Jupiter rules as celestial monarch


Jupiter was the Roman king of the gods. The god’s namesake will be king of the night sky this fall and winter, dominating the evening as the brightest object besides the moon.

Right now, look southeast just after dark. Jupiter is below the main stars of the constellation Aries, the Ram. Below is the head of Cetus, the Sea Minster. To the right is Pisces, the Fish.

Look to the left, and you will see a small dipper of stars called the Pleiades or seven sisters.

Down and to the left of Jupiter is the head of Taurus the Bull, looking like a “V” on its side.

The tableau of the Pleiades, Jupiter, and the head of Taurus makes a beautiful sight in the early evening sky.

Although the ancients didn’t know it, Jupiter’s size is appropriate to his status as king of the gods.

Jupiter is the largest planet, almost 89,000 miles in diameter compared to the Earth’s 8,000. It has more than twice the combined mass of all the other solar-system planets.

Jupiter is located past the asteroid belt beyond Mars, making it the fifth planet from the sun. Because of its distance from the sun, it takes almost t12 Earth years to complete one orbit.

Jupiter is a nasty, uninhabitable planet.

It might have a solid rocky core, but recent studies suggest it is more diffuse.

Around the core is a thick layer of “metallic” liquid hydrogen under so much gravitational pressure that it conducts electricity like a metal.

That layer is covered with a deep ocean of liquid hydrogen that envelops the planet’s entire surface. But even that is not what we see in telescopes.

A 44-mile-deep atmosphere of hydrogen and helium gas blankets the planet. Given the planet’s girth, the atmospheric layer is as thin as the skin of an apple.

Part of the hydrogen in the atmosphere has combined with other elements to form ammonia, water, and methane, often in crystalline or “snowflake” form. Those floating crystals give Jupiter its striking colors — reds, yellows, and browns occasionally visible in amateur telescopes.

Jupiter’s day lasts only 10 hours. That means it rotates once on its axis in less than half Earth’s 24-hour day. Consequently, the clouds on the planet’s surface are rotating so rapidly that some of them are pulled into long bands stretching all the way around Jupiter.

Those cloud belts, the two most prominent of which are located right above and below the equator, are easily visible in a small telescope.

Because of the intense density of its hydrogen beneath the surface clouds, Jupiter generates more heat than it receives from the sun.

Jupiter’s rising and falling heat and rapid rotation create complex and violent weather systems. Often, the clouds form into violent storms, giant cyclones that can rage for thousands of years or dissipate within a few days. You’ll see them as small white ovals, most easily visible silhouetted against the dark bands.

The famous Great Red Spot (GRB), a giant oval below its lower band, is the granddaddy of such storms. Most telescopes invert their images, so look for it floating on the upper band.

Because of Jupiter’s fast rotation, the GRS is usually visible in a telescope at some time during the night.

Over the past few years, the GRB has shrunk dramatically. It was once over 8,000 miles along its short axis, so big that Earth could fall right into it without touching the sides. Currently, its short-axis width has shrunk to 6,525 miles.

In recent years, the GRS’s color has changed from orange to white to pink and back to orange. It gets its color because it extends at least 300 miles down into Jupiter and dredges up colored particulate matter from down below.

A small telescope or even binoculars will reveal the Galilean moons, the four brightest of Jupiter’s 95-plus satellites.

Jupiter’s four rings are composed primarily of dust ejected from some of Jupiter’s moons by high-velocity impacts.

Despite (perhaps because of) visits by spacecraft, many of the statements I’ve made here are oversimplified or controversial. Even Jupiter’s diameter depends on where you measure it, for example.

Over the coming weeks, I’ll break down some of those controversies.

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

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