Scientifically speaking, the cosmic phenomenon happening Monday night is simply the alignment of two planets. Astronomers call such meetings "conjunctions," and they happen a lot among objects in space. The moon, for example, passes conjunction with every planet once a month.
"Think of it this way," said Jack Pitkin, operations manager of the University of Tennessee at Chattanooga's Clarence T. Jones Observatory. "You are sitting on the porch watching cars on a two-lane highway. Every once in a while, two go by door-to-door and look about like one car."
Still, this is no ordinary conjunction. This one is a "great conjunction," the name given to the alignment between the two largest planets in our solar system, Jupiter and Saturn. Greater still, this will be their closest observable meeting in nearly 800 years. The alignment of our two brightest planets will form what appears to be an ultra-bright star in the night sky, and its Christmas week timing, especially in such a bleak year, is reminding Christians of a manger and Magi.
Whatever its connotations, star gazers can expect a celestial show.
Retired educator Bobby Thompson has been watching the approach over the past several days from his backyard observatory and is eager to see the culmination.
"The really interesting thing to me is that you'll be able to see both planets in a telescope at the same time, which is a really rare experience," he said. "Our astronomical telescopes have a very narrow field of view. To get both in the same field of view is very unusual."
To understand what's happening, it helps to know a little more about the two gas giants in the galaxy. Jupiter is the fifth planet from the sun and the largest planet in our solar system with a radius almost 11 times the size of Earth. Saturn, sixth from the sun, is about nine times the Earth's radius.
Conjunctions of the two planets are not uncommon — they happen every 20 years. But at the last one, in 2000, the two planets were near the sun and difficult to observe. Other factors also can affect how bright and how close their alignment appears. Both have slightly oval orbits, and the orbits' closeness to the sun's equator can cause fluctuations in the planets' proximity to each other.
How to see it
NASA.gov provides these tips for viewing the great conjunction:
* Find a spot with an unobstructed view of the sky, such as a field or park. Jupiter and Saturn are bright, so they can be seen even from most cities.
* An hour after sunset, look to the southwestern sky. Jupiter will look like a bright star and be easily visible. Saturn will be slightly fainter and will appear slightly above and to the left of Jupiter until Dec. 21, when Jupiter will overtake it and they will reverse positions in the sky.
* The planets can be seen with the unaided eye, but if you have binoculars or a small telescope, you may be able to see Jupiter’s four large moons orbiting the giant planet.
Note: UTC’s Clarence T. Jones Observatory is closed to visitors during the coronavirus pandemic.
"During some great conjunctions, the two worlds appear to come so close as to practically hug each other; during others, they seem to approach no nearer than arm's length," writer Charles Q. Choi explained in an article for ScientificAmerican.com.
In space, of course, distance is measured in hundreds of millions of miles, so they'll be "nowhere near colliding," Pitkin said. "Their combined gravity will not do anything to Earth either."
But their relative closeness in the southwestern sky, he said, "will look like they will form one big, bright, slightly football-shaped star."
To some, the effect may be reminiscent of the Bethlehem star, often represented with eight points and an elongated tail, that Christians believe signaled Jesus' birth.
Dan MacDougall, professor of biblical studies at Covenant College, cautions that identifying the current astronomical phenomenon with the star of Bethlehem "is to go beyond the information at our disposal."
The New Testament contains "very limited information" about the star of Bethlehem, he said. References in Matthew 2 tell of wise men from the East (the Magi), who are inspired by the star to travel to Jerusalem in search of the Christ Child.
Still, in a year that has claimed the lives of more than 300,000 Americans, isolated families, collapsed the economy and disrupted nearly every aspect of life, he understands the longing for a beacon of hope.
"This conjunction of two planets, which may look similar to a large star, can still remind us of that star long ago that announced the birth of Jesus, who is God with us," MacDougall said. "His birth assures us that God cares about our world and also about our present situation."
Also worth noting, the conjunction culminates on the winter solstice, meaning the bright beacon shines on the longest night of the year.
The great conjunction will be visible to the naked eye, though a telescope or binoculars will bring more detail into view. In case of cloudy skies, the pairing can be seen for two days on either side of the peak.
According to NASA, the last time the planets passed this close to each other was 1623. This was 13 years after Italian astronomer Galileo trained his telescope on the night sky and discovered Jupiter's moons. Modern astronomers believe the planets were too close to the sun for that great conjunction to have been visible. For the closest observable conjunction, we have to go back to the Middle Ages, medieval times, 1226.
Other great conjunctions will occur on Oct. 31, 2040, and April 7, 2060, but it will be 2080 before the two planets will be so near to each other again — the "hug" that Choi referenced. That conjunction happens on March 15, coinciding with Easter, meaning it's possible for youngsters to experience both a Christmas star and an Easter star in their lifetimes.
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