Many celestial objects have the property to turn around their center of gravity. Galaxies, stars, planets, asteroids, and comets, seem to have the same property. For asteroids and comets it is not possible to predict the rotation speed on the base of the geometry or the mass of the object. The rotation can be measured by monitoring the (often) miniscule variations in the brightness. Indeed, most minor bodies have an irregular shape. The surface reflectance of the bodies may also be irregular, so that some parts appear brighter than other parts. If such an irregular body turns around its center of gravity, an observer may measure a variating magnitude. These magnitudes can be plotted in a graph, with the observation time in the abscissa (x-axis), and the magnitude in the ordinate (y-axis).
The plot above shows hypothetic magnitude/time data for an asteroid. The rotation period for an asteroid is typically some hours to over one day.
To my knowledge the first paper mentioning the rotational lightcurve of Chiron was published by Bust et al. in 1989 (refer to the Chiron bibliography for further details). Other papers covering the same subject were published as follows:
author(s) year of date of rotational rotational
publi- obser- period amplitude
cation vation (hr) (R mag.)
Bus et al. 1989 1986 5.9181 0.088
+/- 0.0003 +/- 0.003
1986/88 5.9180
+/- 0.0001
Luu & Jewitt 1990 1990 5.91780 0.045
+/- 0.00005
West 1991 1990 5.91783
+/- 0.00005
Dahlgren et al. 1991 1990 5.92 0.07
Marcialis & 1993 1991 5.917813 0.04
Buratti +/- 0.000007
From these papers one learns that Chiron's rotational lightcurve looks more or less as follows:
The rotation period of Chiron is the time between two pronounced peaks, or, equivalently, the time between two consecutive smaller peaks. Some confusion is possible between the two peaks (the pronounced and the smaller ones).
Denis Bergeron from Canada was so kind to send me the original ST6 files which he used to produce the Chiron images of 1995 March 27/28 and 1995 March 28/29. I wanted to compute precise positions and magnitudes for Chiron (Denis did not have Astrometrica yet at that time). I also realised that Denis had misplaced (mirrored) the two previously mentioned images, so that the two "asteroids" that he had indicated in fact belonged to a same asteroid. But that is another story: read all about this success leading to 1995 FC21 too!
So, I started reducing the images. I took advantage of the fact that the two observing runs were at two consecutive nights. Working with Astrometrica and the Hubble GSC, I selected the same reference stars for the two sets of images. It was soon clear that astrometry was easier done than photometry. After some time a to my feeling "stable set" of photometric values was obtained. But I could not get rid of some slow "movement" in the photometric curve. I was suspicious about Denis's instruments, or at least about the atmospheric conditions during the two observing runs: a drop in the magnitudes seemed to appear in the middle of the two runs, at almost the same time. After some time, I hypothesised that maybe Chiron itself was responsible for the magnitude variations. Indeed, in the literature some authors had been publishing about the rotational lightcurve. But that lightcurve caused only a variation of some hundreths of a magnitude. I finally concluded that my photometric data were reliable, and I transferred both the astrometric and photometric data to Brian Marsden. I added the remark the possibly some trait of a rotational lightcurve were present in these data. I asked Marsden about his opinion, but he kindly replied to me that he was not a photometrist, and could thus not judge. By the way, these data have been promptly published in M.P.C. 25405-25406. I contacted several professionals, among them Alan Stern and David Jewitt. Stern replied that he too was not well placed to judge, but that if this were true, that I would have caught "a nice fish" (this is my own transcription of the contents of his electronic mail message). Jewitt, being in Paris at the moment that I wrote my message, but his remote mail on the server in Hawaii, immediately replied but told me that he was hindered by the French (AZERTY keyboard). Once he was back in Hawaii, he again took a look at my data, and mailed me that probably the lightcurve was there, however rather weakly visible. He added the word "remarkable". I was, of course, exited about this reply... Later on Jane Luu also sent me an electronic mail message. Se too congratulated us for "the fine work", and encouraged us to continue.
The following table gives the precise astrometric and photometric data that I reduced with Astrometrica and the GSC. The following table is exactly the one that I sent to Marsden by electronic mail.
Observations by Denis Bergeron -- Meade LX-2000 (10 inch, F10), ST4/ST6 + CCD.
Measurements by Patrick Vanouplines -- GSC reference stars (J2000.0).
