Night watch.

In one of the most beautiful of Japan's national parks, Takasaki's symbolic and inspiring astronomical museum encourages heavenly contemplation and civic pride.

Masaharu Takasaki's weird, visionary projects and buildings are expressions of a private philosophy that admits both natural and atavistic forces and humanity's potential for civilisation and progress. Characteristically containing symbolic representations of landscape ('I try to find geometries that communicate the nature of the site'), they inhabit the outer reaches of architecture, intersecting at some fantastic cusp with Japanese folklore and a kind of reinvented futurism.

The Kihoku Astronomical Museum, at the southernmost tip of Kyushu in south Japan, is one of Takasaki's latest buildings. Since its purpose is heavenly contemplation - which has since ancient times induced mystical and philosophical musings - the project encapsulated visionary expression.

The museum has its own observatory and galleries devoted to exhibitions about space, as well as an internal theatre of extraordinary primitive drama. It is no wonder that the place is popular. The site alone is an inspiration and of classic Japanese beauty. The museum is set on the summit of a 550-metre hill in the Kihoku Uwaba park, between the Kirishima mountains to the north and Shibushi bay to the south. Looking west, there are views of Kinko bay and the active volcano, Sakurajima. Seen from a distance, particularly when illuminated at night, the strange structure seems unearthly, barely anchored to its terrestrial site.

Takasaki's elemental design is heavy with primal metaphor, and counterpoises the twin themes of earth and water against those of the polar and celestial. The building is uniformly constructed of grey concrete so that its several distinct elements are combined into a single articulated body. Depending on the point of view, the body takes on different guises; the north-east elevation has a distinctly biomorphic cast but on the south-east the structure looks mechanical, like some mad space station. The images, protean and insistent, keep on coming.

To the north an ovoid sprouting antennal blossom is aimed towards the northerly sky. Pierced through its conical nose by shafts of light and fixed by asymmetrical columns that thrust through the shell, this monumental form describes the great volume of the theatre (rather prosaically known as the training room). It is an austere space of great gravity and simplicity, encircled around the base by tiers of wooden seats like steps, more akin to the timeless structures of the ancient world than a modern museum.

It is set into a sharply indented platform supported on a forest of splayed legs. To the rear, and balancing the composition, a domed helmeted block summons up Samurai ghosts (or in Western imaginations, some latter-day anthroposophical Goetheanum). This part, too, is sunk into the platform, bellying down into the concrete forest to create a children's house. Stacked up above on three levels are a Star Dome, an exhibition gallery and at the top, the observatory.

You reach the platform and entry to the various parts of the museum by clambering under the lowering concrete cliffs up frail-looking stairs that lead from ground level - the terrestrial zone riven by channels meandering through the concrete forest and terraced to symbolise earth mass and water.

The powerful structure, sculpted out of a single material, balances weightlessness and gravity, the static and dynamic. It has a socio-economic purpose as well as an instructive one, for it commemorates the area's award for 'Most Beautiful Night Sky in Japan'. Such quaintness has proved efficacious: direct consequences of the award have been the large numbers of visitors attracted to the area, and the restoration of civic pride to a town depressed by emigration and struggling to support an ageing population. Architecture, Takasaki insists, is a 'social art'.