Kepler and Jesus’ Star

Posted on 2011/12/12


Feature Image - Kepler and Jesus' Star

Who vagrant transitory Comets see,
Wonders, because they are rare: but a new starre
Whose motion with the firmament agrees,
Is miracle, for there no new things are.

From John Donne’s ‘To the Countesse of Huntingdon
[Appears in Arthur Koestler (1959)
'Sleepwalkers: A History of Man's Changing Vision of the Universe']


In 1604, while astronomers were observing a rare and mystically significant event, a miracle occurred. A new star appeared in the night’s sky, persisted for a few months, and eventually faded back to black. This one bright dot provided a stunning contradiction of the widely accepted astronomy of Aristotle, and the literalist view of Genesis that held the heavens were completed after the first six days of creation. For almost 2000 years, it was believed that the stars were unchanging and eternal, forever moving in unison around the Earth.

For one unique observer, the event had a deeper, religious revelation. He believed they were witnessing a modern Star of Bethlehem. An event that could help him demonstrate the true birth-date of Jesus.

This is a story of when an imaginative genius encountered an extraordinary coincidence. To understand the story, we must first understand the man.


Everyone has heard of Isaac Newton, if only for his rumoured encounter with an apple. The same goes for Galileo Galilei, except he had the full force of the Catholic Church land on his head. René Descartes is probably less well know, but some will have heard his phrase ‘I think therefore I am’.

Yet, hardly anyone has heard of Johannes Kepler, the 4th horseman of classical mechanics. During the 1600s these scientists established the fundamental ideas for modern physics and theories that would shape the world for the next three hundred years.


Kepler’s work was significant for two reasons.

Firstly, he discovered patterns in the motion of the planets around the sun that were vital to Isaac Newton’s research into gravitation. Newton dedicated a large portion of his ‘Philosophiæ Naturalis Principia Mathematica‘ to demonstrating that Kepler’s Laws can be derived from his, universal, law of gravitation. In other words, Newton mathematically proved that if his law was true, you’d expect the planets to move exactly as Kepler describes them. Without these ideas, Newton would have little data to test his theory, and little inspiration to formulate it at all.

Secondly, Kepler was one of the first people to propose theories that modern physicists would recognize as scientific. Kepler’s Laws were descriptive physical theories; guesses at the relationships between certain physical variables. They didn’t suggest a reason, or a cause, or a purpose for what was observed. They simply joined the dots between observations and started a type of thinking that is still at the heart of today’s finest ideas about the cosmos.

Kepler himself was not clear about the distinction between scientific and pseudo-science, often proclaiming wild assertions based on numerology and astrology. He also practiced as an astrologer to supplement his income. This was often disingenuously, but only because he rejected types of astrology, often critiquing the conclusions of other astrologers. Yet, he was a firm believer in the fundamental assumption of astrology: that the positions of the planets influence one’s fate.


Kepler was also a Christian, with a very individual faith. Living at the time and location of the reformation, Kepler’s life was complicated by religious persecution. Deciding not to adhere to the requirements of Catholicism, Lutheranism or Calvinism, he was perceived as an enemy to all three churches at some point in his life.

His religiosity was primal to his view of the world, and was the motivation for all his intellectual inquiries. For Kepler, God was as the Pythagoreans saw Him:

“Geometry existed before the Creation, is co-eternal with the mind of God, is God himself (What exists in God that is not God himself?); geometry provided God with a model for the Creation and was implanted into man, together with God’s own likeness — and not merely conveyed to his mind through the eyes.”

Kepler (1619) ‘Harmonices Mundi’

No observation could shake Kepler’s faith. Rather, it was faith that inspired him to observe.


In 1603, Kepler observed a conjunction between Jupiter and Saturn that was considered to be astrologically significant. A ‘conjunction’ simply refers to when two planets appear very close together in the sky. It was considered significant because of its rarity and its location. Astrologers divided the signs of the zodiac between the 4 elements of Empedocles. ‘Fire’, ‘earth’, ‘air’ and ‘water’ were each associated with three zodiacal signs. These groups of signs are distributed alternately, so that the three signs of each element form an equilateral triangle (as pictured below).

In 1603, the conjunction of Jupiter and Saturn (which cycle around the sky) moved into Sagittarius, a fiery sign thought to be at the beginning of the zodiac.


The following year Kepler he observed a conjunction between Mars and Saturn. Thirteen days later, on the 9th of October 1604, there was a conjunction between Mars and Jupiter. During this time, these 3 planets were very close together in the sky, an phenomenon known as a ‘massing’. This massing occurred in the same fire sign as the previous year and involved the 3 ‘fiery planets’ of Jupiter, Saturn and Mars.


With modern computing, it is possible to simulate exactly what Kepler saw on those days. This article uses a open-source planetarium software called ‘Stellarium’ and I encourage the reader to download the software to recreate the images themselves.

To start, you need to know that Kepler was living in Prague during these observations. This is the geographical data you will need:

Here  is the data of the conjunctions of 1603 and 1604. You can still click on the images to see what Kepler saw, even if you aren’t using Stellarium.