(2060) C1995 03 28.14194 11 24 22.68 -01 06 10.2 16.08V 999
(2060) C1995 03 28.14795 11 24 22.59 -01 06 09.6 16.14V 999
(2060) C1995 03 28.15258 11 24 22.51 -01 06 09.0 16.12V 999
(2060) C1995 03 28.15721 11 24 22.45 -01 06 08.3 16.06V 999
(2060) C1995 03 28.16183 11 24 22.35 -01 06 07.5 16.17V 999
(2060) C1995 03 28.16646 11 24 22.29 -01 06 07.0 16.17V 999
(2060) C1995 03 28.17109 11 24 22.22 -01 06 06.5 16.15V 999
(2060) C1995 03 28.17572 11 24 22.15 -01 06 05.7 16.16V 999
(2060) C1995 03 28.18034 11 24 22.06 -01 06 05.2 16.15V 999
(2060) C1995 03 28.18427 11 24 22.00 -01 06 04.7 16.17V 999
(2060) C1995 03 28.18959 11 24 21.92 -01 06 03.9 16.15V 999
(2060) C1995 03 28.19423 11 24 21.84 -01 06 03.5 16.13V 999
(2060) C1995 03 28.19884 11 24 21.76 -01 06 02.8 16.13V 999
(2060) C1995 03 28.21441 11 24 21.51 -01 06 00.7 16.12V 999
(2060) C1995 03 28.21904 11 24 21.44 -01 06 00.0 16.11V 999
(2060) C1995 03 28.22367 11 24 21.36 -01 05 59.6 16.09V 999
(2060) C1995 03 28.22829 11 24 21.29 -01 05 59.1 16.11V 999
(2060) C1995 03 28.23292 11 24 21.22 -01 05 58.5 16.11V 999
(2060) C1995 03 28.23755 11 24 21.15 -01 05 57.9 16.12V 999
(2060) C1995 03 28.24216 11 24 21.07 -01 05 57.3 16.10V 999
(2060) C1995 03 28.24679 11 24 20.99 -01 05 56.6 16.16V 999
(2060) C1995 03 28.25142 11 24 20.91 -01 05 56.0 16.12V 999
(2060) C1995 03 28.25605 11 24 20.84 -01 05 55.4 16.14V 999
(2060) C1995 03 28.26067 11 24 20.77 -01 05 54.7 16.17V 999
(2060) C1995 03 28.26530 11 24 20.70 -01 05 54.2 16.16V 999
(2060) C1995 03 29.15063 11 24 06.82 -01 03 58.8 16.22V 999
(2060) C1995 03 29.15525 11 24 06.76 -01 03 58.2 16.15V 999
(2060) C1995 03 29.15988 11 24 06.68 -01 03 57.5 16.12V 999
(2060) C1995 03 29.16451 11 24 06.61 -01 03 56.9 16.17V 999
(2060) C1995 03 29.17851 11 24 06.40 -01 03 55.1 16.18V 999
(2060) C1995 03 29.18314 11 24 06.32 -01 03 54.5 16.18V 999
(2060) C1995 03 29.18777 11 24 06.26 -01 03 54.0 16.17V 999
(2060) C1995 03 29.19240 11 24 06.18 -01 03 53.3 16.18V 999
(2060) C1995 03 29.19701 11 24 06.10 -01 03 52.7 16.15V 999
(2060) C1995 03 29.20095 11 24 06.02 -01 03 52.0 16.19V 999
(2060) C1995 03 29.20627 11 24 05.95 -01 03 51.3 16.19V 999
(2060) C1995 03 29.21090 11 24 05.88 -01 03 50.7 16.17V 999
(2060) C1995 03 29.21553 11 24 05.78 -01 03 50.0 16.18V 999
(2060) C1995 03 29.22016 11 24 05.73 -01 03 49.6 16.15V 999
(2060) C1995 03 29.22478 11 24 05.64 -01 03 49.0 16.14V 999
(2060) C1995 03 29.22941 11 24 05.57 -01 03 48.2 16.12V 999
(2060) C1995 03 29.24275 11 24 05.34 -01 03 46.6 16.14V 999
(2060) C1995 03 29.24739 11 24 05.29 -01 03 46.0 16.16V 999
(2060) C1995 03 29.25200 11 24 05.22 -01 03 45.5 16.18V 999
(2060) C1995 03 29.26057 11 24 05.06 -01 03 44.2 16.19V 999
(2060) C1995 03 29.26589 11 24 04.98 -01 03 43.4 16.17V 999
(2060) C1995 03 29.27052 11 24 04.91 -01 03 42.9 16.17V 999
(2060) C1995 03 29.27514 11 24 04.84 -01 03 42.4 16.19V 999
(2060) C1995 03 29.27977 11 24 04.76 -01 03 41.7 16.19V 999
(2060) C1995 03 29.28440 11 24 04.69 -01 03 41.0 16.18V 999
(2060) C1995 03 29.28903 11 24 04.62 -01 03 40.3 16.15V 999
(2060) C1995 03 29.29366 11 24 04.55 -01 03 39.8 16.18V 999
In order to give all evidence, I also add a few lines that I had also sent to Marsden:
"Magnitudes are not reliable for the four first observations of 1995 03 28, not for the three first observations of 1995 03 29, due to the vicinity of a star. The magnitudes for Chiron are given with two decimals. However, Astrometrica reports a dmag in the range 0.02 to 0.10. I have the impression that the magnitudes are precise to +/- 0.03 (see the 'stability' of the values). It is remarkable to observe the drop in magnitude. This might be related to the rotation of Chiron. Indeed, taking the highest magnitudes during the two observation runs, i.e. at resp. 1995 03 28 05:27 UT, and at 1995 03 29 05:19 UT, and assuming that Chiron has undergone four rotations during that period (based on Luu and Jewitt, 1990), then one might deduce a rotational period of 5.97 +/- 0.04 h, which in agreement with the values given by Bus e.a. (1989) and Luu and Jewitt (1990)."
And Marsden replied (on 1995 June 26 -- I will have to celebrate this the next years...):
" I congratulate you on your nice consistent measurements of (2060) and the new object, which we are calling 1995 FC21. We are giving to Val-des- Bois the observatory code 819, ...".
So, we (Bergeron and I) had three things to celebrate:
The following days I will provide here a graph representing the detailed magnitude data. I will also continue my work on the rotational light curve. I already have some things in mind, and on paper -- I just have to rework them to present them online. I preferred to make available the above already now, before the page was fully completed. I hope you don't mind...
URL of this page is: [http://www.vub.ac.be/STER/www.astro/chirot.htm]
Last modified: 1995 October 02. These pages are ©1995.