Worlds Are Colliding!!!

Tomorrow, December 21st, we will get a once-in-a-lifetime planetary alignment:

Skywatchers are in for an end-of-year treat. What has become known popularly as the “Christmas Star” is an especially vibrant planetary conjunction easily visible in the evening sky over the next two weeks as the bright planets Jupiter and Saturn come together, culminating on the night of Dec. 21.

In 1610, Italian astronomer Galileo Galilei pointed his telescope to the night sky, discovering the four moons of Jupiter – Io, Europa, Ganymede, and Callisto. In that same year, Galileo also discovered a strange oval surrounding Saturn, which later observations determined to be its rings. These discoveries changed how people understood the far reaches of our solar system.

Thirteen years later, in 1623, the solar system’s two giant planets, Jupiter and Saturn, traveled together across the sky. Jupiter caught up to and passed Saturn, in an astronomical event known as a “Great Conjunction.”

The planets of the Solar System all orbit in roughly the same plane in the sky, what we call the ecliptic. This is an artifact of their formation in the extended flat disk left over after the Sun formed. The planets also move at different speeds around the Sun, with their orbital period being correlated with their distance. This is known as Kepler Third Law. As a result, one planet will occasionally overtake a more distant planet in the sky. Jupiter, for example, overtakes Saturn about once every twenty years. During these conjunctions, the two planets appear very close to each other from our two-dimensional perspective (while actually being many hundred of millions of miles apart).

It’s rare to have an alignment this precise though. As I noted, the orbits of the planets are roughly aligned. The difference in the orbits is small but enough so that they rarely line up exactly on top of each other. But this passage is particularly close, at about six arc-minutes.¹ They have not passed this close for over 400 years. They will not again until 60 years from now.

At their closets passage, the planets may look like one object but they are easily separable with a telescope or even a pair of binoculars. A decent telescope or a great pair of binoculars might make the rings of Saturn or the moons of Jupiter visible as well. It’s a spectacular event and, were it not for COVID, every observatory in America would be having an event for it. But you don’t need a pro to enjoy this. You may not even need particularly dark or crystal clear skies. Just go out after sunset and look southwest. Just above the sunset, you’ll see a very bright object (or two) with a noticeable size. There’s a long arc through the sky that connect the Sun, the two planets and, to some extent, the Moon.². This is the plane of the Solar System.

Enjoy the “Christmas Star”. The ancients thought such conjunctions were portent. Let’s hope this one portends a better 2021.

1. An arc-minute is about a 30th of the diameter of the full moon or the Sun.
2. The Moon’s orbit is tilted to the ecliptic. It’s sometimes close, though.