The Boyne River, in County Meath, Ireland, may be the site of an ancient astronomical computer. Giant mounds at three prehistoric complexes appear to track the motions of the Sun and Moon. And one of those mounds could be the home to the oldest known map of the Moon.

The mound at Knowth is more than 200 feet in diameter and three stories tall. Two long passages — one aimed east, the other west — lead to a central chamber painted with elaborate geometric shapes.

At one of the other complexes, passages align with the sunrise and sunset on the equinoxes. At Knowth, though, the Sun shines down these passages a few days or weeks before or after the equinoxes. Some researchers argue that the passages were designed to align with the Moon instead.

Many of the pictures inside the mound appear to have a lunar connection. The calendar stone is decorated with circles and crescents that may represent the cycle of lunar phases. The layout may be an effort to depict a 19-year cycle in which the motions of the Sun and Moon synchronize.

Another stone contains three groups of curved lines and dots. These may be depictions of the lunar disk, with the lines and dots representing the Moon’s dark markings. Since the site is about 5,000 years old, that would make the painting the oldest known lunar calendar.

Look for the Moon early tomorrow, with a bright companion close by: the planet Jupiter. They’re side by side at first light.

Script by Damond Benningfield