# See end of file for documentation
YYYY MM DD Lon  Source/comments
3019 11  1 24320 (extrapolation: 24 deg/yr over 1000 yr = 24000 deg; see comments)
2019 11  1 320  (2010-present from http://jupos.privat.t-online.de/rGrs.htm)
2019  9  1 315
2019  7  1 313
2019  5  1 309
2019  1  1 300
2018 11  1 296
2018  6  1 288
2018  4  1 288  seems to be a (temporary?) 'halt' in Apr/May 2018
2017 10  1 280
2017 08  1 276
2016 12  1 258
2016  9  1 253
2016  4  1 244
2015 12  1 236
2015  5  1 227
2015  1  1 226
2014  9  1 216
2014  4  1 212
2014  2  1 209
2013 10  1 202
2012  8  1 182
2012  1  1 174
2011  8  1 168
2011  2  1 163
2010 10 10 156
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
1986  7  1  19  http://jupos.privat.t-online.de/img/oRS19b.gif
1984  6  1  28  http://jupos.privat.t-online.de/img/oRS19b.gif
1983  7 27  39  http://jupos.privat.t-online.de/img/oRS19b.gif
1982  5  2  51  http://jupos.privat.t-online.de/img/oRS19b.gif
1982  1  1  54  http://jupos.privat.t-online.de/img/oRS19b.gif
1980 12  6  53  http://jupos.privat.t-online.de/img/oRS19b.gif
1980  5 13  58  http://jupos.privat.t-online.de/img/oRS19b.gif
1980  1 12  56  http://jupos.privat.t-online.de/img/oRS19b.gif
1979 10 21  61  http://jupos.privat.t-online.de/img/oRS19b.gif
1978  9 26  57  http://jupos.privat.t-online.de/img/oRS19b.gif
1978  4 14  55  http://jupos.privat.t-online.de/img/oRS19b.gif
1977  2  9  48  http://jupos.privat.t-online.de/img/oRS19b.gif
1976 10  7  41  http://jupos.privat.t-online.de/img/oRS19b.gif
1976  3 15  49  http://jupos.privat.t-online.de/img/oRS19b.gif
1975 11 13  45  http://jupos.privat.t-online.de/img/oRS19b.gif
1975  6 21  35  http://jupos.privat.t-online.de/img/oRS19b.gif
1975  1 30  29  http://jupos.privat.t-online.de/img/oRS19b.gif
1974 11  9  21  http://jupos.privat.t-online.de/img/oRS19b.gif
1974  5 27  14  http://jupos.privat.t-online.de/img/oRS19b.gif
1973 10 14   7  http://jupos.privat.t-online.de/img/oRS19b.gif
1972  8 29   0  http://jupos.privat.t-online.de/img/oRS19b.gif
1972  6  8  -1  http://jupos.privat.t-online.de/img/oRS19b.gif
1971  9 15   7  http://jupos.privat.t-online.de/img/oRS19b.gif
1971  2  2  11  http://jupos.privat.t-online.de/img/oRS19b.gif
1971  1 12  16  http://jupos.privat.t-online.de/img/oRS19b.gif
1970  7 30  22  http://jupos.privat.t-online.de/img/oRS19b.gif
1970  4 19  19  http://jupos.privat.t-online.de/img/oRS19b.gif
1969  7 25  24  http://jupos.privat.t-online.de/img/oRS19b.gif
1969  5  4  21  http://jupos.privat.t-online.de/img/oRS19b.gif
1968  6 10  21  http://jupos.privat.t-online.de/img/oRS19b.gif
1968  2 29  27  http://jupos.privat.t-online.de/img/oRS19b.gif
1967  3 15  28  http://jupos.privat.t-online.de/img/oRS19b.gif
1966  1 10  22  http://jupos.privat.t-online.de/img/oRS19b.gif
1965  3  6  21  http://jupos.privat.t-online.de/img/oRS19b.gif
1964 11  4  17  http://jupos.privat.t-online.de/img/oRS19b.gif
1964  9 24  18  http://jupos.privat.t-online.de/img/oRS19b.gif
1962 12 31  17  1965MNRAS.130..423P (eyeballed on graph)
1962  8 31  14  1965MNRAS.130..423P (eyeballed on graph)
1957 04 13  -52 From JUPOS database : single observation by Voigt
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.

   As of September 2017,  the GRS longitude appears to be increasing
at about 24 degrees per year (with plenty of fluctuations).  I've
taken the most recent data point, added a thousand years and 24000
degrees to it, and added it as an extrapolated value.  This is 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.  (Previous versions of this file used
different drift rates.)

   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.

   Data from 1964 to 1987 came from

http://jupos.privat.t-online.de/img/oRS19b.gif

   I was able to measure pixel positions in PaintShop Pro and write a
little code to convert (pix_x, pix_y) to year and GRS longitude,  so
these will be at least passably accurate.  Getting the underlying data
would be even better,  of course,  but I've had no luck on that yet.

   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.  I had hopes of going back before 1831,
but this is unlikely;  the GRS may have been first observed in that year,
by Samuel Heinrich Schwabe.  It seems likely that Robert Hooke observed
it in 1664 and Cassini (as a "permanent spot") between 1665 and 1713 :

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

   If it were possible to figure out a passably exact GRS longitude from
these data,  however,  we'd still have trouble filling in the gap
between 1713 and 1831.

   You'll also notice a gap between Peek's data (which ends in 1952) and
the 'jupos' image, which only goes back to 1964.  It should be possible to
fill that gap.

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
2013 Oct 6:  added another GRS longitude from jupos.org,  corrected
   the extrapolated value
2013 Oct 8:  added 1964-1986 data from jupos.org
2013 Oct 12:  revised section about pre-1831 observations after comments
   from Jari Suomela
