Jupiter is the biggest planet in our Solar System. Its four largest moons (Io, Europa, Ganymede, Callisto, also named GalileanMoons) are orbiting around Jupiter in different orbits. Phenomena such as "transit", "eclipse" and "occultation" occur fairly often as the relative positions of the Sun, the Earth, Jupiter and Galilean moons change.

2021 is a bit special for Jupiter. It is spring for Jupiter and rare "MutualPhenomena" between Galilean moons would be possible. You can understand the orbits and the phenomena with an everyday example: a roundabout.

The orbital planes of the Galilean moons are almost on the same plane (the road surface of the roundabout), and the inclination of the moon's orbital plane will vary by plus or minus three degrees as Jupiter revolves. In Jupiter's winter or summer, it is like we watch the roundabout from the footbridge looking at the moons' orbit from above; in Jupiter's spring (2021) or autumn, it's like we watch the roundabout on the road where the moons' orbits are on the same plane to our view, resulting in obstructing of vehicles (moons).

Some of the mutual phenomena would be observable (via a large telescope) in Hong Kong near mid-year this year. A series of transit, eclipse, occultation, mutual occultation and eclipse will happen within a few hours on 15-16 August. We will introduce it in detail soon. Stay tuned!