This fragmented group of sculptured figures shows a terrifying moment in a story from Greek mythology. A young girl is about to be sacrificed on the altar of Artemis, goddess of hunting, the moon and women. At the instant the sacrificial knife is about to descend, the goddess herself suddenly appears and, whirling the girl away with one hand, casts a young deer down on to the altar with the other. (A deer’s head forms part of the sculptural group shown here, but it was too fragile to include in the exhibition currently at the Royal Academy. Artemis’s thumb is visible on the deer’s antler.) A photographic reproduction cannot do justice to the dynamic force and delicate carving of these fragmented marble figures, but the drama and realism conveyed in what remains of their bodies is such that anyone viewing the piece is whirled into the action and led to envisage the completed forms.

Ancient Greek sculpture has often survived in beautiful and tantalising fragments that evoke a world of lost gods and heroes. It is as if their brilliant but fragmentary nature represents our forever incomplete knowledge of the extraordinary culture that created them. The missing faces allow us also to recognise the artist’s skill, since so much expression has been conveyed solely through the bodies’ forms.

The story behind this heavenly intervention is as follows. The fleet of Agamemnon, king of Mycenae, is becalmed as it is about to sail against the city of Troy. Artemis is infuriated by Agamemnon’s unintentional slaughter of one of her stags, and decrees that only if the king sacrifices his beloved daughter Iphigeneia will the winds blow again, allowing the fleet to sail into battle. Iphigeneia agrees to die when her father explains that the Greek army will be so enraged if it cannot go to war that it will rise up and kill their whole family. As Iphigeneia awaits her death on the altar, Artemis herself, touched by the girl’s heroism, decides to save her and, replacing her with a deer, spirits her off to be one of her priestesses.

Holding us in the grip of the moment, the sculptor gives the remarkably vivid impression that we are watching a real event as it unfolds before our eyes. In spite of the stationary nature of the sculpture, we have an illusion of violent movement. The regal vertical of the goddess’s figure standing behind, the delicately carved pleats of her dress slightly ruffled, contrasts with the diagonal instability of Iphigeneia’s body as it is wrenched up from the ground, the movement swishing her tunic against her. Our mind unconsciously completes her body’s trajectory, thereby experiencing the completed motion.

This artistic realism was developed in classical Greece because artists needed to create empathy in the viewer and so vitalise the powerful stories of the gods and heroes. These artists revolutionised the function and the language of art so that it would show particular psychological moments, feelings, moods and relationships, with realistic-looking figures. To do this, they needed to explore and understand body-language and create a style that would represent a kind of perfected reality, so that their audience could feel it was witnessing actual events.

It was no accident that during this period Greek theatre reached heights of psychological realism. The playwright Euripides told Iphigeneia’s story in his play Iphigeneia at Aulis (produced this summer at the National Theatre in London) although the action we witness in the sculpture occurs off-stage in the play and is represented in a witness’s report.

Freestanding sculptured groups were extremely rare in ancient art, since they were so vulnerable to the ravages of time, as witnessed by the fragmented state of this group. This sculpture’s impact must have been astonishing when it was perfect, since it is so beautiful in its fragmented state and we can read so much from what remains. It would have been fascinating to see the full choreography of the figures, to say nothing of the expressions on the faces of Artemis and Iphigeneia.

Although it is in the more long-lasting medium of sculpture that the skill of Ancient Greek artists is preserved, there are some remains of wall paintings (for example, in the tomb at Vergina in Macedonia), in which their consummate skill in representing perspective and form on the flat surface is apparent, even in the deteriorated state of the paintings.

This and the other works in the exhibition are housed in the Ny Carlsberg Glyptotek in Copenhagen, at present undergoing refurbishment, enabling the Royal Academy to exhibit so many of its most magnificent pieces. There are fascinating comparisons to be made across the centuries, since the exhibition includes sculptures from Ancient Egypt, Greece, Etruria and Rome, paintings and sculptures from the Danish Golden Age and from the French 19th and 20th centuries, including works by Manet, Monet, Rodin, Maillol, Degas, van Gogh, Gauguin and Matisse.

Annie Harris is head of education at the Royal Academy, London