Dominant Jupiter comes to opposition

Mighty Jupiter, the king of the planets and the most observer-friendly planet of all, comes to opposition on the night of 19/20 August (Thursday night/Friday morning) at around midnight BST. It lies 4.013 astronomical units (AU) from Earth, a distance of around 600.3 million kilometres (373 million miles).

Jupiter imaged from the UK on 11 August 2021 at 00:25 UT. The dark red ovals in the north of the NEB are prominent, as it the turbulence around the GRS, which is just rotating out of view to the lower-right. Image: Damian Peach.

Superior planets such as Jupiter (planets that orbit the Sun outside of Earth’s orbit) are best observed when they reach opposition and Earth lies directly between it and the Sun. At this time, the planet lies directly opposite the Sun in our sky, rises at the Sun sets and can be observed for a large chunk of the night.

Where do I look for Jupiter?

Jupiter is located around 13 degrees south of the celestial equator, straddling the boundary between Capricornus to the west and Aquarius to the east. The gas giant actually lies marginally inside the former’s territory.

Anyone who has been out observing at around the midnight hour in recent weeks will already know how dazzling Jupiter is to the naked eye, as the planet totally dominates its part of the sky and overshadows nearby Saturn in the process.
On opposition night, Jupiter shines a magnitude –2.9, making it by far the brightest object in the night sky except for the Moon and Venus (Mars at its perihelic oppositions can rival Jupiter’s brilliance) You can’t miss it when looking to the south from late evening onwards, though its southerly declination means its elevation in the sky across the UK is far from ideal. Jupiter traces a relatively shallow arc across the sky, which limits Jupiter’s observing window to a few hours around culmination.

Town and city dwellers will want to make sure their observing site has a reasonably unobstructed view from the south-east around to the south-west.

At what time can Jupiter be observed?

From Edinburgh, Jupiter rises at about 8.30pm BST (19:30 UT) and achieves a decent 20-degree altitude as it culminates at about 1.15am BST (00:15 UT). Observers in London will see Jupiter cross the southern meridian (culminate) at about 1am BST, when it lies at a reasonable 25 degrees above the horizon and stays at or above the important 20-degree-altitude barrier from about 11pm to 3am BST (22:00 to 02:00 UT).

To get the best from any observing session, try to observe Jupiter within an hour either side of culmination.

What can I see with binoculars?

Grab a pair of 10 x 50 or binoculars and Jupiter will show a small, perceptible disc, which at opposition spans a very impressive 49.1 arcseconds, almost three times larger than Saturn’s globe (not including its rings) appeared at its opposition at the start of August.

You should also notice that Jupiter’s globe bulges outwards owing to Jupiter’s rapid rotation rate, which is about nine hours and 50 minutes at the equator and slightly slower at Jovian latitudes above 15 degrees.

One of Jupiter’s great appeals to observers is its four bright Galilean moons, Io (I), Europa (II), Ganymede (III) and Callisto (IV). Since Galileo Galilei turned his primitive telescope toward Jupiter in January 1610, countless observers have been observing their eternal dance around their parent. All four moons are easy objects to see through a pair of 10 x 50 binoculars as they shine between fifth and sixth magnitude, though Io, which lies closest to Jupiter, is best seen when lying at its furthest east or west of the planet.

What can I see through a telescope?

Jupiter doesn’t present a solid surface as a gas giant planet. The surface we see is the outer layer of its atmosphere in the form of major dark belts and bright zones, well-punctuated by among other features, numerous dark and bright spots, or ovals, coming and going on a regular basis.

A small telescope of as little as 60mm in aperture, operating at a magnification of 30x, can show Jupiter’s major bright zones and dark belts. From UK shores in 2021 though, you’ll probably want to observe through at least an 80–100mm (~three- to four-inch) telescope owing to Jupiter lowly position in the sky.

Usually, the two equatorial belts, the North and South Equatorial Belts (NEB and SEB) are the most prominent; presently a striking feature of the Jovian cloud tops is the markedly ochre, or rusty colouring of the northern part of the Equatorial Zone. There’s also a series of elongated oval-shaped dark red spots populating the north of the NEB and much turbulence is visible in the wake of the Great Red Spot (GRS).

Jupiter imaged from Chile, South America, on 1 August 2021 at 05:28 UT. The dark orange coloration of the northern part of the Equatorial Zone is striking, while Oval BA, a feature present in the South Temperate Belt (STB) since 2000, is well seen. Image: D.Peach/Chilescope.

Observe the Great Red Spot

Jupiter’s Great Red Spot (GRS) is its most famous feature, a long-lived anticyclonic storm that has been raging in the planet’s South Tropical Zone (STrZ) for possibly 350 years, but it’s not a permanently fixed feature. Down the decades it has been observed drifting steadily in longitude, though barely in latitude by comparison (Astronomy Now has monthly listings of the GRS’s appearances on the Jovian disc).

It hasn’t always exhibited a vivid red colour; at times in the past it’s been tough to make out at all before recovering some or all of its lustre. Presently, it sports a subtle orange colouration.

One thing for sure is that the GRS is shrinking. In the late nineteenth-century, it measured more than 40,000 kilometres along its major, east–west axis. In 1995, Hubble Space Telescope images showed the GRS with a diameter of just under 21,000 kilometres. By 2009, it had shrunk to around 18,000 kilometres and today it spans just over 15,000 kilometres. A 150–200mm (six- to eight-inch telescope will give a good view of the GRS at this opposition, though you may get lucky to spot it through a smaller-aperture telescope in good seeing conditions.

Watch the moons and their shadows

We’ve already mentioned that Jupiter’s four Galilean moons are visible through binoculars. Turn a telescope on Jupiter though and you’ll be able to enjoy some of the exciting and fascinating moon
events (phenomena) that take place every day.

Jupiter’s Great Red Spot (GRS) transits the planet’s central meridian in the image shot from Curaçao, an island in the southern Caribbean Sea, on 12 August 2021 at 04:39 UT. Europa can just be spotted to the right of the GRS, though its jet-black shadow is prominent. Image: Eric Sussenbach.

It’s easy to see, even through binoculars, the moons disappearing or reappearing from behind Jupiter (occultation) or moving in and out of its massive shadow (eclipse), though perhaps the most appealing of all the phenomena is the appearance of the moon’s pitch-black shadows, often accompanied by the moons themselves, traveling across the face of the giant planet (the moons are much harder to see, especially when they cross one of Jupiter’s bright zones). A small- to medium-aperture telescope in the 100–150mm (four-to six-inch) class should be up to the task, though a much larger telescope or an imaging set-up is usually needed to detect the moons themselves.

Observing and imaging the pitch-black shadows of the Galilean moons as they are seen crossing the face of Jupiter is one of the highlights of observing the gas giant. A fascinating consequence of opposition, with the alignment of the Sun, Earth and Jupiter, is that a moon and its shadow can lie very close together. On opposition night (19/20 August), Io actually occults its own shadow when in transit.

On 19/20 August, the night of opposition, just before 3am BST (02:00 UT) Io sits right on top of its own shadow as it begins to transit its parent’s cloud tops. On the evening of 21/22 August, Io lies very close to its shadow as it leads it in a transit which lasts from about 9.20pm BST (20:20 UT) until about 11.30pm BST (22:30 UT). Finally, on 28/29 August, Io and its shadow are seen in transit again from 11pm to 1.30am BST (22:00 to 00:30 UT).