Event Planets Date Time Image
Single Conjunction Jupiter and Saturn 1603/12/17 7:00  
Massing of Jupiter, Saturn and Mars Saturn and Mars 1604/09/26 19:00  
Jupiter and Mars 1604/10/09 19:00  


During the time of these observations, something miraculous occurred. A new star (called a nova from the Latin for ‘new’) appeared right next to the massing. It is believed that Kepler saw it on the 17th (as he was experiences cloud) and some claim it was visible as early as the 3rd.

Kepler sketched what he saw in his 1606 book ‘De Stella Nova‘:

Comparing this to what the planetarium software simulates, we see that Kepler made a high fidelity drawing. You can see Jupiter (orange), Mar (red) and Saturn (yellow) with the new star (white),marked with an ‘N’ in Kepler’s diagram, looks very similar to modern expectations of the event.


This new star caused much debate, and many turned to the learned Kepler to offer his view.

Patrick J. Boner’s 2010 book Change and Continuity in Early Modern Cosmology‘ considers the ideas of Johannes Fabricius, a contemporary of Kepler. In letters exchanged between the two men, Fabricius explains his speculation that the nova signifies the rejuvenation of the Holy Roman Empire and a reunification of Christianity.

This bold conjecture arose from Fabricius finding a pattern between astronomy and history. As most astronomers of the time, he knew that the conjunction of 1603 and the massing of 1604 only occurred in that part of the sky around every 800 years. This was the time it took for this Great Conjunction of Jupiter and Saturn to precess around the zodiac, like a pen in a spirograph.

The last time the planets were in this position was around 800AD, when Charlemange conquered and Christianized most of Europe. The time before that, the Roman Empire was newly established, and Jesus was born. Additionally, Fabricius thought the appearance of the nova in 1604 corresponded with the appearance of a nova around 800 (now thought to be a comet appearing near Venus, recorded by Albumasar) and the Star of Bethlehem appearing at the birth of Jesus.

Kepler was more cautious in his approach. He rejected most of the speculation in the writings of his contemporaries, instead focusing on the physical characteristics of the nova.

However, while writing his observations and speculations, he came to read a the Doctoral thesis of Laurentius Suslyga. In the short work, Suslyga asserts that, due to mistakes in the formation the Julian and Gregorian calendars, Jesus was actually born in 4BC.

This new analysis inspired Kepler to speculate whether the Star of Bethlehem arose through events similar to those he had just observed.

Kepler calculated when the conjunction and massing occurred. This new historical insight combined with his far superior astronomical data and extraordinary mathematical ability, Kepler was able to investigate Fabricius original claim with rigor. His calculations showed that the events he just witnessed were very similar to the state of the sky around the supposed birth of Jesus.

There were a few differences. Firstly, the conjunction was a triple conjunction - a consequence of the apparent retrograde motion of Jupiter and Saturn. Secondly, the conjunction occurred in Pisces and the massing occurred in Aries. The latter of which was a fiery sign.


For the purposes of using the planetarium, we need to pick a location in Persia where we assume the Maji originated. The difference in the sky’s appearance will be minor, so I have arbitrarily chosen the present capital of Iran for the simulation. Here is the relevant data:

And these are the dates of the triple conjunction and massing.

[Note: Stellarium includes '0AD' as a year. However, the concept of zero was not in the formation of the original calendar. Jesus' birthday was originally set as the 25th of December -1BC, and the following years was 1AD. Due to the quirk, the dates shown appear to be one year ahead of Kepler's speculations.]

Event Planets Date Time Image
Triple Conjunction Jupiter and Saturn 1st -6/05/30 23:30  
Jupiter and Saturn 2nd -6/10/03 23:30  
Jupiter and Saturn 3rd -6/12/08 21:00  
Massing of Jupiter, Saturn and Mars Saturn and Mars -5/02/19 16:15  
Jupiter and Mars -5/03/04 16:15  


Modern observations add strength to the historical record of Kepler. When we point telescopes at the location of Kepler’s 1604 Nova, we see this:

Kepler’s Nova was an exploding star, and we can see the remnants of the explosion. It was not a new star, just a star that was too faint to see until it exploded.
When we point our telescopes at suggested locations for the Star of Bethlehem, we see no such supernovae remanent. Indeed, the only reference to the supposed star appears in the Gospel of Matthew:

“Now after Jesus was born in Bethlehem of Judea in the days of Herod the king, behold, wise men from the East came to Jerusalem, saying,
Where is He who has been born King of the Jews? For we have seen His star in the East [or at its rising] and have come to worship Him.”
Matthew 2:1-2 (NKJV)


What would Kepler make of the modern data? Without ventriloquizing him too much, I think it likely that he would abandon his claims about the Star of Bethlehem. He would probably deny the existance of the star at all. I do not know if he would be a Christian in light of our modern scientific advances, but neither does anyone else.

What we can be sure about is that Kepler was a man who’s imagination was only restricted by the facts.

“When [Kepler] found that his long cherished beliefs did not agree with the most precise observation, he accepted the uncomfortable facts. He preferred the hard truth to his dearest illusions. That is the heart of science.”

Carl Sagan (October 12th 1980) ‘Cosmos: Episode 3 – The Harmony of the Worlds’