Look east after sunset — away from the sun, toward the Organ Mountains if you are in Las Cruces. A very bright white star will be shining in the constellation of Pisces, the fishes, and visible for most of the night as it slowly travels west.
But that’s not a star. It’s Jupiter, the king of the planets, in a rare muscle flexing.
Jupiter is over three times more massive than the next biggest planet, Saturn. In fact, Jupiter is more massive than all other planets combined. The fifth planet from the sun, Jupiter is on average 5.2 times more distant to our star than we are: 478 million miles, compared to our 92 million. Although Jupiter is enormous, it is far enough to be outshined by smaller nearby objects. Usually, Jupiter is the third brightest object in the night sky, after the moon and Venus, the closest planet to us. Yet, Venus is currently very close to the sun, and the moon is in new phase. As a result, Jupiter is stealing the throne and single-handedly commanding the sky as the brightest object upon nightfall, brighter than all the stars.
What’s going on is what astronomers call a perihelic opposition. The fancy expression simply means the closest that Jupiter ever gets to Earth. Opposition means that Earth is exactly between the sun and the planet, the three of them neatly in a line. We’re seeing Jupiter in full phase (like the full moon), reflecting as much sunlight toward us as it possibly can.
The perihelic bit is Greek for “closest (peri) to the sun (helios)”. This refers to the fact that the orbits of the planets are not circular, but ellipses, oval figures. Perihelion is the point in the ellipse where the planet is closest to the sun. Earth’s orbit is not too distant from a circle. Along the year, we vary our distance to the sun by a little more than 3%. Jupiter, in contrast, varies it by almost 10%. Given its orbital distance, that’s a change of almost 50 million miles. At this closest approach, which happens once every 70 years, Jupiter shines twice as bright as usual.
A rare occasion, but some seem to be under the impression they will never get the chance to see Jupiter again. That’s not the case. Jupiter is in opposition every 13 months, and every opposition is a striking sight to behold. Jupiter will be shining bright again in November 2023, December 2024, and January 2026. It just so happens that a perihelic opposition is even more of a show-stopper. Jupiter is always bright. This year it’s just extra bright.
When this close, the iconic banded structures of Jupiter are visible even with binoculars. With a small telescope, at any time Jupiter is visible, one can see the Great Red Spot, a hurricane of planetary dimensions going on at least since humanity first began observing Jupiter with the telescope 400 years ago. But the grandstand is reserved for the parade of the moons: Io, Europa, Ganymede and Callisto. Jupiter has 80 satellites, but the majority are little moonlets no bigger than an asteroid. In contrast, these four large satellites are of planetary size, enclosing over 99.99% of the mass of all Jupiter’s moons. Put them in independent orbit around the sun and astronomers would have all reason to call them planets. Of the four, only Europa is smaller than Earth’s moon.
These moons are collectively called Galilean moons, because they were discovered by the Italian astronomer Galileo, a major contributor to the Scientific Revolution of the 16th and 17th centuries. At the time of Galileo, the Aristotelian system with the Earth as center of the Universe was under way to being replaced by the heliocentric system, centered on the sun, formulated two generations earlier by the Polish astronomer Nicolaus Copernicus. Galileo’s discovery of Jupiter’s moons was a watershed moment: For the first time, astronomical bodies were shown beyond doubt to orbit something else than Earth. This discovery paved the way to the widespread acceptance of the heliocentric system, galvanizing the onset of our modern era.
These moons orbit Jupiter in very short orbital periods. Io in less than two days, Europa in less than four. One can see them visibly moving around Jupiter over the course of a single night. A beautiful sight that evokes so much inspiring history. With its moons and storms, Jupiter is an amazing sight, a planet that played such a prominent role in the development of astronomy. And even today still gets astronomers to scratch their heads at its unsolved mysteries. So these last days of September, grab a pair of binoculars, your backyard telescope, or come to the Astronomy Department Campus Observatory at NMSU, and gaze up to a world of wonder.
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This article originally appeared on Las Cruces Sun-News: Star News: Jupiter, the king of the night