What’s Out Tonight?

A general ASTRONOMY site to get you started exploring the night sky


Facts about Jupiter 

• Fifth planet from the Sun 
• It’s a Gas Giant & the largest planet 

Diameter: 88,844 miles 
Mass: 318 Earth’s mass 
Density: 1.33 where water = 1 
Gravity: 2.53 times that of Earth 
Albedo (% of Sunlight reflected): 52% 
Rotation on Axis: 9.84 hours 
Inclination of Axis to Orbit: 3.1° 

Distance from Sun: 483,630,000 miles 
Revolution about Sun: 11.86 years 
Inclination of Orbit to Earth’s Orbit: 1.31° 

Atmosphere: 90% Hydrogen, 10% Helium plus traces of other elements 
Moons: Although Jupiter has over 5 dozens moons, only the 4 Galilean moons can be seen with amateur telescopes (see more about these moons at the bottom of this page). 

Most interesting features in a small telescope: With any small telescope at just 100x, you can see the 4 Galilean moons, the two equatorial cloud belts and the Great Red Spot. The Great Red Spot is currently paler than in the past, so it is a little harder to discern. The Galilean moons are fun to watch because they are bright and create different patterns from day to day. It is possible to see movement of the two closest moons in as little as 5 minutes.

The colors have been enhanced to bring out the beautiful swirling clouds. Yes, that big oval thing below center is the Great Red Spot. but it is not as apparent as in this image. It was more reddish and very easy to “spot” about 50 or so years ago—don't ask me how I know this.

The Planet Jupiter

Jupiter was given its kingly name because it is consistently the brightest Planet in the sky, and unlike Venus, it can be seen throughout the night. It is just a coincidence that Jupiter turned out to be our largest Planet. 

Galileo became the first scientist to study this Planet early in 1610. It was his observations of the revolution of Jupiter’s four brightest moons that validated the heliocentric or Sun-centered concept of the Solar System. In his honor, these four moons are called the Galilean moons. 

Jupiter is the largest of the four Gas Giants, namely Jupiter, Saturn, Uranus and Neptune. The Gas Giants do not have a familiar surface like the Earth and other Terrestrial Planets. They are mainly balls of hydrogen gas, creating enormous pressures that increase with depth, eventually turning the hydrogen into a liquid. And because Jupiter is so large, near its core, the liquid hydrogen is finally compressed to make “solid” metallic hydrogen. 

Jupiter is almost synonymous with the Great Red Spot, a giant hurricane-type cloud structure located at the South Equatorial Belt. Its width is about 14,000 miles (22,500 km) and wind speeds top 270 miles/hour (435 km/hour). The “eye” has been a feature of this Planet since it was noticed around 1665. It is thought that this vortex could dissipate but it would probably form again. The spot itself is higher than surrounding clouds. Its “color,” size and shape do vary. 

In July of 1994, comet Shoemaker-Levy 9 slammed into Jupiter’s atmosphere, creating dark patches in the clouds. This collision provided direct evidence that Jupiter may have served as a “cometary magnet” during the early evolution of our Solar System, sparing Earth from huge impacts and giving life the chance to develop. 

Locating & Observing Jupiter 
This amber jewel is easy to find in the sky because it is always bright and prominent. It shines boldly above magnitude –2 most of the time (Remember, Sirius, which is the brightest star in the sky, shines at magnitude –1.4). Jupiter varies little in size and magnitude compared to some of the other Planets. The What’s Out Tonight? monthly star charts indicate Jupiter’s position and magnitude but you can follow its movement with any planetarium software program. 

Jupiter is best observed in a telescope with magnifications ranging from 50x to 200x, depending on the turbulence in the Earth’s atmosphere. At 50x, you will be able to easily see the four Galilean Moons and at least two of the cloud belts. The Great Red Spot might be harder to discern because it has become pale in color, blending in with the surrounding cloud belts, and sometimes the spot is on the other side of the planet.

This is a fair representation of what Jupiter looks like through a moderate size telescope on an exceptionally clear night. However, the two darker thick bands are visible in any size telescope. They are called the North and South Equatorial Belts. The thinner belt above the North Equatorial Belt is usually visible and is called the North Temperate Belt. Lately, the Great Red Spot has not been as “red” as shown in this picture, but paler, making it harder to discern.

This is a good representation of what Jupiter looks like through a telescope at 100x. Often, the air is turbulent and so the “surface” of Jupiter is not very sharp but you can almost always see the two darker parallel bands. The four Galilean moons in this image are in a line and sometimes show create pretty patterns. And, there are other times when only 3 or even 2 are visible because one could be behind Jupiter or even in front. 

The Four Galilean Moons

The four Galilean moons are easily visible with well-focused binoculars but better observed with a small telescope with magnifications of 50x to 150x. The brightness of the Galilean moons combined with their rapid revolution around Jupiter creates beautiful, ever changing patterns. Movement of these inner moons can be noticed in as little as 5 minutes. There are also transits, that is, the passing of these moons in front of Jupiter.

During a transit, the moon’s shadow can be seen moving across the cloud belts; however, the actual moon is usually more difficult to see. When Jupiter is visible in the sky, the popular monthly astronomy magazines publish a graph indicating the daily positions of the four moons. However, planetarium software programs provide positions accurate to the minute.

These four moon are all around the same size as our Moon which has a diameter of 2,160 miles.

2,255 miles
Revolution about Jupiter:
1.77 days
Distance from Jupiter:
262,000 miles

1,950 miles
Revolution about Jupiter:
3.55 days
Distance from Jupiter:
417,000 miles

3,270 miles
Revolution about Jupiter:
7.16 days
Distance from Jupiter:
665,000 miles

2,980 miles
Revolution about Jupiter:
16.7 days
Distance from Jupiter:
1,171,000 miles