 76 (Mar 2001 S&T) 2000 12 1 76 (Feb 2001 S&T) 2000 11 1 76 (Jan 2001 S&T) 2000 10 1 74 (Dec 2000) 1999 12 1 66 (Jan 2000) 1998 11 1 64 (Jan 1999) 1997 5 1 62 (July, 1997) 1996 9 1 61 (November, 1996) 1996 8 1 51 (Oct 1996) 1996 7 1 51 (September, 1996) 1995 9 1 49 (November, 1995) 1995 6 1 42 (Aug 1995) 1995 5 1 42 (July, 1995) 1994 6 1 40 (Aug 1994) 1994 5 1 41 (July, 1994) 1994 4 1 40 (June, 1994) 1993 5 1 45 (Jul 1993) 1993 2 1 38 1992 7 1 33 1992 6 1 33 1992 5 1 30 1992 4 1 30 1992 3 1 34 1992 2 1 34 1991 6 1 32 1991 5 1 32 1991 4 1 32 1991 3 1 27 1991 2 1 27 1991 1 1 27 1990 12 1 27 1990 4 1 22 1990 3 1 22 1990 2 1 22 1990 1 1 30 1989 11 1 30 1989 2 1 16 1989 1 1 16 1988 12 1 16 1988 11 1 16 1988 1 1 14 1987 12 1 14 1987 11 1 14 1986 11 1 19 1986 10 1 19 1968 6 1 26 1975AN....296..241R (eyeballed on graph) 1965 7 1 24 1975AN....296..241R (eyeballed on graph) 1962 8 31 14 1965MNRAS.130..423P (eyeballed on graph) 1962 12 31 17 1965MNRAS.130..423P (eyeballed on graph) 1952 11 10 -89 Bertrand Peek: The Planet Jupiter (see notes) 1951 10 7 -104 1950 9 3 -110 1949 7 30 -119 1948 6 27 -126 1947 5 24 -130 1946 4 21 -143 1945 3 18 -148 1944 2 14 -177 1943 1 11 -188 1941 12 8 -191 1940 11 5 -187 1939 10 2 -194 1938 8 29 -213 1937 7 26 -221 1936 6 22 -218 1935 5 19 -201 1934 4 16 -180 1933 3 13 -157 1932 2 10 -131 1931 1 7 -103 1929 12 3 -76 1928 10 30 -45 1927 9 27 -16 1926 8 24 21 1925 7 21 90 1924 6 18 172 1923 5 14 230 1922 4 11 256 1921 3 8 284 1920 2 5 322 1919 1 2 370 1917 11 29 439 1916 10 25 490 1915 9 22 533 1914 8 19 564 1913 7 16 621 1912 6 13 664 1911 5 10 688 1910 4 6 728 1909 3 3 737 1908 1 30 743 1906 12 27 738 1905 11 24 748 1904 10 21 745 1903 9 18 754 1902 8 14 762 1901 7 11 766 1900 6 8 760 1899 5 5 751 1898 4 2 742 1897 2 29 735 1896 1 25 727 1894 12 22 720 1893 11 19 718 1892 10 16 716 1891 9 13 714 1889 7 6 714 1887 4 30 733 1884 1 21 755 1880 10 11 848 1878 8 5 976 1876 5 28 1117 1873 2 19 1304 1869 11 10 1485 1867 9 3 1607 1864 5 24 1782 1863 4 21 1849 1860 1 11 1996 1858 12 8 2014 1857 11 5 2046 1856 10 2 2075 1855 8 28 2109 1852 5 19 2246 1851 4 16 2297 1850 3 13 2345 1845 10 30 2549 1840 5 14 2816 1832 9 22 3220 1831 8 19 3308 End This file gives the GRS longitude as a function of time. Guide computes the System II GRS longitude for any date by interpolating within this table. Data are given from most recent to oldest. For each date, a year, month, day, and System II longitude are given, followed by the source of the data. For dates after the most recent data point, the last GRS longitude is used; for dates preceding the table, the oldest GRS longitude is used. The GRS longitude currently appears to be increasing at about eight degrees per year (with plenty of fluctuations). I've added a totally fictitious longitude for 2017 based on that drift, just to get around the fact that otherwise, the GRS would remain at the last listed value, which is almost certainly even less realistic than the assumed drift. 1986 to present longitudes came from _Sky & Telescope_, which in turn got the data from ALPO (Association of Lunar and Planetary Observers). I checked a random sampling of my back issues of that magazine to compile the table. Because S&T is published about two months before the date given on the cover, I've assumed that the longitude given in (for example) the August issue of a given year is the one that was actually measured in June, and have arbitrarily chosen the first of the month. Jari Suomela found some data in Bertrand Peek's book _The Planet Jupiter_ for years 1831 to 1952. See 'grs_peek.cpp' for details. Other sources would be welcome. Seems to me as if somebody, somewhere, must have assembled a table going back to Galileo's observations... 17 June 2006 version: created by BJG 6 June 2007: updated with new data and 2017 "observation" added 9 Mar 2008: added 1968 and earlier data from Peek via Jari Suomela; see 'grs_peek.cpp' for details