The dots represent different star groups

Astronomers are intrigued by a set of cave paintings discovered on the island of Guahan in the Pacific. One of the paintings shows 16 dots arranged both vertically and horizontally. It is thought these may represent an ancient calendar which divided the year into 16 months of unequal length. The months would have been based on the movement of different constellations across the night sky.

Although the authenticity of the drawings has been established by archaeologists, their exact age is not known. However, Guam is believed to have had settlers for at least 3,500 years - perhaps even 4,000 years.

Scientists are interested to know how the ancient Chamori people of the region gained their astronomical skills. The knowledge developed quite separately from that in ancient China or Europe.

The use of the stars to mark out the months rather than the moon is especially interesting in this respect.

Stick-shaped humans

Two other paintings show a stick-shaped human figure looking towards a constellation. In one, the figure points to the Southern Cross, in the other, to Cassiopeia. Being bright and easily identifiable, these constellations in the northern and southern sky would have been important markers.

Professor Rosina Iping from the University of Guahan has been studying the cave paintings. She says their interpretation has been helped by the special knowledge of an old navigator who lived on a nearby island.

"The 16 dots, I am pretty sure, are a calendar," she told BBC News Online.

"It's supposed to be the 16 months they are still using on the nearby island of Puluwat. An old navigator who lives on Puluwat told me how they use the months, and how they navigate by the stars."

Year start

The beginning of a month is marked by the appearance of a star belonging to a particular group at about 45 degrees above the horizon just before dawn in the east. The year starts with the rising of Antares and ends with the Corona Borealis.

Because different constellations occupy a greater or lesser part of the sky, the months are not of uniform length.

Ancient Chamori petroglyphs can be found in several places on the island of Guahan. The paintings studied by Professor Iping were found at Ritidian point on the north coast.

Some have been carved out of the cave walls and are now on display at the Bishop Museum in Hawaii.

The sketches show human figures, animals and weapons. A few of the figures seem to resemble Chinese characters, which originated from pictorial images.

The colours are white, brown and black. Rosina Iping presented her interpretation of the paintings to the 193rd meeting of the American Astronomical Society in Austin, Texas, on Saturday.