Ask the Expert: The Great Conjunction
EAST LANSING, me. – As the world continues to turn during the COVID-19 pandemic, many of us are preparing for a holiday season that has been unparalleled in recent times. One that at first glance seems to be missing the usual magic of holidays in the past. This would be the case if the one-time major conjunction did not take place on December 21st. A great conjunction that many call the “poinsettia”. Shannon Schmoll, director of the Abrams Planetarium at Michigan State University, offers a glimpse into the astronomical event that will brighten the holidays.
What is a conjunction? Or what qualifies something as a conjunction?
Basically, a conjunction is when two objects such as planets pass each other in the sky. The more specific answer, however, has to do with coordinates. We use a coordinate system that basically consists of longitude and latitude and is called Right Ascension (RA) and Declination (Dec), respectively. Stars maintain the same stellar latitude and longitude as a city on Earth. Planets move between stars, which means that their RA and Dec will change over time. A conjunction occurs when two objects have the same RA or star length. Another way to think about it is when you get a bird’s eye view of the solar system that the planets are lined up together.
Where does the name “Great Conjunction” come from? Is it specific to this event or has it happened before / will it happen again?
We call a great conjunction a conjunction between the two planets Jupiter and Saturn. These two planets are the furthest away naked eye Planets from the sun and their connections have been observed since ancient times. Because they are farther from the sun, they move around the sun more slowly and take longer to go around the sun once. This means that they line up less frequently in our sky than other planets and therefore were the rarest connections that humans can see with the naked eye. Jupiter and Saturn are conjunct roughly every 20 years. That said, not all are easy to see. The last one was in 2000 and from our point of view they were so close to the sun that it was difficult to see. The last time we had an easy-to-spot conjunction was in 1981!
Why do some call this a “poinsettia”?
In the Bible, the star of Bethlehem marked the birth of Jesus and made the kings travel to find him. Over the years, astronomers have tried to figure out which astronomical event might have been the star of Bethlehem. At 7 BC There was a threefold conjunction of Jupiter and Saturn. A triple conjunction is when the planets connect three times in relatively close succession. This is because slower planets appear to be moving backwards, as if they were passing a slower car on the highway. This backward movement is known as retrograde movement and can cause the planets to appear to happen three times in a year. However, this was not a particularly close connection and would not have been particularly noteworthy.
In the year 2 BC Jupiter had a very close connection to Venus and would have looked like a particularly bright star. It also passed the star Regulus twice this year. Both Regulus and Jupiter were associated with kings and could have had a special meaning for the magicians. We don’t really know what the star of Bethlehem was, but because it was very likely a conjunction, some refer to that conjunction as a “poinsettia”.
How rare is this occurrence?
Jupiter and Saturn meet in our sky roughly every 20 years. What is special about it, however, is how close they appear to each other in the sky. Because the orbits are not perfectly aligned, the planets will be connected most of the time, but will still be about a degree apart. This can still be seen close and neat, but they are still further apart than the moon is far. They continue to appear as two different objects in the sky. This conjunction is Really shut down. The planet will only be about a tenth of a degree apart. You can actually see both in one telescope at the same time! This conclusion of a conjunction is quite rare. In 1961 we had one that was a quarter of a degree apart, which is close but not as close as this year. You have to go back about eight centuries to find this. The next one in this vicinity won’t be too far, just in about 60 years on March 15, 2080!
Do you have any tips for people looking to view this event?
All you need is a clear view of the lower part of the sky in the southwest. The tops of parking garages or large, wide-open parking lots are generally good spots for when you don’t have a clear view from your home. Jupiter is very bright so you shouldn’t have to worry too much about lights, but still try to stay away from lots of really bright lights. Also, don’t go out only on the 21st, the day of conjunction. Go to conjunction as often as you can and the following week to see how they happen to one another. If you have a telescope or binoculars, it is worth taking them out to get a closer look at the planets and their bright moons.
What other heavenly events should watchers look for during this time?
We have just passed the climax of the Geminid meteor shows. So the best days are over, but that doesn’t mean there aren’t any meteors (aka shooting stars) here right now. Mars is high in the south right after sunset all winter. Look for a brightest orange star. Similar to Jupiter and Saturn, you can watch it move across the constellations. It will run near the Pleiades star cluster in late February / early March.
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