# See end of file for documentation
YYYY MM DD Lon  Source/comments
2030 10 10 339  (extrapolation; see comments)
2012  8  1 182  (From http://jupos.privat.t-online.de/rGrs.htm)
2012  1  1 174  (From http://jupos.privat.t-online.de/rGrs.htm)
2011  8  1 168  (From http://jupos.privat.t-online.de/rGrs.htm)
2011  2  1 163  (From http://jupos.privat.t-online.de/rGrs.htm)
2011  2  1 163  (From http://jupos.privat.t-online.de/rGrs.htm)
2010 10 10 156  (From http://jupos.privat.t-online.de/rGrs.htm)
2009  7 15 138  (From Sky & Tel Web site)
2008  7 15 127  (From Sky & Tel Web site)
2008  3  1 121  (Mar 2008 Christopher Go value)
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

   Comments can be added freely after the above 'End' line.

   At one point,  the GRS longitude in Guide was set by the user.
But the GRS moves;  what is really needed is a table giving the
GRS longitude as a function of time.  This file provides that table.

    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.

   Note that,  while coverage is good for years since 1986,  the data
gets a little sparser for previous dates.

   The GRS longitude currently appears to be increasing at about
nine degrees per year (with plenty of fluctuations).  I've added a
totally fictitious longitude for 2030 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.

   Data since 2009 has come from JUPOS,  the database for object
positions on Jupiter.

   1986 to 2009 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 Cassini's observations...

http://articles.adsabs.harvard.edu//full/1987JBAA...97..215F/0000218.000.html

   shows a drawing of an "observation of Jupiter made in Rome on 1665
July 9 by Divini, Fabri, Gottigniez,  Serra et. al... This is the first
recorded observation of the Red Spot,  which was seen to accompany the
shadow of satellite III..."

   Which is fortunate,  because the shadow of satellite III can be measured
to have been at longitude 208 at the time.  I haven't added that to the
above table,   because there could have been several rotations around the
planet between 1665 and 1831.  If that gap is adequately filled, though,
the above observation might suddenly become very useful.

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
21 Sep 2009:  added GRS longitude for August 2009 from S&T site.
15 Oct 2010:  added GRS longitude from jupos.org
