Modern passive house it may be, but as its name suggests, the showstopper at Equinox House in Bulgaria harkens to ancient times when humans built buildings in veneration to heavenly bodies. A narrow aperture in the roof transforms the house into a solar calendar leading it's designer, Ignatov Architects, to refer to the house as a "celestial instrument."

Essentially a shrunken skylight, the oculus creates a concentrated sunspot inside the living room on clear days. Because the sun describes a higher arc across the sky as winter turns to summer, the position of the sun, and hence the resulting sunspot, at a particular time of day can be used to tell the time of year. At noon each day, a calendar scale built into the living room does precisely this.

The calendar is the basis for the house's depth, so that in the summer months, the noon sunspot hits the floor and in the winter months the wall. At each equinox, when the sun passes directly above the equator and day and night are about equal in duration, the noon sunspot falls on the corner between the two.

The design takes other cues from our relationship to the sun. A prominent sea-facing window is angled downwards, which, combined with the curving green roof gives the house a unique identity. However, the precise angle means that this facade is directly in line with the sun at noon on the summer solstice, when it's at its highest in the sky.

It's both an aesthetic and practical feature which, though not blocking direct sunlight from entering the window, except perhaps for a few minutes per year, does help to minimize solar penetration in the summer months thanks to the Sun effectively "seeing" a small aperture, and to the Fresnel conditions, which describe light's tendency to reflect from a glass surface rather then pass through it when arriving at more obtuse angles – a condition that the angling of the window brings about. It doesn't hurt any that the house's south face is near a declining slope so that there's still plenty of view to see from the window.

The building is cut into the hill and, thanks to the green roof, is apparently rather hard to see when approaching from the north. Another nice touch is the south-facing pool which doubles as a reflector during winter, throwing light onto the house's ceilings in those rather charming patterns you see when light is reflected from water.

The building has been fitted with triple glazing, solar power, rainwater harvesting and waste-water treatment which creates compost and water for irrigation.

It's lovely to see a passive house with more to recommend it than dry performance criteria, as vital as they may be.

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