Get ready for prime-time Jupiter and its multi-moon events in 2019

By Ade Ashford

The first multiple shadow transit of Jupiter’s Galilean moons close to the planet’s 2019 opposition date occurs in the small hours of 5 June for the UK. On this occasion the shadows of Io and Ganymede are cast upon the face of their parent planet from 01:29–02:53am BST (00:29–01:53 UT). This looping animation depicts the motion of Io, Ganymede and their shadows every ten minutes over the course of two hours starting 01:30am BST (00:30 UT). Observers on the Eastern Seaboard of the United States can see the shadows of Io and Ganymede on Jupiter from 11:32pm EDT on 11 June to 12:35am EDT the following morning. AN animation by Ade Ashford/Cartes du Ciel.
Even casual skywatchers in the British Isles and Western Europe cannot fail to notice the bright magnitude -2.6 ‘star’ currently low in the south-southeast around local midnight. The object in question is none other than the solar system’s largest planet, Jupiter, and its prominence is due to the fact that it’s rapidly approaching its closest point to Earth for 2019.

Jupiter reaches opposition on Monday, 10 June when it lies in the constellation of Ophiuchus (the Serpent Bearer) and is visible throughout the night. But the planet doesn’t make its closest approach to Earth for the year until shortly after 4am BST on 12 June, when the distance between our two worlds narrows to 4.2839 astronomical units, or 640.8 million kilometres (398.2 million miles).

This NASA/ESA Hubble Space Telescope image of Jupiter taken on 3 April 2017 reveals the intricate, detailed beauty of the planet’s clouds as arranged into bands at different latitudes. Lighter coloured areas, called zones, are high-pressure where the atmosphere rises. Darker low-pressure regions where air falls are called belts. Constantly stormy weather occurs where these opposing east-to-west and west-to-east flows interact. The planet’s Great Red Spot (GRS, lower left), is a long-lived storm roughly the diameter of Earth. Image credit: NASA, ESA, and A. Simon (GSFC).
With a diameter in excess of 11 times that of Earth, Jupiter’s angular size still attains 46 arcseconds at its minimum distance from Earth in early June. This means that a magnification of little more than 40× is required to enlarge it to the same size as the full Moon appears to the unaided eye.

For observers in the UK this is not a particularly favourable opposition due to Jupiter’s southerly declination of -22 degrees, which means that Southern Hemisphere observers get to see it much higher in the sky.

For an observer in the heart of the British Isles, Jupiter attains a peak altitude of just 14 degrees in the southern sky close to 1am BST. By the end of June, the planet is highest in the south by 11:30pm BST. But don’t let Jupiter’s low altitude dissuade you from looking, for there’s still much to see from northern temperate latitudes on nights of steady seeing.

Jupiter’s Great Red Spot
Quality telescopes with apertures of just 7.6- to 10-cm (3- to 4-inch) at magnifications of 100× or more will readily reveal structures in Jupiter’s gaseous atmosphere. Naturally, larger apertures will reveal more detail (seeing permitting).

The table above lists transits of Jupiter’s Great Red Spot (GRS) visible from Western Europe and the British Isles, with those best seen from the UK highlighted in grey. 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/Guide.
Aside from the darker equatorial cloud belts, look out for an oval-shaped and brick-coloured anticyclonic storm in Jupiter’s atmosphere some 22 degrees south of the planet’s equator known as the Great Red Spot (GRS).

A so-called transit of the GRS occurs when it is seen to cross an imaginary line drawn between the planet’s north and south poles. Great Red Spot transits occur roughly every 9h 56m (Jupiter’s rotation period at that latitude) and the feature is well seen up to an hour either side of the transit time.

See this article for an update on dramatic changes occurring in and around the GRS.

Jupiter’s Galilean moons
The largest of Jupiter’s 79 known natural satellites are the so-called Galilean moons — Io, Europa, Ganymede and Callisto — discovered by Galileo Galilei on 7 January 1610. Europa, the smallest of the ‘big four’, has a diameter slightly less than our own Moon, whereas Ganymede exceeds planet Mercury in size. In fact, the Galilean moons are so conspicuous that quality binoculars (if suitably steadied by leaning one’s elbows on a wall or fence) will reveal them when farthest from their parent planet.

A montage of Jupiter’s four Galilean moons, in a composite image comparing their sizes with Jupiter (part of) and its Great Red Spot. From left to right: Callisto (diameter 4820 kilometres), Ganymede (5262 km), Europa (3122 km), Io (diameter 3642 km). Image credit: NASA/JPL/DLR – NASA planetary photojournal.
Galilean moon events
The orbital motion of Io, Europa, Ganymede and Callisto cause them to shuttle back and forth, occasionally passing in front of (transit) Jupiter or be hidden (occulted) by their parent planet or its shadow (eclipsed).

Furthermore, the shadows of the Galilean moons frequently fall on the cloud tops of Jupiter, visible in modest (7½-centimetre or 3-inch aperture) telescopes of good quality at magnifications of 100× or more as inky black dots slowly drifting across the face of the planet.

Multi-Galilean moon events
The double shadow transit of Io and Ganymede in the small hours of Wednesday, 5 June depicted at the top of the page is followed by a repeat performance on 12 June, but Jupiter is at a very low altitude and sets in the British Isles soon after the event starts at 4:29am BST. However, this event is well seen from the Eastern Seaboard of the United States where the double shadow transit of Io and Ganymede starts at 11:32pm EDT on 11 June.

The next big multi-moon event visible from the UK is that of Io, Europa and their shadows at 5pm GMT on Thursday, 31 October when Callisto is also partially transiting the north polar region of its parent planet. Furthermore, Jupiter is in a close conjunction with a 3-day-old crescent Moon that night.

Other phenomena of Jupiter’s Galilean moons
The table below lists moon and shadow transits, occultations and eclipses visible from the British Isles from 31 May to 1 July 2019. Note that due to the inclination of the moons’ orbits to our line of sight, only events of Io, Europa and Ganymede feature in the table; shadow transits, eclipses and occultations of outermost Galilean moon Callisto start on 1 November 2019.

Phenomena of Jupiter and its bright Galilean moons visible from the British Isles throughout June 2019. Note that all events are given in 24-hour British Summer Time (BST), so subtract one hour to convert to Universal Time/GMT. Computation and data preparation: Ade Ashford/Guide.