Two versions of same subject reunited for first time since 1947 and will be on show for three months
"I've counted six so far," said a visitor examining what will be one of the rarest and most stupendous spot the difference challenges imaginable – two of Van Gogh's sunflower paintings side by side for a once-in-a-lifetime display at the National Gallery.
The two versions of the same subject have gone on public display for three months, reunited in London for the first time since 1947 when they were brought together for a Van Gogh exhibition at what is now Tate Britain.
The reuniting is an undeniably special moment. One is owned by the National Gallery and the other by the Van Gogh Museum in Amsterdam and both are among the most popular paintings they own.
"It has taken 65 years to happen and will probably take another 65 for it to happen again because both paintings are so central to what our museums do," said the National Gallery's curator of post-18th century paintings, Christopher Riopelle. "I'm sure there are people in Amsterdam now swearing because they've gone to the museum and Sunflowers isn't there."
Visitors entering Room 46 of the National Gallery will see the London Sunflowers, one of four that Van Gogh made to decorate a bedroom in his "yellow house" in Arles for his visiting friend Paul Gauguin. On its right is one of the three copies that Van Gogh later made.
Martin Bailey, author of the recently published The Sunflowers Are Mine, said people would inevitably compare the two to see what changes he made.
The differences are both subtle and obvious: the copy is a bit taller for example, because Van Gogh wanted more of a margin at the top. His Vincent signatures are in slightly different places on the pots, to maybe give a better balance, and the colours in the Amsterdam version are more stylised – botanically incorrect but perhaps more interesting.
Bailey said there was no big revelation in having them side by side "but it is more that people will appreciate them in a deeper and more meaningful way".
Senior conservator at the Van Gogh Museum, Ella Hendricks, knows every millimetre of the paintings but even for her seeing them side by side "was also an eye opener". She said: "It is almost like a spot the difference game and the differences became obvious."
The National Gallery bought its Sunflowers direct from the artist's family in 1924 and it has become an integral part of its collection. In the gallery's shop you can purchase nearly 100 different types of sunflowers merchandise – from a single handmade chocolate square (£1.25) to a coaster (£6), a cushion (£35) and a pearl bangle (£125).
Of the four original sunflower paintings, only three still exist after one was destroyed in an American bombing raid on Japan during the second world war. The remaining originals are the one in the National Gallery, another in Munich and a third that has been squirrelled away in a private collection since 1948, the last time it was seen in public.
The three copies are on public display in Amsterdam, Philadelphia and on the 42nd floor of a Tokyo skyscraper.
Getting two together is remarkable. Getting more than that would be "everyone's dream" but unlikely, said Riopelle. "I don't think it's going to happen."
Amsterdam's loan follows a reciprocal one by London last year, marking the reopening of the Van Gogh Museum after renovation. Also on display are the results of recent scientific research by both institutions giving new insights into how he painted his sunflowers and what materials he used.
The gallery is anticipating big demand and, because only so many people can be in the room at one time, will operate a queuing system at busy times.
The series come from Van Gogh's most infamous year – it was 1888 that he had his nervous breakdown, cut off part of his ear and went into an asylum.
People have their own reasons for adoring Sunflowers and it often has something to do with the cycle of life feel to them – there are buds, flowers in full health and dieing ones.
Bailey said: "I hope that people will look afresh at something we all think we know. Superficially it is such a simple image but it has enormous power."
• The Sunflowers are at the National Gallery from 25 January-27 April. Admission is free.