The moons of
JUPITER
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Jupiter is at opposition on 14 July, a time when the planet will be in the opposite part of the sky to the Sun. At opposition the distance between Earth and Jupiter will be at a minimum for the current period of observation. This means Jupiter’s disc will appear slightly larger and brighter than at other times.
Another consequence of this alignment is that the four Galilean moons and their shadows appear to align. When the Sun, Earth and Jupiter are in a direct line, the shadows appear to line up behind their respective moons. A perfect line up requires the difference between Jupiter’s ecliptic longitude and that of the Sun to be 180°.
However, typically the ecliptic latitude will not be 0°, which is the ecliptic latitude of the Sun. This means the moon shadows tend to appear either above or below their respective moons. In addition, Jupiter and the Sun only maintain their 180° opposition position for a short period of time. Catch a moon and its shadow passing across Jupiter’s face on the day before or after opposition and it’s quite evident that the alignment isn’t perfect.
There are other effects too. Jupiter itself casts a large shadow behind it. Obviously this is hidden by the darkness of space, but it is revealed by the Galilean moons. As they head behind the planet they pass into the shadow for an eclipse event. During periods away from opposition the outer moons can pass into the shadow and out of it again on the same side of Jupiter: visually they effectively disappear then reappear again.
As we get closer to opposition so Jupiter’s shadow becomes more aligned with the planet. The moons disappear as they enter it and then remain hidden as they pass behind Jupiter’s disc. They then reappear from behind the planet’s bright eastern limb. After opposition the reverse takes place. The moons disappear behind the planet’s bright western limb then emerge still within Jupiter’s shadow, which is now present on the eastern side of the planet. The moons appear to magically pop into view as they leave the shadow.
At opposition, Jupiter’s shadow lines up with the planet’s disc as seen from Earth, so for a short time the moons disappear behind the planet’s western limb and reappear behind the eastern limb.
There are a lot of moon events taking place this month and we’ve indicated several interesting ones in the main graphic (above). Watching the Galilean moons dance around gas giant Jupiter is fascinating and can be done quite easily with a small telescope.