Jupiter’s 9 May opposition and a month of Jovian events visible from the UK

By Ade Ashford

A simulated high-power telescope view of Jupiter at 11pm BST (22:00 UT) on 9 May 2018, showing the planet’s Great Red Spot with innermost Galilean moon Io superimposed on its shadow – one example of the many Jovian events visible from the UK during May tabulated at the bottom of this page. Observers with Newtonian/Dobsonian telescopes should rotate the image through 180° to match their eyepiece view, while users of refractors and catadioptrics (Schmidt- and Maksutov-Cassegrains) with a star diagonal need to mirror this graphic left-right to replicate their ocular orientation. AN graphic by Ade Ashford/Cartes du Ciel.
The annual viewing highlight for many observers is the opposition of Jupiter, the solar system’s largest planet, which occurs on 9 May this year. So now is the time to arrange a visit to your local astronomical society on an observing night or, if you own your own telescope, to check that its optical components are carefully collimated (aligned) for optimal performance if you wish to see all that the Jovian system has to offer this month.

The equator of Jupiter’s enormous gaseous globe is more than 11 times the diameter of Earth, the planet having a pot-bellied appearance since it spins on its axis in less than ten hours. Note the Great Red Spot anticyclonic weather feature with Earth superimposed at the same scale. The huge storm has been shrinking in recent years, but a new NASA study shows it is moving faster and extending upward. Image credit: NASA
At the beginning of May, Jupiter is already visible low in the southeast an hour after sunset for observers in the British Isles, the brightest object in the twilight sky except for Venus low in the west-northwest and the Moon. The rising full Moon was close to Jupiter late into the evening of 30 April (see this story) and the pair will reunite for a slightly closer conjunction on the evening of 27 May.

With its closest approach to Earth occurring a day after opposition, prime-time Jupiter is as large and as bright as you’ll see it all year, so don’t miss out on any clear nights! Shining at magnitude -2.5, the planet is unmissable even from the most light-polluted town or city. Jupiter’s angular size exceeds 44 arcseconds throughout May, so a telescope magnification of just 50× is sufficient to enlarge it to the same size as an average full Moon appears to the unaided eye. For many of the events about to be described, you’ll need powers of 100× or more and preferably a telescope of 10-cm (4-inch) aperture or larger.

Observing prospects worldwide
Jupiter presently lies in the constellation of Libra some 16 degrees south of the celestial equator – a position that favours Southern Hemisphere observers where the planet reaches a respectable altitude in the northern sky around local midnight. As seen from the Australian capital, for example, Jupiter attains a peak altitude exceeding 70 degrees and lies above the horizon for 13⅔ hours, hence it’s possible to observe more than a full rotation of the planet in a single night.

In the Northern Hemisphere, particularly at the temperate latitudes of the British Isles, the planet’s viewing prospects are best described as fair. For an observer in the heart of the UK, Jupiter’s currently highest in the southern sky around 1:30am BST where it attains a maximum altitude of 20 degrees – about the span of an outstretched hand at arm’s length. At such low altitudes, high-magnification observations of Jupiter will be more susceptible to turbulence in our atmosphere, but steady views (‘good seeing’) can still be had.

However, even in good seeing, unequal refraction at low altitudes will impart some false prismatic colour to the upper and lower limbs of Jupiter, hence an atmospheric dispersion corrector is a wise investment – especially since Saturn at opposition in June and Mars at its closest for 15 years in July will also be low in the sky as seen from the UK. But for now, as viewed from the centre of the British Isles, Jupiter lies above the horizon for just nine hours.

A montage of Jupiter’s four Galilean moons, in a composite image comparing their sizes and that of Jupiter and the Great Red Spot. From left to right: Callisto (diameter 4820 kilometres), Ganymede (5270 km), Europa (3120 km), Io (diameter 3640 km). Image credit: NASA/JPL/DLR – NASA planetary photojournal.

Observing Jupiter’s moons
As befits the solar system’s largest and most massive planet, Jupiter is accompanied by an impressive retinue of 67 natural satellites at the last count. The largest and easiest to see of these are the so-called Galilean moons — Io, Europa, Ganymede and Callisto — discovered by Galileo Galilei in 1610. The Galilean moons are so conspicuous that they are visible in quality 10× binoculars if you steady the view by resting your elbows on a low wall or fence.

Io, Europa, Ganymede and Callisto continually play cat and mouse as their orbital motion cause them to alternately pass in front of (transit) Jupiter, be hidden (occulted) by their parent planet or its shadow (eclipsed). While the transit of a Galilean moon across the face of Jupiter can be challenging to see, their shadows also frequently fall on the cloud tops of Jupiter and are much easier to view in modest (4-inch, or 10-cm aperture) telescopes of good quality at magnifications of 100x or more, seen as inky black dots slowly drifting across the face of the planet.

Note that in all of this article’s diagrams that show compass directions, the Great Red Spot, Galilean moons and their shadows cross Jupiter from east to west. When they are occulted (hidden) by Jupiter or eclipsed by the planet’s shadow they are on the far side of Jupiter from our perspective and moving from west to east.

Jupiter’s south pole is currently tilted towards Earth, hence the outer Galilean moons Ganymede and Callisto appear to the north of the planet’s equator at inferior conjunction – so much so in the case of Callisto that it passes above Jupiter’s disc. Weather permitting, UK-based observers studying Jupiter at 2:30am BST (01:30 UT) on Monday, 14 May can see the Great Red Spot, Ganymede and its shadow superimposed on the planet’s northern limb while Callisto sails above it. AN graphic by Ade Ashford/Cartes du Ciel.
Since Jupiter’s south pole is currently tipped slightly towards Earth, the orbit of outermost Galilean moon Callisto carries it north of its parent planet rather than transiting like the other three. A fine example of this for UK-based observers occurs around 2:30am BST on Monday, 14 May when Callisto at inferior conjunction (between us and Jupiter) passes north of Jupiter’s disc, while Ganymede and its shadow appear superimposed on the planet. It follows that at superior conjunction on the far side of Jupiter, Callisto passes south of the planet while the remaining Galilean moons are occulted or eclipsed.

The table below lists all the Great Red Spot transits (when it crosses an imaginary line joining Jupiter’s north and south poles), Galilean moon shadow transits, occultations and eclipses visible from the British Isles throughout the month of May. Note that the Great Red Spot is well shown for up to an hour either side of the predicted transit time.

Phenomena of Jupiter and its bright Galilean moons visible from the British Isles (including large parts of Western Europe) throughout May 2018. Note that the times of all events are given in British Summer Time (BST), so subtract one hour to convert to Universal Time/GMT. Computation and data preparation: Ade Ashford.
Predictions of Jovian phenomena for any given date may also be obtained through our interactive online Almanac. To see the satellite events for any given day, ensure that the ‘Add phenomena of Jupiter?’ checkbox is ticked. Like the Great Red Spot predictions, all Galilean moon events are in Universal Time (UT). For help using the Almanac, see this article.