The Sky Guide challenge
Jupiter’s four Galilean moons are easy to spot, but how many others can you find?
With Jupiter now clearly visible low in the south, this month’s challenge is to capture some of its fainter moons. 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 four of them are easy to spot. These are the four so called Galilean moons, Io, Europa, Ganymede and Callisto. The reason for the apparent disparity between the total moon count and the number seen through the eyepiece is down to size. The four Galileans are huge and appear bright, the remaining 75 are small and quite faint.
However, four is not the limit for amateurs and with a bit of vigilance, others may be seen or imaged. The moons which 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 for them 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 which show their positions, including the free Jupiter Guide, the freeware version of Stellarium and the popular freeware planetarium program Cartes du Ciel which may be downloaded from www.ap-i.net/skychart/en/start. 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 potential amateur 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. As ever, if you manage to capture one of these dim objects, we’d be delighted to see your results.