After a year unlike any other, 2020 will conclude with a spectacle of celestial proportions. On the winter solstice, Jupiter and Saturn will dance closer together in the sky than they have for centuries. Perhaps a concluding memo from the cosmos putting an emphatic stamp on 2020.
While this late December pairing will come closest on Dec. 21, the sight of Jupiter and Saturn appearing so near to one another in the night sky can be enjoyed throughout late December. The planets will appear as close together as the diameter of a full moon for a few weeks. This extended viewing period will provide some relief to stargazers in case of cloudy skies or inclement weather.
This conjunction of planets will be plainly visible with the unaided eye, while binoculars or a small telescope will help reveal the moons of Jupiter and the rings of Saturn to celestial viewers.
When Jupiter and Saturn align with the Earth, the event is called a great conjunction. This happens about every 20 years because of the characteristics of Jupiter’s 12-year and Saturn’s 29-year orbits around our sun. However, not every great conjunction is viewable and not every great conjunction is quite as close as this year’s exceeding rare event. The planets will appear within a mere tenth of a degree this time around.
A great conjunction last happened with the two planets this close in 1623 (three years after the initial English settlement in Plymouth, Mass.), although that event would not have been easily viewable by humans because of the planets’ apparent proximity to the sun. A trip back in history to 1226 (around the time of the construction of the prehistoric indigenous settlement site in Natchez, Miss.) reveals a similarly close conjunction that would have been plainly visible to humans. The next viewing opportunity with this close an approach of the two planets is expected in 2080.
To view the 2020 great conjunction of Jupiter and Saturn, one should look towards the southwest at evening twilight, within about 15 degrees of the horizon. Except for the moon, the conjuncting planets will be the brightest objects in the night sky.
For Middlebury, the sun will set at about 4:18 p.m. on Dec. 21. Conditions will be best for viewing the conjunction beginning about 45 minutes after sunset. However, the optimal viewing window of opportunity will only last about an hour.
For Middlebury, Jupiter and Saturn will then set at about 6:34 p.m. on Dec. 21. However, this 5 p.m. to 6 p.m. ET window at dusk should be an appropriate viewing window from Addison County throughout late December, weather permitting, for enjoying these gas giant planets in the night sky.
Observers with binoculars or telescopes will enjoy the additional treats of moons and rings.
With binoculars, the Galilean satellites of Jupiter will be visible. These include Io, Europa, Ganymede and Calisto — the four largest moons discovered by Italian astronomer Galileo Galilei in 1609–1610, shortly after the Dutch invention of the telescope.
With a small telescope, atmospheric bands on Jupiter and the rings of Saturn will be visible. These rings were also first noticed by Galileo, but not identified as such until 1655 by Dutch astronomer Christaan Huygens.
Both Jupiter and Saturn were formed about 4.5 billion years ago, just tens of millions of years after the formation of our solar system. Both planets are gas giants, meaning they largely consist of hydrogen and helium in a gaseous state (just like our sun), but also potentially harbor metallic elements and rocky materials in a liquid or solid core. Jupiter and Saturn are tens of times larger and hundreds of times more massive than our own planet.
While the distances from Earth vary based on orbital position, Jupiter and Saturn are hundreds of millions of miles away. During the 2020 great conjunction, Saturn will be nearly twice as far away as Jupiter. Starlight takes roughly an hour or so to travel between these outer planets and Earth.
In spite of our occasionally grey New England winters, our dark Vermont skies should hopefully provide some fantastic opportunities for viewing this once-in-a-lifetime conjunction of Jupiter and Saturn.
For those looking forward to other amazing celestial event observing opportunities, Champlain Valley residents should also note the total solar eclipse of April 8, 2024. From Middlebury to Montréal, the path of totality will grace our northern latitudes in a rare event. With some weather luck, Middlebury will enjoy an entire minute of a total solar eclipse, with the full eclipse lasting about two and a half hours. Don’t forget your eclipse viewing safety glasses and be sure to mark your calendars now.
Editor’s note: Jonathan Kemp is Telescope and Scientific Computing Specialist at Middlebury College.