If you’ve been looking up in the evening over the last several months, you may have seen some rather bright objects in the southern sky.
Those are the planets Jupiter and Saturn, and on Dec. 21, they are going to appear closer together than people have seen in nearly 400 years.
Conjunctions are events where celestial bodies are aligned, and they are among the most entertaining and easily seen events in astronomy.
Since the planets and the moon all orbit in nearly the same plane, we often see them approach each other in the sky as they revolve at different speeds.
If they line up perfectly, eclipses occur or planets transit in front of the sun.
The moon’s motion across the sky each month makes it appear to come close to several planets and bright stars, making for nice photographic opportunities.
Since the planets take much longer to orbit the sun, conjunctions of two planets are much more rare.
We only see Jupiter and Saturn line up with each other once every 20 years.
On Dec. 21, the two largest planets in the solar system will be about one-tenth of a degree from each other, which will make them indistinguishable, unless you use binoculars or a telescope.
For a size comparison, the moon’s width is about half a degree in the sky, or roughly half the width of your fingernail when held at arm’s length.
Jupiter and Saturn will be closer together than the width of the moon between Dec. 17 and 25.
Despite their apparent proximity, Saturn is nearly half a billion miles away from Jupiter in space.
Jupiter and Saturn will appear to be a single, bright point of light on the evening of the solstice, which may remind you of the Star of Bethlehem from the Nativity story.
Some astronomers have attempted to link the appearance of the star to historical conjunctions, including one between Venus and Jupiter in 2 BCE.
Giotto’s painting “The Adoration of the Magi” included a depiction of the Star of Bethlehem as a comet, which may have been inspired by the appearance of Comet Halley in 1301.
A wonderful benefit of knowing how most objects in the solar system move is being able to understand how the sky looked in the past and being able to predict similar events in the future.
Throughout the next month, you will be able to enjoy the fruits of one of these predictions.
Look for Jupiter and Saturn low in the southwestern sky after sunset.
If you miss out, these planets will have their next conjunction in 2040.
Staerkel Planetarium offers virtual programs to schools and to the public.
The schedule of “Prairie Skies” and the speakers for the James B. Kaler Science Lecture Series can be found at parkland.edu/planetarium.
Contact the staff at
to book a show or to inquire about renting a telescope.
The Champaign County Museums Network has information at champaigncounty
Erik Johnson is the director of the William M. Staerkel Planetarium at Parkland College. He can be reached at firstname.lastname@example.org.