THE BIG THREE
The three top sights to observe or image this month
JOVIAN EQUINOX BEST TIME TO SEE: 6, 14 and 29 May at specified times
Jupiter reaches equinox on 2 May, a position where the geometric centre of the Sun’s disc appears to cross the equatorial plane of the planet. Compared to Earth, Jupiter’s axial tilt is quite small; 3.3˚ compared to our planet’s 23.4˚. For this reason, seasonal variations on Jupiter are relatively minor. Of more significance from our perspective, the four Galilean moons Io, Europa, Ganymede and Callisto, have orbits which are only slightly inclined to Jupiter’s equatorial plane.
At those times away from an equinox, if the orbits of these moons were directly visible, from the Sun they would appear like narrow ellipses. Near to a solstice – a position where one of the planet’s poles points maximally towards the Sun – the orbit ellipses would appear to have maximum width. Near a Jovian solstice, the edges of the orbital ellipses would all intersect Jupiter’s disc with the exception of Callisto’s. However, near to a Jovian equinox, all of the ellipses would appear so narrow from the perspective of the Sun that they would be lines representing the moon orbits seen edge on.
From Earth this creates interesting opportunities, as our view of the orbital ellipses isn’t that different to the Sun’s. Near a Jovian equinox we see the Galilean moons shuffling back and forth either side of the planet in an almost straight line. The alignment relative to the Sun allows the shadows of each moon to interact with other moons. Around a Jovian eclipse it’s possible to witness moon-moon transits, occultations and eclipses – which are collectively known as mutual events.
On 6 May, Io’s shadow eclipses Europa from 04:26-04:32 BST (03:26-03:32 UT).
Then on the 14th, it’s Ganymede’s shadow which eclipses Io between 04:43-04:53 BST (03:43-03:53 UT). Then, from 03:2704:17 BST (02:27-03:17 UT) on 29 May, Ganymede’s shadow takes another swipe at Io and covers the majority of Io’s disc.
The visibility of these events varies depending on where Jupiter is in the sky. The event on 6 May occurs under brightening sky conditions with Jupiter just 8˚ above the southeast horizon. The 14 May event fares a little better in terms of altitude, Jupiter appearing 13˚ above the southeast horizon by the time it starts. The event on 29 May occurs with Jupiter 11˚ up under lightening dawn twilight skies.