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If you’ve been looking up into the evening for the past few months, you may have seen some pretty bright objects in the southern sky.
These are the planets Jupiter and Saturn, and on December 21 they will appear closer than people have seen in nearly 400 years.
Conjunctions are events in which celestial bodies are aligned and are among the funniest and most easily observable events in astronomy.
Since the planets and the moon all orbit almost the same plane, we often see them approaching each other in the sky as they rotate at different speeds.
If they align perfectly, eclipses occur or planets pass in front of the sun.
The movement of the moon across the sky each month makes the moon appear to approach different planets and bright stars, providing great photo opportunities.
Since planets take much longer to orbit the sun, conjunctions of two planets are much rarer.
We see Jupiter and Saturn align with each other only once every 20 years.
On December 21, the two largest planets in the solar system will be about a tenth of a degree from each other, which will make them indistinguishable, unless you use binoculars or a telescope.
For a size comparison, the width of the moon is about half a degree in the sky, or about half the width of the fingernail when held at arm’s length.
Jupiter and Saturn will be closer than the width of the moon between 17 and 25 December.
Despite their apparent proximity, Saturn is nearly half a billion miles from Jupiter in space.
Jupiter and Saturn will appear as a single bright spot 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 star’s appearance to historical conjunctions, including that between Venus and Jupiter in 2 BC.
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 Halley’s comet in 1301.
A wonderful benefit of knowing how most objects in the solar system move is being able to understand what the sky looked like in the past and being able to predict similar events in the future.
During 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 dark.
If you get lost, these planets will have their next conjunction in 2040.
Staerkel Planetarium offers virtual programs to schools and the public.
The “Prairie Skies” program and speakers for James B. Kaler’s science lecture series can be found at parkland.edu/planetarium.
Contact the staff at
to book a show or to request information on renting a telescope.
The Champaign County Museum Network has information at champaigncounty
museums.org.
Erik Johnson is the director of the William M. Staerkel Planetarium at Parkland College. He can be reached at [email protected].
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