When Galileo first used the telescope to examine the heavens, he saw things no one in history had seen before: moons of Jupiter, the phases of Venus and the surface features of the moon, just to name a few.
Galileo also discovered blemishes on the surface of objects in the solar system, in particular spots on the sun. These appeared and disappeared over days and weeks and were carried across the sun’s face by its rotation. Today, we know them as sunspots.
In the following centuries, astronomers discovered a similar blot on the face of Jupiter, eventually known as the Great Red Spot, though unlike sunspots this one seemed to be much more permanent, lasting centuries — which is why astronomers are fairly surprised to find it vanishing before their eyes today.
After its discovery astronomers soon realized that, unlike sunspots, the Great Red Spot was a storm. Jupiter, along with Saturn, Uranus and Neptune, are gas giants of the solar system. This means they’re huge spheres of mainly hydrogen (in the case of Saturn and Jupiter) or hydrogen compounds such as methane, ammonia and — as scientists are just now determining — perhaps an exotic form of frozen water (in the case of Uranus and Neptune).
Unlike the rocky bodies in the solar system, the features of the gas giants are not dominated by geology or impacts. This means they don’t have features such as volcanoes, craters, canyons, etc. Instead, their visible “surfaces” are composed of weather patterns, clouds and wind.
And the huge rotational speeds of these worlds (Jupiter, for instance, rotates in only nine hours, almost 30 times faster than Earth’s rotational velocity its the equator) means those weather patterns can be immense. On Jupiter, those patterns have generated and sustained the Great Red Spot, a storm larger than our entire planet, for at least the past 200 years.
Semipermanent storm systems have formed on other gas giants as well.
When Voyager 2 passed Neptune in 1989, astronomers realized that planet had a Great Dark Spot. However, this storm had dissipated and disappeared when the Hubble Space Telescope images the planet in 1994.
The Cassini mission, which for the first time gave us detailed views of the polar regions of Saturn, confirmed a bizarre hexagon-shaped cloud structure ringing the planet’s north pole. But the Great Red Spot is by far the best-known and apparently longest-lasting weather feature in the outer solar system.
Scientists have realized that it has been very slowly shrinking since careful observations of the storm has been kept, but right now, this process seems to be accelerating.
Astronomers aren’t sure what’s causing the Great Red Spot to shrink and why this process is accelerating, just as they can’t explain entirely why it has endured so long. This month, however, amateurs are in a perfect position to witness this process unfolding (or “unfurling” as some have described the storm’s changes) themselves as Jupiter reaches opposition, or its closest approach to Earth on Wednesday (yesterday).
For the days leading up to and following this date, Jupiter is nearly opposite the sun, passing overhead near midnight, very large and bright in the evening sky. (Jupiter is low in the southern sky, above the hooked stinger of Scorpio, and easy to spot because it is by far the brightest object in the sky.)
A good backyard scope will show the Great Red Spot as just that: a brownish-red spot on Jupiter’s face. Of course, this is assuming the spot is on the side of Jupiter turned toward Earth when you’re looking. (The JupiterMoons app from Sky & Telescope can calculate this for you, or you can use their free online calculator at skyandtelescope.com/observing/interactive-sky-watching-tools/transit-times-of-jupiters-great-red-spot/).
Larger scopes will give more detail, and amateurs with advanced equipment can add and compare their observations at sites such as JUPOS.org to see how the storm is rapidly changing day by day.
With the spot getting smaller all the time, this month might be your last, best chance to catch a glimpse of one of the solar system’s wonders: an extraterrestrial hurricane so large it can (for now) be viewed across the solar system and so old it has been observed by generations of astronomers. Take the chance to see it now though, because there might not be another huge storm like this visible on Jupiter’s face for a very long time