Observing Jupiter

The Solar System is an amazing place, but there is one world in particular that stands out as a target for observers, the king of the Planets, Jupiter.

Jupiter is by far the largest of the planets – it would take 1,321 Earths to fill the volume of Jupiter. It also has a large system of about a hundred moons. The largest of these, Ganymede, is bigger than the planet Mercury.

The planet offers amateur astronomers the best opportunity to see atmospheric features in the Solar System.





Amateur observers of Jupiter can make original, genuine contributions to planetary astronomy. This does not necessarily require any expensive, specialist equipment.

Jupiter is hard to miss in the night sky. It is a very bright object – usually the fourth brightest object in the sky. The brightest objects are, of course, the Sun, then the Moon and Venus, sometimes Mars is also brighter. Jupiter can vary in visual magnitude from as bright as −2.9 at opposition, down to −1.6 during conjunction with the Sun. The planet has a large angular diameter which varies from 50.1 to 29.8 arc seconds, again depending on its orbital position.

Jupiter is best observed around opposition, which occurs about once every thirteen months.

The location of opposition in Jupiter's orbit is shown at the bottom

Conjunctions of Jupiter with the Moon and other planets such as Venus provide a great display visually and are a fantastic opportunity for an interesting photograph.

What Can You See With Binoculars?

With decent binoculars – say 7 x 50′s – you will certainly see the Galilean moons, but not necessarily all four at the same time.  This is because they pass in front or behind the Jovian disk or may be too close to each other to differentiate using binoculars. If you observe the Moons on different nights you will notice that their positions have changed, relative to the planet.

The Galilean moons are Io, Callisto, Ganymede and Europa and are named after Galileo, who first observed them.

Your view will be much improved by resting the binoculars on a wall or fence. Better still a tripod offers greater stability and flexibility. You should be aware that when viewed with binoculars, Jupiter itself will only appear as a large, slightly oval-shaped disc. You won’t see any surface detail without a telescope.

View of Jupiter and the Galilean satellites through a typical telescope

What Can You See With a Telescope?

New astronomers are sometimes disappointed when they first look at Jupiter through a telescope. You are NOT going to see images like the amazing pictures like the one above, taken by the Voyager spacecraft. However, Jupiter is still an exciting view in just about any telescope. Even a small, beginners scope will reveal several cloud belts and the Galilean moons. Shadow transits, eclipses of the Sun cause shadows on the surface of Jupiter by the Galilean moons, can also occasionally be seen.

Jupiter is dynamic — you never get the same view twice. Changes happen on a short timescale, this is the main appeal of the planet to Jupiter enthusiasts.

A full rotation takes less than ten hours which allows us to observe the whole planet during a long winter night.

Gas-giant planets like Jupiter exhibit differential rotation. This means they rotate more rapidly at the equator than they do at the poles. Jupiter’s observable “surface” has two general systems of rotation that differ by approximately five minutes. The Equatorial zone rotates in 9 hours 50.5 minutes, but the rest of the planet takes9 hours 55.7 minutes to rotate.

The Jovian disk is crossed by bands of light and dark clouds. These are known as zones and belts respectively. Astronomers use a standard nomenclature when describing these zones and belts.

If the atmospheric seeing is favorable the Jovian disk will reveal its often subtle colors and the interfaces between the light-colored zones and the dark belts will show numerous irregularities, caused by turbulence.

There is a lot to be seen on Jupiter, but it takes a trained eye to see most of it. The contrast on the planet is actually low, making the detail difficult to discern.

As ever, your observing will improve with experience. As you progress, you will be become  familiar with where the features are located and what to look for. Your technical knowledge of the tools and techniques available will allow you to bring out the more subtle details like the festoons, barges and ovals on the “surface” of the planet.

Jupiter's Great Red Spot

The Great Red Spot

The Great Red Spot is a persistent storm in the Jovian atmosphere, 22° south of Jupiter’s equator. It has lasted for at least 181 years and possibly longer. The storm rotates counterclockwise, with a period of about six Earth days.

New observers can sometimes been misled into thinking the Great Red Spot must be easy to see on the planet. Unfortunately, as mentioned above, the Jovian contrast is very low, so the detail is subtle and it may be harder to discern the Great Red Spot than you may have anticipated.

When looking at the central meridian of Jupiter Watch for the Great Red Spot as it crosses the center of the disk. The local time for impact transits over the central meridian can be found from astronomical websites, which is a great help in locating the Great Red Spot.

Which Telescope?

Refractors and long focal length Newtonian reflectors will generally give the best views of Jupiter. In years gone
by Schmidt-Cassegrains or Maksutov-Cassegrains may not be have been as suitable because of the reduction in
contrast caused by their large secondary mirrors.  However, recently CATs with high quality optics have been able to
improve their results by compensating with a large aperture.

Magnification

Some degree of magnification is required but don’t overdo it – although Jupiter is big and bright, unlike other planets it doesn’t tolerate high magnification well. Try to find the highest magnification that doesn’t degrade the colors. As a general rule 40x per inch of aperture is usually enough. Also magnification is limited by the seeing. Increasing the magnification in poor seeing conditions can only make resolving details more difficult.

Eyepieces

The quality of the eyepiece is just as important as the telescope. You don’t necessarily need to spend a lot of money on a ultra-wide field view eyepiece – these may have a reduced transparency. A high-quality Plössl or orthoscopic eyepiece is a better option for observing close detail.

Filters for Observing Jupiter

Filters can make features more obvious, but they do not enhance detail. In fact they reduce the detail. However  some colored filters such as light blue (80A), very light blue (82A) and light green (56) may help you pick out features. A good light pollution filter can also be helpful to enhance the contrast.