Galilean moon shadows
BEST TIME TO SEE: 4, 8 & 13 June at the times shown on the graphic
The four giant Galilean moons of Jupiter present an ever-changing ballet for those of us lucky enough to be able to view this spectacular world through the eyepiece of a telescope. As they pass around the planet, they tend to stay close to the gas giant’s equatorial plane. Passing between Jupiter and the Sun, they cast huge, impressive dark shadows on the Jovian atmosphere below.
Before opposition, the geometry of this arrangement is such that a shadow precedes the moon casting it. After opposition a shadow follows its moon. If you’re a regular observer of Jupiter this shifting order is very evident.
However, there’s a third arrangement which occurs close to opposition and is quite
spectacular. Here, the alignment is such that the moons line up with their shadows. As each moon transits across the planet’s disc, so its shadow marches along in sync. The relative position of Jupiter, the Galilean moons and the Sun normally means that the shadow can still be seen either north or
south of the moon as the pair transit. When Jupiter is close to an equinox (the next will happen in 2021) the alignment is much tighter and, the moon can overlay its shadow.
Jupiter reaches opposition on 10 June and there are several transits worth looking for during this period as shown in the graphic.
Europa and its shadow in transit from 23:53–02:32 BST (22:53–01:32 UT), 7 and 8 June
Io and its shadow in transit from 21:45–00:06 BST (20:45 –23:06 UT), 13 and 14 June
Double moon and shadow transit, from 00:30–03:52 BST (23:30–02:52 UT), 5 June