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An eclipse of Europa by Ganymede on 2009 August 12.

The BAA Handbook predicted there would be an interesting mutual event of Jupiter's satellites during the night of August 11/12 this year, at the height of the Perseid meteor shower. The moon Europa would be totally eclipsed by Ganymede with mid-eclipse at 1h 53m 20s UT on the morning of August 12. The exact time of the eclipse is of interest to those who develop dynamical models of the motion of the Galilean satellites, since the predicted time is sensitive to the details of the different models. I was surprised to learn that these models can differ in their predicted times of mid-eclipse by more than 10 seconds. For example reference 1 lists two predicted mid-eclipse times based on different ephemerides as 1h 52m 55s and 1h 53m 7s, both adjusted from TT to UTC.

I decided to try to measure the time of mid-eclipse by making photometric measurements of the combined brightness of the two moons in comparison to Callisto, the only other object of similar brightness within my CCD camera's field of view. The brightest star in the field was about 6 magnitudes fainter, too large a difference for accurate photometry.

As the eclipse approached, Ganymede and Europa were close enough that their images could be included within a single photometry aperture. Ganymede and Callisto, which lay on opposite sides of Jupiter, were very close to opposite edges of the chip in my SXV-H9 CCD so I had to guide carefully to ensure both objects remained far enough inside the chip to be able to carry out photometry on their images. I started observing at 01:22 UT to establish the out-of-eclipse level of brightness of the combined images of Ganymede and Europa. Because their combined brightness was about 4th magnitude, I used a B filter to reduce the light level and exposed each image for only 0.5 sec to avoid saturation. (Alternatively I could have stopped down my 355mm SCT but this would have necessitated taking new flat fields).

Figure 1 shows the configuration of Jupiter and its moons at 01:28 UT. Jupiter is grossly overexposed while Io is in transit and invisible. At that time Jupiter was only 20[degrees] above the southwest horizon and slowly sinking. After about 9 minutes clouds rolled in completely blotting out Jupiter and I was ready to give up the attempt. However I left the camera running and 17 minutes later the cloud thinned sufficiently to give usable images of Jupiter and its moons. By this time the eclipse was underway and the combined image of the two moons, no longer resolvable, was starting to dim. Fortunately the clouds remained thin until the eclipse was almost over, before thickening and again intermittently blotting out the planet.



According to the Handbook, there was also due to be a partial occultation of Europa by Ganymede centred on 02:11 UT. I had hoped to record that event as well, even though the drop in brightness would be much less than for the eclipse, but the poor conditions held out little prospect of success. I finished the run at 02:32 UT having collected about 1000 measurable images.

I used differential aperture photometry in AIP4WIN to measure the combined magnitude of Ganymede and Europa assuming a magnitude of 5.65 for Callisto as given by NASA. (2) The short exposures meant that scintillation caused large random variations in the derived magnitudes. To compensate for this, groups of 10 consecutive magnitude measurements were averaged giving a mean magnitude measurement approximately every 30 sec. These mean magnitudes are plotted in Figure 2. The mean combined magnitude of Ganymede and Europa before the eclipse was 4.11. The theoretical combined magnitude of Ganymede and Europa, using the individual magnitudes 4.61 and 5.19 respectively given in reference 2, is 4.15.

The data were of much poorer quality towards the end of the run and as suspected the occultation was not detectable in the noisy light curve after the eclipse. At the deepest point of the eclipse, when only Ganymede was visible, the mean magnitude was 4.62, in good agreement with its magnitude in ref. 2. The eclipse itself was recorded sufficiently well to enable a measurement of the time of mid-eclipse by fitting a quadratic curve to the data points. This gave the mid-eclipse time as 1h 53m 16 [+ or -] 5s UT, in good agreement with the ephemeris in reference 3 based on the most recent model, which gives the predicted mid-eclipse time as 1h 53m 13s UT.


(1) Arlot J.-E., A*A, 478, 285-298 (2008)


(3) ephemerides/generateur/saimirror/ nssephe.php
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Title Annotation:Observers' Forum
Author:Boyd, David
Publication:Journal of the British Astronomical Association
Date:Oct 1, 2009
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