Observational geometry reveals a location and likely date
137 years is extremely brief in astronomy time and the motions of celestial bodies can be calculated precisely, which means scientists can reconstruct the sky at the time of Van Gogh’s painting in fine detail.
To do it, Kim and colleagues Suna Kang, Jungwoo Song, and Ho Jin calculated the positions of the Moon and Venus with astronomical computing tools in Python, and verified the results against NASA JPL’s ephemerides (a mathematical model of the Solar System that shows the time, position, and velocity of points within the path of planets, planetary satellites, comets, and asteroids around the Sun). “We first defined the local time used back then, before standard time was established, and calculated which stars would have been visible to the naked eye as the dawn twilight progressed,” he says.
Observational geometry calculates where Van Gogh stood, which direction he looked, and which part of the sky he could see. “We constructed the painter’s field of view by overlaying photographs taken onsite onto his drawing,” says Kim. “This is similar to astrometric plate solving, a technique that automatically identifies which part of the sky a telescope image shows.”
The scientists matched the size and orientation of the star arrangement of Van Gogh’s painting to those of the real stars, overlaid them, and measured the degree of resemblance as a number.
“I’ll never forget the moment in Saint-Rémy when we found the spot where Van Gogh’s drawing F1541v was made, overlaid a photograph taken there on the drawing, and saw the outlines match exactly,” Kim says. “I was standing where Vincent stood 130 years earlier.”
Another memorable moment was discovering that “in the pen drawing Van Gogh later made after the painting (F1540), the shape of Hyades is even more accurate than in the original,” he adds. “Mechanically copying an unknown shape merely inherits the errors of the original and doesn’t move closer to the real thing. This means he kept the real arrangement of the stars in his mind as a reference.”