The photometric observation of the quasi-simultaneous mutual eclipse and occultation between Europa and Ganymede on 22 August 2021

• Published in: Icarus (New York, N.Y. 1962) N.Y. 1962), 2023-03, Vol.392, p.115348, Article 115348
• Main Authors: So, Chu Wing, Luk, Godfrey Ho Ching, Chung, Giann On Ching, Leung, Po Kin, Hui, Kenneith Ho Keung, Cheung, Jack Lap Chung, Chan, Ka Wo, Yuen, Edwin Lok Hei, Lee, Lawrence Wai Kwan, Lau, Patrick Kai Ip, Cheung, Gloria Wing Shan, Chan, Prince Chun Lam, Pun, Jason Chun Shing
• Format: Article
• Language: English
• Subjects: Astrometry; Eclipses; Jupiter; Occultations; Photometry; Satellites
• ISSN: 0019-1035; 1090-2643
• DOI: 10.1016/j.icarus.2022.115348

Mutual events (MEs) are eclipses and occultations among planetary natural satellites. Most of the time, eclipses and occultations occur separately. However, the same satellite pair will exhibit an eclipse and an occultation quasi-simultaneously under particular orbital configurations. This kind of rare event is termed as a quasi-simultaneous mutual event (“QSME”). During the 2021 campaign of mutual events of jovian satellites, we observed a QSME between Europa and Ganymede. The present study aims to describe and study the event in detail. We observed the QSME with a CCD camera attached to a 300-mm telescope at the Hong Kong Space Museum Sai Kung iObservatory. We obtained the combined flux of Europa and Ganymede from aperture photometry. A geometric model was developed to explain the light curve observed. Our results are compared with theoretical predictions (“O-C”). We found that our simple geometric model can explain the QSME fairly accurately, and the QSME light curve is a superposition of the light curves of an eclipse and an occultation. Notably, the observed flux drops are within 2.6% of the theoretical predictions. The size of the event central time O-C’s ranges from −14.4 to 43.2 s. Both O-C’s of flux drop and timing are comparable to other studies adopting more complicated models. Given the event rarity, model simplicity and accuracy, we encourage more observations and analysis on QSMEs to improve Solar System ephemerides. •A satellite can be eclipsed and occulted by another satellite quasi-simultaneously.•This paper presents the 2021 observation of such an event between jovian satellites.•A simple geometric model is developed to explain the event.•The model yields astrometric solutions consistent with theoretical predictions.