The Great Christmas Conjunction: Jupiter-Saturn Conjunction of 2020

Merge of multiple frames - Saturn (L) and Jupiter (R) on 25th December 2020. The angular separation is about 25'. Shot with a Canon 450D on a Nexstar 6SE. Equipment courtesy of Eranga Ganegala,

The year of 2020 was not a great year in many ways. But it was not without its own silver lines. Back in July 2020, we saw the brightest comet in 23 years, the Comet NEOWISE (C/2020 F3) and further before in June, some parts of the world were able to witness an annular solar eclipse. On December 14th, people in Chile and Argentina were able to view a total solar eclipse, which would be the last total solar eclipse till 2024. The best of the events however, is probably the Great Conjunction of Jupiter and Saturn.

A conjunction is when two or more astronomical bodies get very close to each other from a visible point of view. Given that the stars are in a fixed position (unless you observe for over a period of say, ten to hundred thousand years), this refers to objects that are moving across the sky, i.e. planets, Moon and the Sun. 

The importance of the Jupiter-Saturn conjunction in 2020 was that it's the closest these two planets got this close since 1623. Nearly a 70 years before that, in 1563 a conjunction brought the two planets almost as close as it happened in December 2020. The next conjunction where the two planets will be nearly as close will be again in 2080 and then it will be again in 2417.

However, it is important to note that the conjunctions themselves are not very rare. It's about how close you can see them together. A conjunction of these two planets occur nearly every 20 years. However, this time around, they got as close as 6.1 arc minutes. This is about 1/5th of the apparent width of the Moon. In the previous conjunction we witnessed back in 2000, the two planets were nearly twice as apart as the width of the Moon. It was the same in 1980 and will be even more apart in 2040.

 

Single shot of Jupiter and Saturn on 25th December 2020. Canon 450D Samyang 500mm mirror lens. 0.6s at ISO 3200.

Given the usual horrible weather we have been experiencing, no one I personally knew was able to witness the conjunction live on it's closest approach on 21st December. Few days later, we had one day of somewhat clear skies on the Christmas Day. By this time the planets were more than 4 times apart than the closest they were, but still less than one Moon-width.

Some of the biggest challenges in taking photographs of two planets together is in the form of the angular separation and the exposure levels. 

Jupiter is massive and being closer as well, easily burns out when trying to bring out it's moons and Saturn. At this time, Saturn's largest moon, Titan was also visible and we would've been able to capture it, but then Jupiter would've been completely burned out. The second photo shot from the Samyang 500mm should've actually shown the Jovian moon Ganymede right between Jupiter and Europa, but the burnout from Jupiter is so much that you can't see the moon at all. You can also see from the first photo where there is some amount of detail on Saturn and its rings, how burned out Jupiter still is.

Single shot of Jupiter and Saturn on 25th December 2020. Canon 450D on Nexstar 6SE. 0.25s at ISO 3200.

But as can be seen in the photo above, it's not just the exposure that has a say in that. The 6SE which has more than 2x the diameter (and hence over 4x the light gathering power) was able to capture a decent image of Saturn with some definition of its rings and all four of Galilean Moons with exposure slightly less than half. (Here's how the math works, larger the diameter, larger the light gathering area. Since area is given by pi*r*r, 2x the radius means that  light gathering area is 4x. This means if you want to keep the same amount of "light" two other parameters, ISO or the exposure has to compensate. The image above has the same ISO and only half the exposure. Effectively it gathered nearly 2x much "light" as the Samyang).

The second challenge and the sad part of our adventure yesterday was the equipment owner not being able to capture the photos properly. My friend Eranga came over to shoot over from my apartment as there is hardly any obstacle to the west of us. The 6SE is his. However the angular separation of the two planets were way too great for his ZWO ASI to capture both in the same frame even with a focal reducer. The option was to capture them in separate frames and later compose them or to use his mobile phone to capture an eyepiece projection photo. While I could capture them both in the Canon 450D on the same equipment without a focal reducer, the owner had to give up :/ If not for him, I would not have been able to capture these images at all, my Samyang wasn't nearly as good as what I took from his 6SE.