Jupiter and Saturn line up at the ‘great conjunction’

Jupiter and Saturn lined up on December 21, so close together they looked like a bright star. Many referred to her as the “Christmas Star”. It’s the closest the two planets have appeared together in about 800 years, and it won’t happen again until 2080.

This conjunction of Jupiter and Saturn may have an even closer link to the biblical story of the birth of Jesus Christ than its occurrence so close to Christmas this year. As Johannes Kepler pointed out in the 17th century, a similar conjunction occurred in 7BC. C. and could be the astronomical origin of the Star of Bethlehem that guided the wise men.

But there are notable differences between the two events, and the whole story has several interesting links to the history of astronomy, beginning with the origins of the word “planet”, which comes from the Greek word for “wanderer.”

Planets have always been recognizable to astronomers, not only because they are relatively bright points of light between the stars, but because of their unique wandering nature. This posed a problem for ancient astronomers, which lasted for more than 2,000 years and was only solved during the Scientific Revolution.

The movement of the planets

As the Earth rotates on its axis every 24 hours, the sun, moon, stars, and planets appear to move across our sky, rising in the east and setting in the west. But because the planets orbit the sun, all traveling counterclockwise when viewed from above the North Pole, to us on Earth the sun and planets appear to move relative to the stars of background.

As the Earth moves around the sun, the sun in turn appears to move slowly eastward, about one degree each day as it travels through the constellations of the zodiac. Mercury and Venus move from one side of the sun to the other as they surround it. And the outer planets of the solar system (Mars, Jupiter, and Saturn are visible to the naked eye) appear to move eastward through the stars as they orbit the sun.

But something peculiar happens with the positions of the outer planets when the Earth passes between them and the sun: they seem to briefly reverse direction and travel west, against the background stars. This apparent retrograde motion is caused by a parallax shift that occurs for the same reason that your thumb jumps back and forth if you hold it in front of your face and wink with one eye first, then the other; it is an optical illusion caused by a change in our perspective.

While the ancient Greeks had considered this explanation for the retrograde motions of the planets, they mainly preferred an alternative Earth-centered model in which the planets move around a fixed Earth in small circular orbits, the centers of which revolved around an Earth. larger, centered on Earth. circles. Therefore, when the planets orbited an empty point in space while that point orbited the Earth, the planets occasionally stopped and moved backward in their motion against the background stars.

Astronomers mainly described the solar system in this Earth-centered way until Nicolaus Copernicus proposed a sun-centered theory in 1543. Copernicus’s theory did not do a better job of describing planetary motion than Earth-centered models, but the idea gained traction. .

The 17th-century German astronomer Johannes Kepler finally found the key to describing planetary motion in a system centered on the sun. Rather than orbit the sun in circles, Kepler discovered that the planets moved in ellipses, a distinction that allowed him to accurately predict their observed positions.

Christmas star

A conjunction is said to occur when two astronomical objects intersect due to their motion along the direction of the stars’ daily rotation. Since not all objects in the solar system move exactly within the same plane, conjunctions with wide separation can sometimes occur. Since Jupiter orbits the sun every 11.9 years, while Saturn’s orbit takes 29.5 years, it happens that a conjunction of Jupiter and Saturn, called a “grand conjunction” due to its rarity, occurs approximately every 20 years.

Most of the large conjunctions are not particularly noticeable. But every now and then, like this year, Jupiter and Saturn intersect so close to each other that they can barely be distinguished with the naked eye. Or sometimes the two planets intersect when they are opposite the sun, so their apparent retrograde motion results in a triple conjunction, as was the case in 7 BC. C.

In 1604, while working in Prague, Kepler observed the close arrangement of three planets (Mars, Saturn, and Jupiter) and a new bright star, a supernova, that would slowly fade over the course of a year. This fact inspired him to consider a similar set of events that could have brought the Magi to Bethlehem in time for the birth of Jesus Christ.

Knowing that Herod the Great had died in 4 BC. C., placed the birth of Christ before that date. And using his knowledge of planetary motion, he discovered that Jupiter and Saturn experienced a triple conjunction in 7 BC. C., that the conjunctions of Mars with each planet in the year 6 a. They were followed shortly after by conjunctions of the planets with the sun. Kepler suggested that these solar conjunctions aligned with the conception of Christ and that the sages arrived the following year to witness the birth of Christ under the Star of Bethlehem.

Meaning of the great conjunction

On December 21 of this year, Jupiter and Saturn were only one tenth of a degree apart, within the field of view of any telescope. With this year’s event, it is worth noting the historical significance of the previous conjunctions.

Kepler’s fascination with planetary motion led him, just a few years later, to his discovery that planets follow elliptical paths around the sun. And Kepler’s discovery, before the end of that century, would inspire Newton’s work on his most important contribution, the great Mathematical principles of natural philosophy, where he exposed his ideas on the law of gravity, and that forever changed the world of science.

Without fear of exaggeration, it is possible to link the wandering motion of the planets, never more clearly on display than when we can simultaneously see the rings of Saturn and the Galilean moons of Jupiter through a telescope, with the discovery that the Earth is a planet within a solar system. system in which movements are dominated by a universal gravitation that acts between all massive bodies.

This is a corrected version of a story originally published on December 21. The above story said a conjunction aligned with the Immaculate Conception rather than the conception of Christ.

Daryl Janzen, director and instructor of the observatory, astronomy, University of Saskatchewan. This article is republished from The Conversation under a Creative Commons license. Read the original article.