The absolute premier celestial event this month in the Rochester sky is something that hasn't been seen from this planet since Galileo was alive.

It's the historic conjunction of the bright planets Jupiter and Saturn that culminates on Dec. 21. A conjunction is defined as any close arrangement in the sky between the moon, planets or bright stars. Many conjunctions occur through any given year, but this conjunction between Jupiter and Saturn is super special.

This week, you can easily see the bright planets remarkably close together in the low southwest sky from around 5 p.m. in the early-evening twilight until shortly after 7 p.m., when they set below the horizon. You can't miss them, even in a light-polluted sky.

Jupiter is the much brighter of the two planets, and is also one of the brightest star-like objects in the sky. Just to Jupiter's upper left is Saturn — not nearly as bright as Jupiter, but easily seen, although you might have to strain your eyes to see it in a heavily lit area.

At the start of this week, the largest planets of our solar system are less than 2 degrees apart in the low southwest sky. That's less than the width of two of your fingers held together at arm's length. By the end of the week, they will be less than a degree apart. Over the next couple weeks, they'll draw even closer to each other from night to night.

Newsletter signup for email alerts

On Dec. 21, they'll be "touching" each other, only a tenth of a degree apart. This is the closest conjunction of Jupiter and Saturn since 1623! They won't be this close to each other again until March 2080.

Even though Jupiter and Saturn will appear as a "double planet," they are nowhere near each other physically, but they are almost precisely in the same line of sight from Earth. Because of that, they move at different orbital speeds.

They “catch up” with each other in the sky every 20 years. They are usually nowhere as close to each other in the sky, but this time around, they'll be in the right place at the right time from our view this coming Dec. 21.

This monumental event is fantastic with just the naked eye, but seeing it through a small telescope or even binoculars can make it even sweeter. Both planets can fit in the same field of view. Unfortunately, though, since they're so close to the horizon, they will appear fuzzy. That's because Earth's atmospheric layer is much thicker the closer you get to the horizon from our point of view.

You'll be able to see up to four of Jupiter's largest moons, which will be lined up diagonally on both sides of Jupiter. They constantly change their position relative to Jupiter because they orbit the giant planet in periods of two to 17 days.

Some nights, one or more of the moons may be hiding behind Jupiter or camouflaged in front of it. You should also be able to see a fuzzy version of Saturn's ring system, and maybe some of its moons, especially Titan, Saturn's largest moon.

Make sure you glance at the Jupiter-Saturn show every clear night you can between now and Dec. 21. Again, you're seeing something that hasn’t been seen since Galileo saw it in 1623!

Mike Lynch is an amateur astronomer and professional broadcast meteorologist for WCCO Radio in Minneapolis/St. Paul and is author of the book, “Stars, a Month by Month Tour of the Constellations” published by Adventure Publications. Send questions to mikewlynch@comcast.net.

The Rochester Astronomy Club welcomes new members and puts on public star parties. Their website is rochesterskies.org.