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A New Zealand researcher has found - 2000 years after it was built - that the ancient Roman temple known as the Pantheon may play the role of a colossal sundial.

The temple in Rome, completed in AD128, is a cylindrical chamber topped by a domed roof with a skylight in the top which lets through a dramatic shaft of sunlight.

But New Scientist reported Otago University's Professor Robert Hannah - an expert in Roman art, "archaeo-astronomy" and ancient calendar systems - has discovered the Pantheon may have been more than just a temple.

Professor Hannah has just published a book, Time in Antiquity, with a case study of the Pantheon.

He shows that for the six months of winter, the light of the noon sun entering through the skylight, or oculus, traces a path across the inside of the domed roof.

In summer, with the sun higher in the sky, the shaft shines on to the lower walls and floor.

At the two equinoxes, in March and September, the sunlight coming in through the hole strikes the junction between the roof and wall, above the Pantheon's grand northern doorway.

A grille above the door allows a sliver of light through to the front courtyard - the only moment in the year that it sees sunlight if its main doors are closed.

Professor Hannah has shown this was no coincidence, because smaller hollowed-out domes were made in Roman times to act as a type of sundial to show the time of year.

He said that by marking the equinoxes, the Pantheon was intended to elevate emperors who worshipped there into the realm of the gods.