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The Return of Jupiter


Last updated 9/2/2021 at 10:01am

Jupiter at opposition.

Anyone stepping outdoors at dusk this week will have a hard time missing the brilliant planet Venus glistening low in the western sky. It'll appear there for a few more months and, believe it or not, will become more than twice as bright by early December.

Then, in the southeast, there's everyone's favorite planet Saturn, which offers those with even a small backyard telescope a stunning view of its famous ring system. Check out last week's column for more on this celestial beauty.

The "star" of the show this week, however, is Jupiter. This great world is now in our evening sky and will reach its official "opposition" point on the night of Aug. 19. Not only is this when the planet appears in our sky opposite the sun (rising in the east at sunset and remaining visible all night long), it's also when it lies closest to Earth and, therefore, appears larger and brighter than at any other time of its orbit.

So, Jupiter, which is always impressive to view through a small telescope, will be especially impressive all month. In fact, it will be quite a sight throughout much of the summer and autumn.

Jupiter has always been one of my favorites because it's a planet that actually appears to change fairly quickly. Here's a world that's 11 times the diameter of Earth, yet it rotates on its axis once every 10 hours or so. This means that its Earth-facing side changes completely in just five hours and, with patience, early evening sky watchers with a small telescope can easily watch its pastel cloud bands and, sometimes, its Great Red Spot, spin completely around in just one long evening of stargazing.

Equally amazing is knowing that Jupiter is made entirely of gas held together by gravitation; there is no surface on which to stand. Hypothetical astronauts trying to "land" on the planet would just sink deeper and deeper into its murky atmosphere until they'd become crushed beyond recognition by its tremendous weight.

Perhaps the most enjoyable part of watching this planet is keeping up with the antics of its four largest moons. These are known as the Galilean satellites – Io, Europa, Ganymede and Callisto – because it was the Italian astronomer Galileo who discovered them and their motions back in January of 1610. They appear to do-si-do around the planet and change their positions from night to night – sometimes even from hour to hour! And sometimes, if the sky is steady and you've got a large enough telescope, you can see one or two of them casting their shadows onto the cloud tops of Jupiter!

Much of the fun of watching these moons is knowing all their names. You can identify them by using an app for your smart phone or tablet. My favorite is called "Gas Giants," but there are certainly others available. With Jupiter back in our early evening sky, accompanied by Venus and Saturn, try aiming a telescope in their directions or contact your local astronomy club or science museum to learn when they'll be hosting a free "star party" so you can get a close-up look at these alien worlds.

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