The Great Conjunction

The Great Conjunction

The visual spectacle that is the night sky never ceases to amaze. In the last couple of weeks of December 2020, a seemingly bright star was visible in the western sky just after twilight. On closer observation, it turned out that this was a combination of two stars, so close together that to the casual observer, it looked like a single source of light.

Incidentally, these weren’t stars. Jupiter and Saturn, the two gas giants which make up almost 90% of the mass of the solar system outside of the sun, were in a rare conjunction – and we were in a historically enviable position to witness it!

As objects move around in the night sky, their apparent paths may tend to cross. Periodically, one of them comes close to another, creating spectacular illusions. The conjunction between Jupiter and Saturn depends on the relative positions of the Earth, Jupiter and Saturn. Each of these revolves around the Sun with different time periods – Jupiter takes 11.87 Earth years and Saturn takes 29.46 Earth years. Once in 20 years, they’re both on the same side of the Sun, and appear close to each other from the Earth. The conjunction of Jupiter and Saturn is the rarest among all planetary conjunctions, and hence is it called the Great Conjunction.


Conjunction Two celestial objects are in the same right ascension (i.e., the equivalent of longitudes or vertical lines on the sky when imagined as a sphere around the earth). In general, the term is used when the objects are apparently near each other.
Transit A smaller object in the night sky passes in front of the larger object. In this case, you see the silhouette of the smaller object against the backdrop of the larger one.
Occultation A larger object passes in front of the smaller one, completely obscuring the smaller one.

An object passes between a source of light and another object, and casting its shadow on the other object.

An eclipse differs from a transit or occultation in that the transit / occultation is an apparent phenomenon – it depends on the position of the observer. An eclipse on the other hand is a physical manifestation – it requires one body to cast its shadow on the other.

Now Great Conjunctions happen every 20 years, but there are two variables that impact their significance. The first is their angular distance – the measure of the angle that separates the two planets when seen from the Earth. The lesser the angle, the closer they are, and the more spectacular the visuals. The second is the duration of the Great Conjunction. In the diagram below, there are three Great Conjunctions, but each of them appears at a different time of night. The first one occurs after sunrise, and cannot be seen. The second rises at midnight, and is visible for half the night. The third one rises at dusk and lasts the whole night.


Great Conjunction appears during the day

Great Conjunction rises at midnight

Great Conjunction rises at dusk

The Great Conjunction of December 2020 was special for both above reasons. Firstly, has the shortest angular distance since the Great Conjunction in 1623. Imagine that – the last time the two planets appeared this close, Galileo was peering through this fancy new instrument called the telescope and the Taj Mahal was a decade away from being commissioned. Secondly, it was visible at night. Incidentally, the 1623 one was not. So the last time there was such a close Great Conjunction visible from the earth was in March 1226. That’s almost nine centuries ago – Genghis Khan was busy running amok at that time. There’s just one more with a comparable angular distance and appearance time that will happen in the next 400 years, and that is in 2080.

Here’s a cool fact – while we can talk about conjunctions of these two planets, an occultation (see Terminology above) is incomprehensibly rarer. The last time Jupiter completely masked Saturn was in 6856 BCE (almost 9,000 years ago), and the next time it happens will be in 7541 CE (5,500 years into the future). That’s one celestial phenomenon we’d be missing out on.

Stargate celebrated the Great Conjunction 2020 as a social event. In partnership with Shiv Nadar University in Noida, Stargate set up a temporary observatory in the college premises for two nights starting on December 20, 2020. The event, “A Date with J & S”, was attended by students, faculty and management of SNU. The guests got to observe Jupiter and Saturn through state-of-the-art 8-inch telescopes, under the guidance of the educators from Stargate. The images were also livestreamed on YouTube for enthusiasts everywhere to live through these stunning moments. Through the 8” telescope, these planets are a sight to behold. The unique experience was observing them in a single frame. Getting a glimpse of them at this level of visual clarity and in a single frame is unprecedented, considering the rarity of the event.




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