Opposition - It's a Good Thing

In everyday use, the word “opposition” has a somewhat negative connotation. But in observational astronomy, opposition is a good thing.

Each planet beyond Earth in the solar system—Mars, Jupiter, Saturn, Uranus, and Neptune—is said to be at opposition when it is opposite the Sun as seen from Earth. In other words, Sun, Earth, and outer planet are in a line, with Earth in the middle.

This happens because, moving outward from the Sun, each planet’s orbit is larger than the one before, and each planet travels slower in its orbit than the one before. Therefore, each planet takes longer to complete its orbit than any of the planets closer to the Sun.

For example, Jupiter was at opposition this past September. Since Jupiter takes longer to chug around the Sun (4,332 days) than Earth does (365 days), Earth catches up to and passes Jupiter about every 13 months, slipping between Jupiter and the Sun.

The significance of opposition to hobby astronomers is three-fold:
1) When a planet is at or near opposition, it is closer to Earth than at other points in its orbit;
2) A planet at opposition is fully illuminated by the Sun, from our vantage point on Earth; and
3) A planet at opposition is visible all night long, rising at sunset and reaching its highest point in the sky around midnight.

In short, a planet at or near opposition is close, bright, and up all night. This combination presents planetary observers with optimal viewing conditions and the opportunity to see detail on the planet’s surface they might not otherwise spot. Not to mention an excellent excuse to stay up all night, no matter what the neighbors think.

Here’s a related question recently dropped into my Ask An Astronomer box at a local science center’s evening event; it was submitted by Tara:

“Jupiter just went into its closest opposition since 1963. A site I found on the internet said it wouldn’t be this close again until 2022. Why the uneven time difference if this is true?”

Wonderful question and observation, Tara. And you need know but one thing to understand why: the orbits of the planets are not perfect circles. Instead, planetary orbits are elliptical, or oval. The eccentricity of each planet’s elliptical orbit differs, that is, the degree to which the oval is stretched out from the circular. In addition, the planets’ orbits are inclined, or tilted, with respect to the plane of Earth’s orbit around the Sun. These variables inserted into the distance-at-opposition equation mean the possible distances separating Earth and another planet when they rendezvous at opposition are, well, astronomical.

Mister Big

Planet Jupiter is, after the Moon, the brightest object in the evening sky right now. The king of the planets is the blazing "star" rising in the southeastern sky. One hour after sunset, Jupiter’s altitude, or distance above the horizon, is about the height of your fist held at arm’s length and oriented with your thumb on top. Over the course of a night, it will appear to move across the sky from east to west, climbing to two or three fist-heights, depending on your latitude. For skywatchers in Fargo, North Dakota, for example, it will climb to about two fist-heights. Folks cruising Old Route 66 through Albuquerque, New Mexico will see it climb three fists high as it arcs across the southern sky.

Credit: NASA, Reta Beebe, and Amy Simon (New Mexico State University)

This is a great time to observe Jupiter. It reached opposition on July 9, so it’s particularly bright right now. Any planet with an orbit farther from the Sun than Earth’s is said to be “at opposition” when the planet, Earth, and the Sun are lined up so that Earth is in the middle. In other words, the planet is directly opposite the Sun, from our viewpoint. The effect for us is that Jupiter’s face is fully illuminated by the Sun.

In addition, a planet at or near opposition is at its closest to Earth. This closeness gives Jupiter a slightly larger apparent size, the amount of sky it covers from our perspective on Earth. The larger the apparent size, the more illuminated surface to shine light in our direction.

Like all the planets, Jupiter doesn’t have its own light source. It is merely reflecting light from the Sun, and therein lies a little irony. You see, Jupiter is a stellar wannabe. It’s hot and it’s massive, but when the solar system was forming, it wasn’t quite massive enough for nuclear fusion to ignite in its core. Nuclear fusion is the engine that makes a star--like our Sun--go. And Jupiter, although impressive, didn’t have star power.

What Jupiter does have is take-no-prisoners gravity. Its colossal size (1,300 Earths would fit inside) and significant mass (greater than the other seven planets combined) make it the shield of the inner solar system, which includes our little blue planet. If it weren’t for Jupiter, Earth might not have survived the chaos of the early solar system. Countless asteroids and other rogue bodies that might have pulverized Earth were sucked in by Jupiter’s strong gravitational pull and neutralized. The mighty planet protects us still. In the early 1990s, it ripped a comet into 21 fragments and then swallowed them all.

Naked eye, Jupiter looks like a brilliant star. But a sky tourist with a telescope--even a modest one--can see the “star” magnified into a planetary disk and can have fun hunting some of the planet’s features. Dense, colored cloud bands on Jupiter’s surface are exciting to spot in a telescope. Ammonia, methane, and other chemical compounds in its upper atmosphere create the striped look.

Great Red Spot (on right) and Red Spot Junior (on left)

Credit: NASA, ESA, I. de Pater and M. Wong (University of California, Berkeley)

Another famous surface feature is the Great Red Spot, a hurricane-like storm that’s been observed swirling high in Jupiter’s atmosphere for at least 300 years. Red Spot Junior, a smaller storm, appeared nearby in 2006. Because Jupiter spins, the Red Spots are not always in view. To calculate when they’ll be visible, visit this web page.

Jupiter and Ganymede
Credit: NASA/JPL/University of Arizona

Like any good kingpin, Jupiter has a large number of lackeys--in this case, more than 60 moons in tow. In a telescope or binoculars, you can easily spot the four largest, the Galilean moons. These are the moons discovered when the 17th century Italian astronomer Galileo first turned a telescope toward the night sky. They’re named Ganymede (GANN-uh-meed), Io (EYE-oh), Callisto (cah-LISS-toe), and Europa (your-ROPE-uh), after characters in Greek mythology. They look like tiny pinpoint stars near the planet. Occasionally, one or more will be hidden behind the planet as they orbit around. To calculate and view their positions at any given time, visit this web page.

Aurora at Jupiter's north pole
Credit: NASA/ESA, John Clarke (University of Michigan)

In the distant future, when interplanetary tourists can book a solar system cruise, Jupiter will be one of the more exotic ports of call. Imagine cruising over the unlit night side of the planet to watch ultra-powerful lightning storms animate the cloud tops. Imagine flyovers of the north and south poles to view immense auroras: pulsating sheets of neon-colored light. Imagine diving through the barely detectable band of microscopic dust that encircles Jupiter, a vast ring system ethereal as smoke. Everything about Jupiter is king-sized.

Hidden under the swirling gasses of Jupiter’s atmosphere is a voluminous sea, but no one will ever build a resort or bathhouse there. The air pressure and temperature are so extreme there that Jupiter’s abundant hydrogen--normally a gas--has become liquid. Beneath the sea of liquid hydrogen is liquid metallic hydrogen, a bizarre substance unknown on Earth. The crushing pressure at that layer is three million times that of Earth’s surface.

Credit: NASA/JPL/Space Science Institute

Jupiter is the first object I ever observed through a telescope. The impact of seeing with my own eyes the surface of another planet--hundreds of millions of miles away, with colored bands and moons to boot--is hard to describe. I can tell you this: the image in my head will stay with me forever. By the way, this was years before I owned my own telescope. Begged and borrowed views are often the best.