The Great Conjunction of Jupiter and Saturn

On December 21, the first day of winter this year, the two gaseous giants in the solar system—Jupiter and Saturn—will put up a spectacular display in the evening sky. They will be so close that they will appear, from our perspective, to overlap completely, creating a rare "double planet" effect. However, while they may appear to the naked eye extremely close, within 0.1 degree of each other, in reality, they are separated by more than 400 million miles. To visualise this distance, a 0.1 degree separation is about the thickness of a dime held at arm's length. This celestial synchronisation, also referred to as the "Christmas Star," has not occurred in nearly 800 years. Last time they were so close together was on March 4, 1226.

The spectacle is a curious effect of their orbits around the Sun. Since Jupiter takes 11.9 years to circle the Sun and Saturn 29.5 years, the faster moving Jupiter catches up with the slower moving Saturn and overtakes it roughly every 20 years. Astronomers call the moment of overtaking "Great Conjunction."

At conjunction, separation between two objects in the sky as viewed from Earth is a minimum. Moreover, at great conjunction, Earth, Jupiter and Saturn align themselves along a straight line so as to make the two Jovian planets appear very close together. Great conjunctions are rare though because more often than not, Earth is not aligned along a straight line with Saturn and Jupiter when they are at conjunction. The duo will then appear to be separated by a few degrees.

Furthermore, the event can happen while the Sun is up blocking the conjunction from view. Indeed, during the last great conjunction on May 31, 2000, the planets never came anywhere as close together as they will this month. Besides, we could not see the overlap because the alignment occurred too close to the Sun and thus was lost in the glare of the twilight. The same was true the time before, in December of 1980.

This time around, after sunset on Monday, December 21, which is also the winter solstice, Jupiter and Saturn will appear to the unaided eye as a single bright object low in the southwestern sky. This dazzling display of two celestial objects kissing each other on the longest night of the year can be seen from everywhere in the world. It will become visible in Bangladesh soon after sunset but only for a short time. The conjoined planets will sink below the western horizon about an hour later. Also, the further north viewers are, the less time they will have to catch a glimpse of this astronomical event.

How can we spot the planets with naked eye? In the weeks leading up to the great conjunction, Saturn will be to the upper left of Jupiter, slowly dancing toward Jupiter. On clear nights, unlike stars which twinkle, Jupiter and Saturn will hold consistent brightness, making it easier to spot them amidst the myriad of objects in the stellar zoo. Although Saturn will be slightly dimmer and smaller in size, yet it will be just as bright as the brightest stars, with a recognisable golden glow. An amateur telescope or a high-power binocular will show the planets in more detail, including the Galilean moons of Jupiter—Io, Europa, Ganymede and Callisto.

We do not have to wait until December 21 to view this dazzling conjunction. They are already a pretty pair in the sky, and will remain so through the entirety of December. Nevertheless, after December 21, Jupiter will start moving eastward, separating from Saturn. Additionally, during early evening hours, the planetary pair will appear lower in the sky, albeit appearing near each other for about a month, giving sky watchers plenty of time to witness the amazing alignment throughout the holiday season.

If you miss the spectacle this year, you should not expect to see it in 2040 or 2060. The next great conjunction, with a separation of about 0.2 degree, will occur on March 15, 2080. After that, it will be 2417 and 2477.

Finally, dating back to Kepler's time in the 17th century, some astronomers hypothesised that the Star of Bethlehem that guided the Three Magi—also known as the "three wise men"—to Christ's birthplace was a conjunction like the one we will witness on December 21. It could be but involving different planets.

Quamrul Haider is a Professor of Physics at Fordham University, New York.