Jupiter has a lot of moons: 79 at the last count. You’d never know looking at the planet through an amateur scope because only 4 of them are easy to spot. These are the 4 so-called Galilean moons: Io, Europa, Ganymede and Callisto.
Watching the Galilean moons dance around gas giant Jupiter is fascinating and can be done quite easily with a small telescope.
- For more about Jupiter’s 4 biggest satellites, read our guide on the discovery of the Galilean moons.
The reason for the apparent disparity between Jupiter’s total moon count and the number seen through the eyepiece is down to size.
Jupiter is at opposition on 19 August 2021, 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.
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.
Also at opposition, the 4 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.
How to see Jupiter’s fainter moons
The 4 Galileans are huge and appear bright, the remaining 75 are small and quite faint. However, 4 moons is not all you can observe around Jupiter and with a bit of vigilance, others may be seen or imaged.
The moons that lie within the magnitude reach of larger amateur scopes are:
- Amalthea at mag. +14.1
- Himalia at mag. +14.8
- Thebe at mag. +15.7
- Metis at mag. +17.6
- And potentially Lysithea at mag. +18.4
The best time to try to spot these Jovian moons is when they are close to an elongation (in other words when they appear furthest from Jupiter).
There are a number of apps and programs available that show their positions, including the free Jupiter Guide, the freeware version of Stellarium and the popular freeware planetarium program Cartes du Ciel.
This program has an option to ‘Show the faint satellites of the outer planets’ (under the ‘Setup: Solar System’ option at the bottom of the ‘Solar System’ tab).
With this checkbox ticked, it will show the positions of the brighter faint Jovian moons.
Some of the targets mentioned above will be compromised due to their proximity to bright Jupiter. Thebe and Metis for example, never appear to wander too far from the planet and this will make imaging them extremely difficult.
Amalthea is a good moon to start with although its maximum elongation from Jupiter isn’t much beyond an apparent Jupiter diameter. Himalia is slightly dimmer but does have the virtue of appearing to move much farther from the planet.
Run a simulation using a program which shows the moons’ positions and determine when Amalthea will be close to elongation at a time when Jupiter is at its highest in the sky.
Unfortunately, from the UK currently, Jupiter’s southern position means that its maximum altitude is quite low, something which will start to improve in subsequent years. A clear, mist free night will work best.
Imaging is the best course of action for capturing these moons and your imaging setup will need to be capable of capturing stars fainter than the magnitude of the moon you’re after.
Pete Lawrence is an experienced astronomer and a co-host on The Sky at Night. This guide originally appeared in the June and July 2020 issues of BBC Sky at Night Magazine.