SYNOPSIS

Johannes Kepler sends copies of Mysterium Cosmographicum (The Mystery of the Cosmos) to dukes, nobles, and professors, hoping to discuss his ideas. No one replies. Only Galileo Galilei sends a polite letter, saying that he found the book interesting but is too busy to collaborate. Encouraged by the single word interesting, Kepler continues writing to Galileo. Whenever Galileo begins questioning one point, another letter arrives that already addresses it.

Galileo initially studies Kepler's theory to prove it wrong. As he continues his analysis, however, he realizes that Kepler's explanation could work under a different model. Galileo proposes that they investigate the remaining contradictions in planetary motion together and stresses the importance of observation. Kepler explains that, because he has poor eyesight, he wears glasses, and that the same principle of adjusting a lens's focal length could be used to observe distant celestial objects. After learning the basic idea, Galileo builds a more sophisticated telescope.

Although they never meet, the two men continue observing the night sky separately from Italy and Germany while exchanging letters. They observe the Moon's uneven surface, Jupiter's four satellites, the phases of Venus, and countless stars around Orion and Taurus. Concluding that the heliocentric model can explain many of these observations, they decide to publish their work.

Galileo warns that openly advocating heliocentrism could lead to charges of heresy, trial, and even execution. He jokingly plays the role of the Grand Duke of Medici and stages a mock trial, forcing Kepler to defend himself. Kepler tries to explain his observations, but his hands shake as he reaches for the telescope. The two laugh about their fear, yet decide to publish their research despite the risk.

The publication causes a major controversy, and the Medici family withdraws its support. Galileo's daughter, Maria Celeste, becomes the subject of gossip at her convent. Church officials question whether she knew about her father's ideas and even whether her own faith has been shaken. As a child, she never understood her father's enthusiastic explanations of parallax and planetary motion, although she remembers lying beside him and looking at the stars. Now she resents him, believing that his actions have made her own religious life difficult.

Galileo is eventually summoned to appear before the Inquisition. At that moment, Kepler travels from Germany to Italy, and the two men meet in person for the first time. Kepler urges Galileo to flee to a region where the Catholic Church has less influence. Galileo appears to agree, but in the end he chooses to answer the Church's summons.

Kepler later returns to Galileo's house and meets Maria. She has been reading the bundle of letters that Galileo had asked her to burn and tells Kepler that she will report him as a heretic as well. Kepler calmly replies that she should do so if she wishes, but asks whether she has ever looked at the night sky through a telescope. Maria then reads her father's book and observes the stars herself. She appears to realize something.

At the trial, Galileo questions the traditional geocentric view by referring to Jupiter's moons, the phases of Venus, and the Moon's uneven surface. Kepler and Maria appear and say that they also contributed to the discoveries, but Galileo insists that all of the research and ideas were his alone, seemingly to protect them. Realizing that scientific arguments will not change the outcome, he abandons his defense, asks for a chance to repent, and is released.

After returning home, Galileo tells Maria that it has become difficult for her to stay at the convent and asks her to move somewhere else with him. At that moment, Kepler rushes in once more and announces that the remaining eight arcminutes of error in planetary motion can be explained by calculating elliptical orbits using Euclidean geometry. Galileo is delighted, saying that he has another problem to solve. In the final scene, Galileo, Kepler, and Maria lie side by side on the stage, looking up at the stars as the curtain falls.


REVIEW

I decided to see a musical while meeting a friend over the weekend and booked Sidereus at the last minute. I knew very little about the story. It was a returning original Korean musical featuring three actors I like, so I had modest expectations. The performance exceeded them. The score, the pacing, and the performances by the three leads were all strong. The score was one of the better scores I have heard in Daehangno.

Whenever I watch a musical based on science, I naturally look for scientific errors or inconsistencies. This time I paid close attention to whether the historical and scientific details matched what is known from the history of astronomy. Sidereus handled them more carefully than I had expected.

The musical explicitly refers to Kepler's Mysterium Cosmographicum (The Mystery of the Cosmos). Near the end, it mentions the remaining eight arcminutes (about 0.13°) between the observed position of Mars and calculations based on circular orbits, leading to Kepler's elliptical orbit. One line even refers to calculating it using Euclidean geometry, suggesting that the production had received some historical or scientific advice.

At the same time, the musical deliberately avoids explaining exactly how Galileo proved heliocentrism. It mentions the Moon's uneven surface, Jupiter's satellites, the phases of Venus, and the many stars observed around Orion and Taurus, but never claims that any single observation directly proved that the Earth moves around the Sun. The tides are mentioned only briefly, without explanation, and the stars revolving around Polaris are shown only through projected images.

Galileo's observations provided several clues that challenged the traditional view of the universe, but they did not constitute definitive proof of heliocentrism at the time. The phases of Venus were fatal to the Ptolemaic system, yet they were still compatible with Tycho Brahe's geoheliocentric model. Jupiter's satellites demonstrated that not everything revolved around the Earth, but they did not directly prove the Earth's motion. The Moon's irregular surface and sunspots likewise challenged the belief that the heavens were perfect and unchanging. Galileo's own tidal theory, which he regarded as strong evidence for the Earth's rotation and revolution, was ultimately incorrect.

The musical includes a line suggesting that simply questioning whether God's creation might not be perfect was enough to make one's faith suspect. Rather than turning these issues into a scientific lecture, it presents the religious atmosphere of the period. I found its decision not to overstate the scientific arguments to be one of the production's strengths.


