1 October 2018
Perihelion Distance (q)
Aphelion Distance (Q)
Click for NASA orbit diagram
Hills Observatory: 1 January 2013 to 30 March 2020
John Grigg (Thames, New Zealand) was engaged in his monthly sky survey when he discovered
this comet in Virgo on 23 July 1902. Grigg was using a 3.5-inch refractor (power=25x) and said
the comet looked like an extremely faint nebula, and was "about twice the diameter of Jupiter."
Grigg was able to catch a glimpse of the comet under mostly cloudy skies on the 24th, but his
solid confirmation did not come until the skies cleared on the 27th.
Grigg was able to again observe the comet on 30 July, 2 and 3 August, the latter of which was
his final observation. Grigg's notices failed to reach the authorities in time, so that he remained
the only observer. Shortly after his final observation he computed a parabolic orbit. Up to that
time he thought he might have caught the return of some other periodic comet, but his orbit
revealed this was a hitherto unknown comet.
James Francis Skjellerup (Cape Town, South Africa) discovered a comet on 17 May 1922. A
notice was sent out and observations were soon made at numerous observatories around the
world. The general concensus during the earliest observations was that the comet's total
magnitude was around 11, while the coma diameter was between 4 and 5 arc minutes. By the
end of the month astronomers had already found that the observations were indicating the
comet was moving in a short-period orbit. Shortly after mid-June, R. T. Crawford and W. F. Meyer
(Berkeley Astronomical Department, California, USA) pointed out the similarities between the
orbits of Grigg's comet of 1902 and that found by Skjellerup. Because of the rough nature of
Grigg's observations, Crawford and Meyer added that identity could not be completely confirmed
until the comet returned in 1927. Identity was fully confirmed in that year.
During 1986 Lubor Kresak (Astronomical Institute, Slovak Academy of Sciences) suggested this
comet had actually been seen in 1808 by Jean Louis Pons. Pons had reported seeing a comet on
6 and 9 February of that year and said it was about one degree in diameter, suggesting it was
near Earth. On the latter date he said it was in Ophiuchus and made a drawing showing its
location near two globular clusters. Kresak said a backward integration of the orbit of Grigg-
Skjellerup by N. A. Belyaev revealed this comet would have passed about 0.12 AU from Earth
during February 1808.
The comet has been observed at every return since its 1922 rediscovery. The maximum
brightness typically reaches 9 or 9.5 during the returns when it passes perihelion during April or
May. This last happened in 1977 and 1982. It can also have rather unfavorable apparitions when
the total magnitude fails to become brighter than 16.
Since 1725 the comet has experienced several encounters with Jupiter which have acted to
steadily increase the comet's perihelion distance from 0.77 AU to the present 0.99 AU.
The current alignment of this comet's orbit allows it to produce a meteor shower around 23 April.
The Pi Puppids were discovered in 1972 following the prediction by H. B. Ridley that the comet
could now produce a meteor shower. Activity is best seen by observers in the Southern
Hemisphere where hourly rates have reached values as high as 42, but only in those years
closest to the comet's perihelion passage. Activity in other years is basically nonexistent.
Close approaches to planets:
0.56 AU from Earth on 23 March 2008
0.63 AU from Earth on 13 March 2029
0.84 AU from Earth on 21 June 2034
0.28 AU from Jupiter on 25 March 2047: will increase perihelion distance from 1.09 AU to 1.34 AU
and orbital period from 5.26 to 5.63 years
0.87 AU from Earth on 13 April 2050
17 Feb 13
2 Sep 13
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