Moment Van Gogh caught the full moon: 9.08pm, on 13 July 1889

The moment when Vincent van Gogh captured a full moon rising over a wheat field in the south of France has been pinpointed by astronomers.

It was 9.08pm on 13 July 1889 that Moonrise - named only years after art historians had initially mistaken it for a setting sun - came into being.

Using aerial photographs, trigonometry, astronomical charts and local knowledge, astronomers concluded that Van Gogh could have only depicted the moonrise at that time and on that day.

Explaining their reasons in the current issue of Sky and Telescope magazine, Donald Olson, Marilynn Olson and Russell Doescher, of Southwest Texas State University, say that Van Gogh was clearly trying to depict accurately something he was witnessing.

His oil painting shows the full moon as an orange disc peeping from behind an overhanging cliff that juts from distant foothills - a cliff that can be seen today from the grounds of the Saint-Paul monastery at Saint-Remy, where Van Gogh was being treated for mental illness.

"We employed topographic maps and aerial photographs to determine that the cliff stood 2,680 metres from Van Gogh's position," the scientists say.

They calculated the position of the full moon - the "declination" - as viewed from that position during the time when Van Gogh was known to have stayed at the monastery. "Our computer calculations showed that from the wheat field in evening twilight, Van Gogh could have seen a nearly full moon rising with this declination on only two dates in 1889: 16 May and 13 July."

And the reaped wheat displayed as golden stacks could not have been painted in May, when the landscape was green, so it must have taken place on the July date.

"Because the moon's disc spent less than two minutes passing behind the overhanging cliff, we can determine a precise time for Van Gogh's Moonrise: exactly 9:08pm local mean time on 13 July 1889," the astronomers say.

Furthermore, as the shadows of the wheat stacks do not coincide with the moon's position, it was clear that Van Gogh continued working in the field long after the moon had risen to a higher point in the sky.

"[This] provides strong evidence that Van Gogh was working from nature - and not from ... memory - when he created Moonrise."

On 13 July next month, a near full moon will be in a similar position to the south-east, much as it was on the same date in 1889 when Van Gogh stood among wheat stacks in the monastery field and captured the scene for posterity.