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Galilean Satellite Mutual Phenomena, 25 September 1997

On 07 January 1610, Galileo Galilei turned his primitive telescope towards Jupiter and saw that it was attended by three little stars. Within a few weeks, he determined by further observations that there were in fact four such objects, and that they were satellites of the planet. The four bodies later became known as the Galileans, and they have been under more-or-less continuous observation ever since their discovery. In order of mean distance from the planet, they are named Io, Europa, Ganymede and Callisto. Table 1 summarises the main data on the satellites and their orbits.

Satellite Orbital Radius
Mag. Orbital Inclination
Orbital Eccentricity Sidereal Period
Io 422,000 3,630 4.9 0.0 0.0001 1.8
Europa 671,000 3,130 5.3 0.5 0.0001 3.6
Ganymede 1,070,000 5,280 4.6 0.2 0.0014 7.2
Callisto 1,880,000 4,820 5.6 0.2 0.0074 16.7

Table 1. Key data for Galilean satellites.

Table 1 shows that the orbits of the Galilean satellites are all very close to perfect circles and all lie very close to Jupiter’s equatorial plane. Approximately every six years, the Earth’s orbit carries it through Jupiter's equatorial plane and, when this happens, for a period of approximately 18 months, it is possible to observe the following mutual phenomena of the satellites:

During 1996-97 the Earth passed through Jupiter's equatorial plane and it was possible to observe mutual eclipses and occultations of the satellites. The Bureau des Longitudes (BDL) of Paris was one of the major astronomical centres disseminating predictions and collating observations of the phenomena in a programme referred to as "PHEMU97" which provided public access to predictions via a website.

Alerted to the event by the BDL website, on the evening of 25 September 1997, I observed an occultation of Ganymede by Io followed by an eclipse of Ganymede by Io. Predicted event times were as follows:

Atmospheric conditions throughout the evening were reasonable: although Jupiter was at an altitude of only some 18° and visible through a layer of haze, the air was very steady and it was possible to glimpse considerable detail in the planet’s main cloud belts. I observed with my 250 mm f10 SCT, using mainly a 12.5 mm eyepiece giving a magnification of 200x.

Figure 1 (a screen dump from Redshift 4) shows the general configuration of the Jovian system at 19:30 UT on the evening of 25 September. Callisto was some 20 Jovian radii to the west of Jupiter, while Europa was closer at approximately six radii west of the planet. To the east of Jupiter, Ganymede and Io were very close together (with Io slightly further out) at approximately five radii distant from the planet. The glare from Jupiter extended to approximately three radii from the planet, so fortunately did not materially affect observations of the satellites.

19970925_Galileans_JMA.gif Figure 1. Configuration of the Jovian system at 19:30 UT on 25 September 1997.

Denoting the predicted central time of the occultation by C, the sequence of events throughout the event was as follows. Io and Ganymede drew slowly closer together, until at approximately C-5 minutes, they appeared to merge. They were then visible as a slightly elongated bar of light until approximately C-2 minutes. From then until approximately C+3 minutes, the two moons appeared together as a single point of light. At C+3, the two moons appeared once more as an elongated bar of light. Finally, at C+8, I was able to discern the two moons as separate points of light. It’s not clear why it was possible to discern the moons as separate until five minutes before C but impossible to separate them again until eight minutes after C. Examination of the satellite’s orbits using the Jovian moon simulator in The Sky ephemeris package suggests that the line-of-sight geometry should have resulted in the moons appearing to separate faster than they appeared to merge.

Following the occultation, later in the evening, Io was responsible for an eclipse of Ganymede. The geometry of the eclipse was such that the moons appeared well-separated as seen from the Earth: Io cast a shadow deep into space which landed on the earth-facing hemisphere of Ganymede. The eclipse was annular. The most noticeable aspect of the moons in the minutes immediately prior to the eclipse was their differing brightness: Ganymede appeared the brighter body. Atmospheric haze made it difficult to be definitive about the colours of the moons and in particular whether there were differences in the colour between the two. As the eclipse progressed, Ganymede grew progressively dimmer. I could not discern the dimming with certainty until approximately a minute before mid-eclipse. At mid-eclipse, the dimming of Ganymede was most noticeable (BDL predicted a reduction in the light flux from Ganymede by 45%). Following mid-eclipse, Ganymede brightened again, regaining its pre-eclipse luminosity after a few minutes.

This was my first observation of mutual events of Jupiter’s Galilean satellites and I found it fascinating! Table 2 lists the most prominent mutual satellite events for the remainder of the 1996-97 season. The table lists only two events, both occurring during November - BDL predicts a very much larger number of phenomena, but many occur under unfavourable observing circumstances, for example the planet is near to the horizon as seen from Ipswich, or the moons lie close to the glare of the planet etc. Glare caused by Jupiter is, in fact, a particular problem. Telescopically, Jupiter creates significant glare which can extend to some four - five radii from the planet’s limb, presenting a major difficulty when attempting to estimate changes in brightness of the satellites. Because of this, one of the main criteria for selecting events to include in table 2 is that the satellites lie beyond the main glare of the planet.

1997 Nov 18

1997 Nov 24

Ganymede occults Europa (partial)
Ganymede occults Io (partial)
Event start (UT)
Mid-time (UT)
Event end (UT)
Drop in light flux
Distance from Jupiter (planetary radii)
Minimum separation of moons (arcsec)

Table 2. Most prominent events for remainder of 1996-97 season.

James Appleton