via BBC Night Magazine

The total solar eclipse was not the last great astronomical spectacle of 2020. There was another event in astral terms in the same year. I’m talking about the planetary conjunction of Jupiter and Saturn – or Christmas Star – that was easily visible in the night sky on December 21, 2020.

At nightfall, Jupiter and Saturn were aligned in such a way that they formed a double planet. While the two are in this position every 20 years, the encounter that night was unique in that it has been almost 400 years since they have been this close to each other, and nearly 800 years since the encounter has taken place at night.

The last time these two planets appeared this close was 397 years ago, on July 16, 1623, when they were separated by only 5 arc minutes. This “closeness” is a matter of perspective since the two planets are separated by 700 million kilometers.

The conjunction of the gas giants occurred on the same day as the summer solstice when the Earth’s semi-axis in the southern hemisphere is most inclined toward the Sun. This is simply a coincidence, based on the orbits of the planets and the tilt of our world.

“Conjunctions like this could occur on any day of the year, depending on where the planets are in their orbits,” said Throop – astronomer of NASA’s Planetary Science Division.


The Great Conjunction: Jupiter And Saturn

via ITV

To ancient skywatchers, Jupiter and Saturn were the two slowest moving planets. Jupiter takes almost 12 years to describe a complete circle in the firmament while spending a year visiting each zodiacal sign in the celestial vault. Saturn, on the other hand, takes 29.5 years to make a complete trip around the Sun. For these reasons and the slow-motion of both, conjunction for the ancients was a rather unusual occurrence.

Usually, when Jupiter and Saturn align, they are separated by one degree (or twice the diameter of the Moon). But on the evening of December 21, they were visible only six arc minutes apart, which is 0.1 degrees (one-fifth the width of the Moon).