Van Gogh painting put on the calendar. (Timing a Moonrise).

Partially hidden by mountains, a glowing orange orb rises over golden stacks of wheat. Vincent van Gogh's "Moonrise" depicts a scene in Provence with the painter's unmistakable palette of brilliant, swirling colors.

Historians have long debated exactly when in 1889 van Gogh created the painting. Astronomical detectives report that they have now solved the mystery: The artist captured the rising moon as it appeared at 9:08 p.m. local mean time on July 13, 1889.

Donald W. Olson and his colleagues at Southwest Texas State University in San Marcos describe their finding in the July Sky & Telescope.

Van Gogh arrived at Saint-Remy-de-Provence on May 8, 1889. For treatment of several medical problems, he moved into a local hospital housed in a monastery. Peering through his window, van Gogh wrote to his brother Theo: "I see an enclosed wheat field ... above which I see the sun rise in all its glory." In another letter, he tells of working in the wheat field on a painting that depicted a moonrise. In late September, he mailed the picture to Theo.

[ILLUSTRATION OMITTED]

But during that summer, van Gogh had become severely ill and didn't paint for 6 weeks. Historians hadn't known whether van Gogh completed "Moonrise" before or after this hiatus.

Computer calculations by the Texas researchers revealed five dates between mid-May and mid-September 1889 when a full or nearly full moon would have appeared over Saint-Remy. To narrow the timing, the scientists journeyed to the town, looking for visual clues--the small house and overhanging cliff depicted in "Moonrise."

They found the site just southeast of the monastery. Observing the sun, moon, and stars for 6 days, the researchers determined the altitude and orientation of the cliff with respect to van Gogh's perspective. A refined calculation then zeroed in on two dates in 1889--May 16 and July 13. The colors in "Moonrise" provided the last clue. The wheat, still green in May, would have looked yellow in July.

Exactly six 19-year lunar cycles have elapsed since 1889, and history is about to repeat itself. On July 13, 2005, the researchers note, "observers during evening twilight [in Saint-Remy] will see a nearly full moon rise in the southeast, much as it did on July 13, 1889, when van Gogh stood among the wheat stacks in the monastery field and captured the scene in his remarkable 'Moonrise'."