How many moons can you see with the naked eye? The answer may surprise you..

Of the hundreds of moons orbiting the planets of our solar system, all but four are too faint to see with the naked eye. One of the four is, of course, Earth’s own Moon. The other three moons orbit the giant planet Jupiter. It is by virtue of their proximity to Jupiter – the fourth brightest object in the sky – that they are ignored by most of us – except, of course, astronomers. 

The moons of Jupiter were first glimpsed four centuries ago when Italian astronomer Galileo Galilei turned one of the first telescopes towards Jupiter and discovered four tiny stars close to the planet (some of Galileo’s drawings of the moons can be found here). Those moons, the first to be discovered around another planet, were subsequently named after mythological lovers of the Olympian god Zeus: Io, Europa, Ganymede and Callisto, with that last one being just under the limit of naked eye visibility. Collectively, they are referred to as “Galilean” in honour of their discoverer.

In their short orbital trek around Jupiter, the moons inevitably cast their shadows on the planet, however making these shadows out requires a telescope. The opposite effect, ie Jupiter casting its own vast shadow on one of its moons, is much easier to see; in that case, the moon simply disappears from sight – an eclipse – in the space of a few minutes.  

Eclipses can also take place between moons. Every six years Io, Europa, Ganymede and Callisto repeatedly eclipse each other when the Sun (and the Earth) line up with the moons’ orbits. Observations of these “mutual events” yield very precise positional measurements of the satellites, used to measure tiny drifts in the orbits due to tides raised by the moons on the planet and vice versa.

Three frames from a sequence of images of Jupiter and its satellites obtained with the Armagh Robotic Telescope (ART) on the night of the 6th April 2015 during an eclipse of Europa by Io. Time advances from top to bottom. In the middle frame, the satellite Io is casting its shadow on the satellite Europa and, as a result, Europa goes into eclipse. In the bottom frame, the eclipse is over and Europa has resumed its normal brightness.

Since 1973, the Institut de Méchanique Céleste et de Calcul des Éphémérides (IMCCE) in Paris, France, has been organising the international campaign of “phénomènes mutuels”, or “PHEMU” for short, where professional and amateur astronomers around the world are invited to record the mutual events and send their observations to IMCCE for analysis. Armagh astronomers participated in the PHEMU campaigns in 2003, 2009 and again in 2015.

In 2015, we had at our disposal the brand-new Armagh Robotic Telescope (aka the ART: see last February’s article by Heather Alexander). Using the ART, a team of Observatory staff and students recorded over twenty such events between December 2014 and April 2015.

The four Galilean moons of Jupiter. From left to right: Io, Europa, Ganymede and Callisto. Io and Europa are roughly the size of Earth’s Moon, Ganymede and Callisto are considerably larger and similar in size to the planet Mercury.

The  Galilean moons are easy enough to spot through a pair of binoculars as tiny stars, huddled close to the bright beacon of Jupiter (for tips on observing and imaging the Galilean moons through binoculars, see this 2020 article by Simon Jeffery). This year’s moon-viewing season will be opening up come August when Jupiter will be shining as a creamy yellow star above the south horizon at midnight.

The moons are unfortunately too far to make out details on their surfaces, even the best telescopes will struggle to resolve their tiny disks. Nevertheless, spacecraft exploration in the 1970s, 80s and 90s showed the moons to be fascinating worlds in their own right, each different from the other. Innermost moon Io has been likened to a pizza and features sulfur-spewing active volcanoes while Europa is covered by a thick layer of almost pure water ice overlying a sub-surface ocean of liquid water. Ganymede, the largest moon in the solar system, is criss-crossed with bundles of parallel grooves as if a giant rake has been dragged across its surface. The outermost moon, Callisto, is quite dark and resembles a huge glass orb, showing the scars of having been stricken again and again by cosmic debris.

It has now been over 20 years since the Galilean moons received a visitor from Earth, but this is about to change. Since 2016, NASA’s Juno spacecraft has been orbiting Jupiter, studying the planet and its atmosphere. In late 2020, Juno was given a new “extended” mission, to fly close to the Galilean satellites and make new, detailed measurements including pictures. It so happens these satellite flybys, the first since the late 1990s, are starting this summer. Juno just flew past the moon Ganymede on the 7th of June and will do so again on the 20th of July. That first flyby is a particularly close shave, with the spacecraft coming to about 1000 km from the satellite’s surface, or one-fifth the diameter of Ganymede itself. Close-up pictures from this flyby have just become available and more satellite visits are planned through to the end of the year and beyond, including close passes to Ganymede’s siblings: Europa in September 2022 and volcanic Io in December 2023.

The Juno flybys will preempt a period of intensive study of the Jupiter system by US and European spacecraft missions later this decade and into the next.


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