Jupiter and Saturn, the solar system’s largest and second-largest planets, respectively, are back in the evening sky.

Around 9:30 p.m., look for Saturn as a pale yellow point of light directly to the south. The planet will be about 1/3 of the way from the horizon to the zenith, the top of the sky. Although the ringed planet is much dimmer than Jupiter, you should have no trouble seeing it in its particularly star-free section of sky.

Jupiter is the extremely bright “star” to the southeast and about the same distance above the horizon.

Jupiter is much brighter than Saturn, mainly because it’s closer to us. Saturn is over twice as far away from Earth as Jupiter right now.

Astronomers like to express distances in the solar system in terms of Earth’s average distance from the Sun, about 93 million miles. Astronomers call that distance an astronomical unit, AU for short. Currently, Jupiter is 4 AUs from Earth. Saturn is 9 AUs.

Another way of looking at the distances is to measure the time it takes the light to get to us. At 33 light minutes away, you are seeing Jupiter the way it looked 33 minutes ago. By comparison, Saturn is about 75 light minutes away.

Because the light from a planet (or anything else, for that matter) spreads out as it recedes from its source, Saturn ends up more than four times fainter than its brighter sky companion.

Both planets are fundamentally different from a planet like Earth. Our planet is primarily a ball of rock with a few metals mixed in.

Jupiter and Saturn are called “gas giants,” which is only partly true. They are indeed made of substances that would be gases if we found them on Earth. They have no solid surfaces because they are composed chiefly of the simplest elements in the universe, hydrogen, with a sprinkling of helium and traces of other elements.

However, those substances can hardly be called gases where Jupiter and Saturn are concerned. Underneath their gaseous atmospheres, the hydrogen is so compressed by the enormous pressure of the planets’ gravity that the planets are mostly liquid, giant drops of fluid spinning rapidly in space.

When I say giant, I mean giant. Jupiter is the largest planet at about 88,000 miles wide, a true behemoth compared to Earth’s 8,000-mile diameter. Over 1,400 Earths would just fit within the confines of Jupiter. Saturn is smaller than Jupiter, but not by much. At nearly 75,000 miles wide, it could hold 860 or so Earths.

Jupiter and Saturn reveal the strengths and weaknesses of the various optical aids available to the amateur stargazer. Of course, both planets are easily visible with the binoculars you were born with, your own two eyes.

Those planets were visible to the first humans to look up at the sky. It took all of human history until 1781 to discover Uranus, the next planet out from Saturn.

No surface details are visible to the unaided eye. The planets may be giants, but they are exceedingly far away.

The first astronomer to resolve the disk of a planet was Galileo. In 1610, he saw Jupiter as a tiny dot, which wasn’t surprising, given the primitive quality of his telescope.

When Galileo first looked at Jupiter with his telescope, he was shocked to find four little dots hovering around the planet.

Even a pair of binoculars will show you those “Galilean satellites” if you can hold your binos steady enough. A small telescope working at medium magnification is required to see the planet’s disk.

You’ll also notice thin, dark stripes stretching in the same direction as the planet’s rotation. They are cloud bands pulled all the way around Jupiter by its speedy rotation. Tiny Earth spins once on its axis in 24 hours. Enormous Jupiter rotates once in only 10 hours.

Following ancient convention, astronomers gave the four brightest moons of Jupiter mythological names. Io, Europa, Ganymede, and Callisto were named after (how shall I say this) intimate friends of Jupiter, the Roman king of the gods.

You can replicate Galileo’s discovery with even the simplest of binoculars. The three or four specks lined up close to the planet are the “Galilean” moons.

The binocular view is remarkable partly because of Earth’s distance from the Jovian system. The moons are, after all, tiny things, averaging about the diameter of Earth’s moon.

We see them so well because they are so reflective of the distant light of the sun. They are covered for the most part with shiny, icy coatings.

We know quite a bit about the moons. Two spacecraft, Voyagers I and II, sped past the planets a few decades back. More recently, the Jupiter-orbiting space probe Galileo provided even more detailed information.

Io is the closest of the moons to Jupiter. Io practically skims giant Jupiter’s surface at a scant 261,000 miles away.

The proximity of the tiny moon to its massive parent planet is terrible news for Io. Jupiter’s enormous gravity bends and stretches Io, like Earth’s moon raises tides on Earth’s mostly liquid surface.

As a result, Io is being torn apart from the inside out. Tidal forces heat the planet’s interior and melt the core of the long-suffering satellite.

The liquid core of the moon spews out from giant volcanoes, which completely resurface the moon in a relatively brief ten million years or so. The volcanoes mostly belch forth hot sulfur, which quickly freezes into a moon-wide dusting of sulfur snow.

Io’s volcanic activity even affects Jupiter’s second moon out, mysterious Europa. Its diameter of 1,925 miles is slightly smaller than Earth’s moon.

Europa’s icy outer surface is smooth as a billiard ball would be if it were almost 2,000 wide. Nowhere on the planet do we find hills over 300 feet tall or craters more than three miles deep. It is also the most reflective of the Jovian satellites.

The most prominent features in Europa’s icy crust are a series of long grooves 600 miles long in some places and as much as 185 miles wide. Initially, the grooves were probably fractures in the ice produced by the same gravitational flexing that causes the volcanoes on Io. These cracks are filled in now, but they once must have been up to 100 miles deep.

Some planetary astronomers suggest that beneath the ice is a deep ocean of liquid water. As with Io, Jupiter’s considerable gravity flexes Europa and heats its water ice into a liquid. The water then oozes up through the cracks in Europa’s surface.

We cannot rule out the possibility of life where there is liquid water and heat. Consequently, some searchers for life in our solar system have pinned their hopes beneath the ice of Europa.

The Galileo spacecraft detected what must be called Europa’s oddest feature — the presence of sulfuric acid covering much of the surface of Europa. As anyone who has had contact with its most common form in our culture, car-battery acid, will testify, sulfuric acid is a corrosive chemical.

The reason for all that acid is still a mystery, but the volcanoes of Io may provide an answer. The volcanoes may spew sulfur and sulfur compounds at such tremendous velocities that some material is thrown into space, delivering a nasty supply of sulfur compounds to beleaguered Europa.

The heavy presence of battery acid may seem to eat away any possibility of simple life on or in Europa, but stranger things have happened in our solar system.

On Earth, for example, certain “extremophile” bacteria think sulfuric acid is just yummy, thank you very much.

Saturn is smaller and farther away, so binoculars won’t help you much.

Galileo’s small telescope revealed two small lumps on either side of the planet. Luckily, telescope technology has improved significantly in the last 390 years. Your small telescope working at medium magnification will resolve the lumps into a bright ring around Saturn.

You are looking at a large field of debris, the cause of which is still undetermined, even with data from an orbiting spacecraft.

Here’s the most plausible explanation. The rings resulted from one or more of Saturn’s moons that shattered into mostly tiny pieces by the planet’s enormous gravity.

Jupiter also has a ring system, but you’ll need a $1.5 billion spacecraft zooming close to the planet to see it. In lieu of that, Saturn’s unforgettable ring system is sufficient recompense.


By Tom Burns


Tom Burns is the former director of the Perkins Observatory in Delaware.