How To Witness Astronomy’s Once-In-A-Lifetime Planetary Alignment On December 21
By: Ethan Siegel
Tags: Astronomy, Jupiter, Saturn, Space, Planets

All throughout the year, two bright lights have stood out in the post-sunset skies.

Earlier this year, on March 31, 2020, Jupiter and Saturn appeared in the southeastern portion of the sky, separated by only 6 degrees and joined by the planet Mars. By the end of December, these two planets will have migrated to the southwestern portion of the sky, and their positions will overlap entirely. Mars is nowhere in the vicinity any longer. (E. SIEGEL / STELLARIUM)

Jupiter, our largest planet, is unmistakably bright in the south/southwest.

The appearance of Jupiter through a high-powered telescope is unmistakable, but a quality amateur telescope can also reveal features such as its horizontal bands, its great red spot, and the largest of its natural satellites: the Galilean moons. (© CORBIS/Corbis via Getty Images)

Nearby, slower-moving, yellower Saturn joins it, just a few degrees away.

Although ground-based telescopic views of Saturn cannot compete with Hubble’s view, the planet’s yellow color, it’s brilliant ring system, and even its largest moon, Titan, can be visible from a humble amateur setup here on Earth. (SSPL/Getty Images)

On December 21, both worlds approach the same skyward position.

Looking towards the southwest skies from the northern hemisphere, an observer would see the sights in the sky shift as shown by viewing the heavens at the same time from November 30 to December 28, 2020: about 1 hour after sunset. Jupiter and Saturn make their closest approach on the night of December 21, 2020. (E. SIEGEL / STELLARIUM)

As Earth overtakes the outer planets, their relative positions shift.

An accurate model of how the planets orbit the Sun, which then moves through the galaxy in a different direction-of-motion. Note that the planets are all in the same plane, and that the inner planets complete their orbits more quickly than the outer planets. From the perspective of an inner world, the outer worlds will appear to be overtaken periodically. (RHYS TAYLOR)

Jupiter passes Saturn, from Earth’s view, just once every 20 years.

First writing in 1606, Johannes Kepler used his new elliptical theory of planetary orbits to predict the upcoming great conjunctions quite accurately: from 1583 and 1603 (which had been observed already) to the next eclipses in the series. These predictions would be borne out by observations taken after Kepler’s death, and his calculations are still valid today. (JOHANNES KEPLER / PUBLIC DOMAIN)

This time, however, they’ll achieve their closest alignment since 1623.

Jupiter and Saturn are the largest planets in our Solar System, and will appear to almost align perfectly with one another as seen from Earth during the night of December 21, 2020. This will be their closest alignment since 1623. (LUNAR AND PLANETARY INSTITUTE)

This year’s great conjunction brings them within just 0.1° of each other.

If you were to view Jupiter, night-after-night, at the same time from December 13, 2020 through December 28, 2020, this would be the view you would see. The other bright point of light is Saturn, which should appear in the same binocular field-of-view or wide-field telescope view during this time. (E. SIEGEL / STELLARIUM)

At magnitude -2.0, Jupiter appears brighter than everything except the Moon and Venus.

While you won’t see views of Jupiter like this from a pair of binoculars or a simple, small telescope, getting to know the night sky and connect with the Universe is an irreplaceable joy. On December 21, 2020, Jupiter and either 3 or 4 of its Galilean moons, depending on when you look and where you look from, will be visible with Saturn appearing very close nearby. (SEBASTIAN VOLTMER (PROCESSING) AND GERRIT KERNBAUER (DATA/IMAGE))

At magnitude +0.64, Saturn shines just 9% as luminous as Jupiter.

Saturn and its spectacular rings, as imaged by the Hubble Space Telescope on July 4, 2020. Saturn itself appears similar to how it will seem to us on December 21, 2020, but the configurations of its moons will be quite different. (NASA, ESA, A. SIMON (GODDARD SPACE FLIGHT CENTER), M.H. WONG (UNIVERSITY OF CALIFORNIA, BERKELEY), AND THE OPAL TEAM)

These worlds will be viewable together through binoculars or a wide-field telescope.

The seven extraterrestrial planets of the solar system: Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune. Each one was independently photographed in 2019 with a Maksutov telescope from Mannheim and Stockach in Germany, with sizes accurate to what’s visible from Earth. For the first time in our lifetimes, Jupiter and Saturn will simultaneously be visible in the same telescopic field of view. (GETTY IMAGES)

But the most spectacular alignment occurs after sundown on December 21, 2020.

After sunset on December 21, 2020, Jupiter and Saturn will emerge, brilliant and extremely close. To many casual skywatchers, they will be indistinguishable from a single point, although those of us with good seeing and excellent visual acuity may be able to “split” this apparent double planet with naked eye vision. (E. SIEGEL / STELLARIUM)

On solstice night only, both planets and their moons will appear in the same high-magnification telescope’s frame.

On the night of December 21, 2020, Jupiter and Saturn will appear so close to one another, just 0.1 degrees apart, that these two worlds and many of their moons will be visible in the same field-of-view of a relatively high magnification telescope. This will be one of the most spectacular astronomical events of our lifetime. (ASTRONOMY CLUB OF ASHEVILLE / ASTROASHEVILLE.ORG)

If you’re gifting a telescope or binoculars this holiday season, give it in advance of this unique astronomical occasion.

The Celestron Firstscope (L) and fellow Forbes contributor Chad Orzel’s (then) seven-year-old daughter (R) using it. Even a small amateur telescope like this will be able to get excellent views of Jupiter and Saturn together, along with their brightest moons: Io, Ganymede, Europa and Callisto (for Jupiter), and Titan (for Saturn). (CELESTRON / CHAD ORZEL)

The next ultra-close conjunction won’t occur until 2080; for some skywatchers, this will be a twice-in-a-lifetime event, after all.