Equinoxes on Jupiter and Saturn in 2009: call for observations

International audienceThe equinox time on the giant planets The equinox occurs on the giant planets as on Earth, when the Sun crosses the equatorial plane i.e. every 6 years for Jupiter, 14 years for Saturn, 42 years for Uranus and 82 years for Neptune. The main large satellites of these planets have their orbits in the equatorial plane and, except Neptune, each planet have several major satellites. Note that at the time of the equinox, the Earth will also cross the equatorial plane of the planet since, as seen from the giant planets, the Earth is very close to the Sun. Then, the satellites will eclipse and occult each other mutually for a groundbased observer during a one-year period around the equinox. These events are observed with photometric receivers since the light from the satellites will decrease during the events. The light curve will provide information on the geometric configuration of the satellites at the time of the event with an accuracy of a few kilometers, not depending on the distance to Earth of the satellite system. Then, we are able to get an astrometric observation with an accuracy several times better than using direct imaging for astrometric positions. The equinox on Jupiter in 2009 The equinox on Jupiter will occur on June 22, 2009 allowing mutual events during about one year around the equinox time (mutual events will occur from January 2009 to April 2010 not regularly due to the small inclinations of the orbits of the satellites on the Jovian equator) as shown on figure n°1. observable events should be numerous. The negative declination of Jupiter will make the observations easier from southern sites but they are not sufficient as seen on figure n°5 and a special effort should be made in the northern hemisphere to make observations even when Jupiter is low above the horizon. The Galilean satellites of Jupiter are very bright (magnitude 5) and their observation is possible even with a very small telescope. Amateur astronomers are able to make such observations of a high astrometric accuracy (similar to space observations): for that purpose they need to follow very precise recommendations. The photometric accuracy should be good enough for further analysis. Such a photometric accuracy may be reached even with cheap camera and we encourage the potential observers to read the Technical notes dedicated to the observation of the mutual events and the recommendations to be followed by the observers on the web site http://www.imcce.fr/phemu09. Note that all the light curves must be dated in Universal Time with an accuracy of 0.1 second of time to be useful for scientific purpose. GPS may be used for that purpose. Such events occur every 6 years and we have experience of these observations. In the past, many campaigns of observations were made [1] providing a large amount of accurate data allowing to improve the dynamics of the system and we know how useful are the observations made by amateur astronomers. The predictions of the mutual events have been made [2] and are also available at the web address given above. We provide below examples of light curves to be obtained on figures n°2 and 3. On figure n°2, the light curve is quite symmetrical (the surface of Europa is quite uniform) but on figure n°3, the light curve is not symmetrical (the surface of Callisto is less uniform) providing information of the surfaces of the satellite and on the laws of reflexion of light. This information allows to improve the astrometric accuracy of the observations by helping to find the center of mass or center of figure of the satellite from the observation of its center of light or photocenter. The equinox on Saturn in 2009 The equinox on Saturn will occur on August 12, 2009 allowing events as for the Jovian system as shown on figure n°4. The mutual events will occur from September 2008 to June 2010. observable events will be less numerous and a special effort has to be made by the observers. The declination of Saturn is favourable for both hemispheres. Such event occurs every 14 years and the satellites are fainter than the Jovian ones. We have experience of these observations. In the past, two campaigns of observations were made [3] providing accurate data allowing to improve the dynamics of the system. The predictions of the mutual events have been made [4] and are also available at the web address: http://www.imcce.fr/phemu09. Call for observations These observations have shown their high interest allowing quantifying the dissipation of energy due to the tidal effects inside the satellites. A constraint on the internal structure of the satellites may be deduced from the analysis of these observations thanks to the theoretical models developed either at JPL or at IMCCE. As we did in the past, we need the participation of as many observers as possible since the events occur at any time and that the satellites must be above the horizon to be observed. Observing sites at all possible longitudes are required. For the last campaigns, we had about 40 sites of observations as shown on figure n°5. More observing sites are required in the southern hemisphere for the Jovian events. The Saturnian events are more difficult to catch (fainter satellites closer to the bright planet) but are observable easily with a CCD camera and a telescope with a not too short focal length. All explanations are provided on Technical notes available on the web site of IMCCE at: http://www.imcce.fr/phemu09. 2009 will be the International Year of Astronomy supported by IAU. We proposed that the opportunity of the equinoxes on Jupiter and Saturn were used to ask professional and amateur astronomers to make together observations of the mutual events presented in this communication. It will be a commemoration of the observations by Galileo four centuries ago. He extensively observed the Jovian system discovering the eclipses by Jupiter. Such an international campaign of observation will gather professional and amateurs for a common scientific work as we did in the past as shown by common publications (see [1] and [3] ). Do not hesitate to join the network of observers to participate to observations! Contact: arlot@imcce.fr Web site: http://www.imcce.fr/phemu09 References [1] Arlot J.E. et al. (2006) Astron. Astrophys., 451, 733-737. [2] Arlot J.E. (2008) Astron. Astrophys., 478, 285-298. [3] Thuillot W.. et al. (2001) Astron. Astrophys., 371, 343-349. [4] Arlot J.E. et al. (2008) Astron. Astrophys., 485, 293- 298