There are no coincidences, they are physics!

Two days ago, three planets in the Solar System aligned and we were able to enjoy how Saturn and Jupiter entered Grand Conjunction and saw each other in the night sky the closest they have been in the last 400 years.

We are not going to talk here about whether it was a single event, which in a way it was not because a conjunction of planets is very common, although in this case the conjunction has been very close.

We are also not interested today whether or not this already happened in 7 BC and would have been a reissue of the Star of Bethlehem.

We are going to focus on what can be learned from these types of events, important aspects that escape a simple and superficial description, in fact many of them have only been explained after centuries of study.

Why does it

happen

that we have had a conjunction of Jupiter and Saturn?

There are multiple aspects that are intertwined for such an event to occur.

To see Jupiter and Saturn very close in the sky, the Earth and those 2 planets must be

aligned

, it must be possible to draw a line that passes very close to the 3 stars.

This happens periodically, which implies that the relative movement of the 3 stars is cyclical, it is repeated with a certain frequency.

Today in school we acquire basic knowledge of physics and we know that the Earth, Jupiter and Saturn revolve around the Sun, each with a different period, due to the effect of the gravitational force.

"Even a child knows", you could say, but that knowledge took us millennia to acquire.

Our physical model of the Solar System did not reach this point until after the Middle Ages, with heliocentric models like those of Copernicus and mathematical descriptions like Kepler's or physical theories like Newton's.

However, the movement of the stars in the Solar System today we know is quite far from that simple vision of planets revolving around the Sun. For example, we do not revolve around the Sun, but the center of mass of the Solar System (which is close to the Sun).

Furthermore, that center of mass, the so-called barycenter, is not fixed in space because all the stars in the Solar System move and influence its position.

Consequently, the planets do not rotate in simple, almost circular or slightly elliptical orbits like those described by Kepler, but they change their path continuously and never close a perfect curve.

Planetary dynamics studies are quite accurate today, including even relativistic effects such as those of the orbit of Mercury.

The eight planets are practically in the same plane, as if it were a merry-go-round, and that causes the alignments of planets to be more frequent than they would be in a

three-dimensional

Solar System

But it is not only necessary that there is a periodic movement of the planets around the Sun to have such common conjunctions as the one we observe these days.

All the planets revolve around the Sun, but also all are more or less in the same plane, which cuts the celestial sphere in what is known as the ecliptic.

The inner Solar System, where the planets are, is a fairly two-dimensional (flat) structure.

Each planet could rotate in different planes, like bees around a honeycomb.

But no, the eight planets are practically in the same plane, as if it were a merry-go-round, and that causes the alignments of planets to be more frequent than they would be in a

three-dimensional

Solar System

.

In fact, Jupiter and Saturn align every 20 years, when the

fast

Jupiter, whose year lasts 12 Terrestrial, catches up (from our point of view) the slower Saturn, whose year lasts 29 Terrestrial.

Chance?

No, there are no coincidences, everything has an origin in physical processes and the existence of an ecliptic plane must have an explanation.

There are many more

coincidences

in the cosmos, sometimes we don't even realize it and they are always there.

The Chinese a year ago achieved something that no one had done until now (unless it has been achieved before by military projects and is classified material), by placing a probe on the surface of the Moon.

Yes, the Americans put the first human on our satellite, but Apollo XI and all of its sisters landed on the visible side.

Approximately a third of our satellite never points towards the Earth, it is what is known as the "hidden side" of the Moon, although some mistakenly call it the dark side, and the Chinese just arrived there just a year ago.

Towards our planet they always give the same seas (the grayer areas) and high areas (the whitish areas).

How is this explained?

Well, because the time it takes for the Moon to circle the Earth, about 27 and a third of a day, is very similar to the time it takes for the Moon to rotate around its axis, 27 days.

One compensates for the other and part of the Moon is never pointed at us.

Will we have to believe in coincidences?

No. There is a physical explanation, which is known as tidal coupling.

Orbit coupling is nothing unusual.

It would have been interesting to know how Galileo and his judges would have interpreted this phenomenon when the great Italian astronomer used the 4 largest satellites of Jupiter, the so-called Galileans today, to show that not everything had to revolve around the Earth, that our place in the universe is more modest than what we often believe.

Thanks to his Galileo telescope he was able to observe for the first time that there were 4 stars that regularly changed their position around the great planet.

It seems that it took him only 3 months to realize that they must be Jupiter's satellites similar to the Moon to us, revolving around the gas giant.

Today it seems normal to us, but it is something not easy to imagine and defend when until that moment everything was considered to revolve around the Earth, which was the center of the universe, for something we should be in it.

The fact that interests us in this case is that the 3 innermost Galilean moons, Io, Europa and Ganymede, easily visible today with amateur telescopes or binoculars, are in a resonance that is the result of a tidal coupling.

Io takes 1,769 days to go around Jupiter, Europa 3,551 days and Ganymede 7,155 days.

That means that, with an accuracy of less than 1%, for every 4 turns of Jupiter that Io turns, Europa turns 2 and Ganymede turns 1. Callisto, the fourth moon, goes more to his ball.

There are many more examples of orbital or resonance coupling: Mercury, Pluto and Charon, Neptune and Pluto, even the Earth's motion is influenced by the Moon (not just the other way around).

The friction makes the affection, and although we explicitly do not touch each other much (tell the dinosaurs), the gravitational field has kept us together for 4,500 million years.

There has been an eternity of connecting in ways that seem implausible, but are not at all casual.

Pablo G. Pérez González

is a researcher at the Astrobiology Center, dependent on the Higher Council for Scientific Research and the National Institute for Aerospace Technology (CAB / CSIC-INTA)

Patricia Sánchez Blázquez

is a tenured professor at the Complutense University of Madrid (UCM)

Cosmic Void

is a section in which our knowledge about the universe is presented in a qualitative and quantitative way.

It is intended to explain the importance of understanding the cosmos not only from a scientific point of view but also from a philosophical, social and economic point of view.

The name "cosmic vacuum" refers to the fact that the universe is and is, for the most part, empty, with less than 1 atom per cubic meter, despite the fact that in our environment, paradoxically, there are quintillion atoms per meter cubic, which invites us to reflect on our existence and the presence of life in the universe.

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