Kepler did not derive elliptical orbits from gravity. Working with Tycho Brahe's precise observations of Mars, he found that circular orbits always left a small but persistent discrepancy. Rather than dismissing it, he continued searching until he found that an ellipse matched the data. It was Isaac Newton who later explained why planets move in ellipses through the laws of motion and universal gravitation. The musical does not attempt to explain these distinctions in detail. Instead, it leaves them as suggestions and images, avoiding explanations that would become inaccurate if taken too far.
Rather than focusing on scientific details, the musical shifts its attention to the friendship between Galileo and Kepler, and to the relationship between Galileo and his daughter, Maria.

In reality, Galileo and Kepler exchanged letters but were never close collaborators, and they never met in person. Galileo never claimed sole responsibility to protect Kepler, nor did Kepler volunteer to stand trial beside Galileo. The idea that they planned a joint publication before Galileo became the sole author is also fictional. Historically, Galileo never accepted Kepler's elliptical orbits, making the final scene, in which he welcomes Kepler's discovery, another departure from history.

Even so, the fictional relationship works within the musical. The two men finally meet just before Galileo's trial and try to share both the credit and the risk of their work. Earlier, Kepler gives up his place as an author. Later, Galileo accepts full responsibility for the research. The relationship is sentimental, but it supports the musical's central idea that scientific discoveries are built through shared effort rather than individual glory.

The musical also places Galileo in the role of the senior figure, with Kepler seeking his approval. Dramatically, this is easy for audiences to follow because Galileo is the more familiar historical figure. In reality, however, Kepler's achievements in mathematical astronomy were considerably ahead of Galileo's. The contrast between the two personalities is effective. Galileo is cheerful, confident, and occasionally boastful, while Kepler is modest but unwilling to compromise his work or his sense of responsibility.

Although the musical is generally careful not to overstate scientific conclusions, Maria's transformation felt like its greatest dramatic leap. After reading her father's book and looking through a telescope, she appears to reach an important realization. The staging naturally leads the audience to believe that she has come to understand the scientific truth of heliocentrism. However, professional astronomers of the period interpreted the same observations in very different ways. It is therefore difficult to accept that Maria could embrace heliocentrism after reading one book and making a single observation.

The scene becomes more convincing if her realization is understood differently—not as a full understanding of astronomy, but as the realization that her father was not rejecting his faith. Rather, he simply could not ignore what he had observed. On stage, however, her expressions and actions seemed closer to scientific enlightenment, and I could not quite accept that interpretation.

Lee Sang-a portrayed Maria's conflicting emotions with a clear and beautiful voice. As a child, she never understood her father's excitement when he spoke about parallax and planetary orbits, although she remembered watching the stars with him. Later, as Galileo's trial placed even her life in the convent under pressure, she grew to resent him while struggling between her faith and her love for her father. Although her scientific awakening felt too simple, her emotional journey remained convincing. Her voice became even more expressive as the role grew more demanding.

An Jae-young is an actor who enjoys improvisation and exaggerated comedy. Some audiences may prefer a more restrained approach, but I look forward to seeing what he will do each time. He stretches scenes freely, yet somehow it always works for me.

One of the funniest scenes came when Galileo, pretending to be the Grand Duke of Medici, staged a mock heresy trial for Kepler. As Kepler explained that he had seen the evidence through a telescope, his hand began to shake while reaching for it. A child sitting near the front reacted aloud. An Jae-young immediately replied that people enjoyed watching Kepler tremble, prompting Jeong Whee to shake even more dramatically. After the scene ended, Kepler crawled back toward the study, muttering, "At least someone in the audience enjoyed it." After an extended performance, An Jae-young suddenly shouted, "Wait!" before pretending to vomit.

I had seen An Jae-young before in The Legendary Little Basketball Team, where he played an unmotivated teacher with remarkable naturalness. This time he portrayed a confident and cheerful Galileo, allowing himself much broader comedy. Jeong Whee also adds small improvisations naturally, but I had not realized how well he could respond to spontaneous moments. Having played lively characters suchs as Oliver in Maybe Happy Ending and Didi in ROGER, he matched An Jae-young's improvisation throughout the performance. The two actors worked together better than I had expected, bringing humor to scenes that could easily have become overly explanatory. Their harmonies were solid, and their contrasting vocal colors blended naturally.

The ending, in which Kepler introduces elliptical orbits and Galileo happily declares that he has another problem to solve, differs from history but fits the emotional direction of the musical. The final image of Galileo, Kepler, and Maria lying together beneath the stars brings together scientific discovery, friendship, and reconciliation between father and daughter.

When I was in middle school, I read the story that Galileo had said, "And yet it moves," after his trial. I wondered whether he had abandoned his scientific convictions in order to survive, or whether he simply had no other choice. There is no historical evidence that he actually said those words. Today, I think I would simply say that whether the Earth moves or not, I would rather stay alive. Being burned at the stake is terrifying. The musical does not portray Galileo's recantation as simple cowardice. Instead, it suggests that he chose to protect Kepler and Maria, survive, and leave behind another problem to solve.

Among the science-themed musicals I have seen, this is the one I would rate the highest. Rather than forcing scientific explanations beyond what can be supported, it leaves difficult questions open while using creative imagination to build the relationships between historical figures. Galileo, cheerful and self-confident; Kepler, modest but steadfast; and Maria, torn between love and resentment for her father, together form a story about friendship, family, and the pursuit of knowledge.

As the story unfolded, I found myself naturally looking for scientific inaccuracies. Instead, I found a production that was much more carefully constructed than I had expected. The score and the performances were better than I had anticipated, and before long I had stopped looking for mistakes and was simply enjoying the musical. What began as a last-minute booking while meeting a friend turned out to be one of the most enjoyable surprises I have had in the theater.