What is 'triple conjunction'? How to watch Jupiter, Mercury and Saturn aligned closest to each other this weekend
In December 2020, the two largest planets in our solar system appeared closer to each other in the sky than they have been since the Middle Ages. If you thought that was impressive, be ready to be treated to another spectacular show this weekend as Mercury, Jupiter and Saturn meet for a rare ‘triple conjunction.’
What does it mean?
When two planets are seen close to each other in the sky the event is called ‘conjunction’ of planets. A conjunction of the giants Jupiter and Saturn is called “great conjunction.”
Triple conjunction occurs when three planets align in the sky. Jupiter (brightest), medium-bright Mercury, and Saturn (dimmest) will be contained within a 5-degree circle from January 8 to January 11, 2021, appearing closest together on Sunday evening, January 10. Mercury, Jupiter and Saturn are expected to appear within roughly 2 degrees of each other in the constellation of Capricorn.
“By definition, a planetary trio consists of three planets fitting within a circle whose diameter is less than 5 degrees in width. For reference, three fingers held together at an arm's length approximates 5 degrees. At their closest, the planets of this planetary trio will fit inside a circle with a diameter of 2 1/2 degrees on January 10 (at 19:00 UTC),” explains Earthsky.org.
The last planetary trio occurred in October 2015. “This planetary trio on January 10, 2021, presents the first of two planetary trios in the year 2021; and the first of four planetary trios in the 203rd decade or the period from January 1, 2021, to December 31, 2030,” says the report.
NASA wrote that from Monday evening (January 11), Mercury will appear to pass first by Saturn and then by Jupiter as it shifts away from the horizon, “visible each evening low in the west-southwest and setting before evening twilight ends.”
How to watch the spectacle?
Clear conditions will be needed to spot the planets, but they will be visible together for several days. It is important to find an area away from bright city lights for the best viewing conditions. Experts strongly suggest binoculars as they will help pick up the planets against the bright twilight sky, and they will be particularly handy for Saturn. Jupiter will be at the top of the triangle, with Mercury and Saturn forming the base angles.
All three will be visible low in the western sky, close to the sunset point on the horizon. But the window for catching this cosmic congregation is not long. The planetary trio will be visible at dusk, so stargazers will need to station themselves at an unobstructed horizon in the direction of the sunset. Hence, it is important to look for the planets ideally in the 30 minutes after sunset, else they could fall below the horizon line. If you live in the US or Canada, you can find out when Mercury, Jupiter, and Saturn will set in your sky with the help of the Old Farmer’s Almanac.
“All the planets of this planetary trio will follow the sun beneath the horizon by nightfall, so it’s imperative that you start your search no later than 45 minutes after sundown,” recommends Earthsky.org.