PHEMU03: "ÉDUCATION"
Learning astronomy through the observation of the mutual occultations and eclipses of the Galilean satellites of Jupiter in 2002-2003

Abstract

Among the numerous types of observations made by the professional astronomers in order to obtain scientific progresses in their scientific research, most of them need the use of large telescopes. Data may be available for educational purpose but data may not be obtained directly by students from high schools. We propose here to present observations which are able to be made with small telescopes without any complex instruments and receptors but, if made with care, useful for scientific projects conducted by astronomers.

We propose the observation of the Galilean satellites of Jupiter which look like a small solar system and which gather most of the problems in celestial mechanics and in planetology. Any amateur astronomer knows the eclipses of theses satellites by the shadow of Jupiter. The observation of these phenomena is interesting, mainly in order to study the historical importance of these events in the discovery of new territories on Earth (these events were a clock to the pioneers explorating new territories and measuring longitudes and latitudes). However, nowadays, these events are no more observed since the accuracy of such observations do not allow any progress in our knowledge of th Galilean satellites.

However, another type of event, the mutual eclipses and occultations, may occur between the satellites themselves because of the configuration of their orbits. Every six years, the Earth and the Sun will go through the common orbital plane of the satellites and during one-year period events will occur. These events are very easy to observe, even with small telescopes associated to CCD cameras, because the satellites are bright (around magnitude 5) and because the light flux drop during an event is sufficient for easy observations. Since the 1970's, it has been demonstrated that a lot of interesting informations may be deduced from these observations. Each observation, even made with a small telescope, will provide astrometric positions of the satellites with an accuracy ten times better than the best regular astrometric observations performed with large professional telescopes. Then, these data will allow to determine the constants in the models of motions of these satellites. The accuracy reaches such a level that we will be able to measure slight shift in the motion of the satellites (accelerations) indicating the level of loss of energy of the satellites because of tide effects. Then , we will be able to constrain the models of the interior of the satellites and provide limits in the possibilities of liquid oceans under the iced crust for the satellite Europe, for example.

We propose to anyone interested, as educational project, to participate to this adventure.

Presentation of the mutual phenomena

Shadows of the Galilean satellites on Jupiter: mutual eclipses concern the shadows of the satellites on another satellite.

Every six years, the Earth and the Sun pass through the equatorial plane of Jupiter which is the orbital plane of the Galilean satellites. Then mutual eclipses occur when the Sun is in this plane and mutual occultations occur when the Earth crosses this plane. As seen by a terrestrial observer, a phenomenon corresponds to the decrease of the light flux received from the occulted or eclipsed satellite. Such events have a duration from a few minutes to more than one hour. The magnitude drop may be large (especially for a total event when a satellite disappears behind another satellite or in the shadow of another satellite). The observation is easy because of the magnitude of the satellites themselves, around 5.

The next favorable occurrence will take place from September, 2002 to July 2003. The observational conditions are especially favorable during this occurrence: the maximum occurrence of events will be at the opposition of Jupiter with the Sun and the declination of Jupiter will be near +17°, a favorable value for the observers of the Northern hemisphere.

Observing these events

Observing these events is easy: as for the eclipses by Jupiter, one will have to record the flux drop during an event using a video or CCD camera associated to a small telescope (at least a 15cm-aperture). These observations allow to record information to be provided to the laboratories studying the Galilean satellites. They also need to be coordinated through the international network of observers gathering professional or amateur astronomers and to understand that all the observations should be related through the timing in Universal Time.

Fig. 2: examples of observations: the recorded light-curves: left, an eclipse of Io by Europa observed with a small refracting telescope with a 15 cm-aperture; at right, an occultation of Io by Europa observed with a 41cm-aperture telescope.

The observations consist in the recording of the variation of the light flux coming from the satellites.

Educational project proposed

The educational goal of this project goal is the understanding of the relationship between the theoretical works on modeling the motions of the Galilean satellites, the nature of the events to be observed, the technique developed for the observation and the information deduced from the observation.

We proposed that groups of students who have yet made some astronomical observations with a small telescope to join the network in order to learn about the galilean satellites of Jupiter, to understand the scientific goal of the project and to participate to the project through some easy observations.

The different groups from several countries will be corresponding in order to exchange their experiences and results.

Available information

The observations of the mutual phenomena will occur from September, 2002 to July, 2003. Please contact us at phemu@imcce.fr for more information and to be included in the network. We will communicate to you the addresses of the other groups of students participating to the project. Dates of the events and interactive software available at the address of the Phemu03 Web site.