Sun Path & Solar Geometry

For all its grandeur, the sun's position at any instant is pinned by just two angles. Altitude is how high it sits above the horizon, from 0 degrees at the horizon to 90 degrees straight overhead. Azimuth is its compass bearing - where along the horizon it sits, measured in degrees from north (so due east is 90, due south is 180, due west is 270). Give a simulation your latitude, the date and the clock time, and it computes altitude and azimuth exactly; every shadow, every patch of sun on a floor, every solar gain through a window follows from those two numbers.

This matters because building performance is, to a large degree, a conversation with the sun. Whether the west rooms bake at 4pm, whether a light-shelf bounces noon sun deep into a plan, whether a photovoltaic array faces the year's radiation well - all of it is geometry first. Learn to think in altitude and azimuth and you can reason about sun and shadow in your head, then let the tools confirm it. The sun is not random; it is one of the most predictable things in the entire simulation.

It is worth being precise about the reference frame, because it trips people up. Both angles are measured from where the building stands: altitude straight up from the flat horizon, azimuth around the compass. Sunrise is at a low altitude in the east, the sun climbs to its daily peak altitude at solar noon (due south in the northern hemisphere), then descends to a low altitude in the west at sunset. The azimuth therefore sweeps through the day from east through south to west. Note too that solar noon - the moment of peak altitude - rarely lines up with 12:00 on your watch, because clock time is set by time zones and daylight saving, not by the sun over your particular meridian. Simulations handle that offset for you, but knowing it exists stops you misreading a sun-path diagram by an hour.

Because Earth's axis is tilted about 23.5 degrees, the sun's daily arc rides up and down across the year, and four dates anchor the swing. At the June solstice the noon sun is at its highest for the northern hemisphere; at the December solstice it is at its lowest; the two equinoxes (March and September) sit exactly between, with the sun rising due east and setting due west everywhere on Earth.

The practical size of the swing is large. At noon, the sun's altitude changes by about 47 degrees (twice the axial tilt) between the two solstices at any location. For a site near 20 degrees N - much of central India - the noon sun stands around 86-90 degrees in June (nearly overhead, so even a small horizontal shade blocks it) but only around 46 degrees in December (low in the south, so it reaches deep under that same shade). That single fact is the physical basis of the south-facing overhang: it rejects the high summer sun and welcomes the low winter sun, all with a fixed piece of concrete. The sun's yearly geometry, not a thermostat, is doing the switching.

A sun-path diagram compresses the whole year of solar geometry onto one drawing. The common stereographic form is a circular plot seen as if looking straight down at the sky dome: the outer circle is the horizon, the centre is the zenith (straight up), concentric rings mark altitude, and radial lines mark azimuth (compass bearing). Onto this the sun's daily path is drawn as an arc for each month - a high, wide arc for June near the top, a low, shallow arc for December, the equinox in between - and hour lines cross them so you can read the sun's position for any date and time at a glance.

The power of the diagram is that it turns 'where is the sun?' into something you can see and design against. Stand at a window on the plan, transfer its orientation onto the diagram, and you can immediately read which hours of which months the sun strikes it - and therefore when you need shade and when you want sun. Overlay the outline of a neighbouring building or a tree and you can read exactly when it will overshadow your site. Ladybug generates these diagrams straight from an EPW, colouring the paths by temperature or radiation so the geometry and the climate appear together. Once you can read a sun path, orientation and shading stop being guesswork.

Solar geometry converts directly into design moves. Orientation: in the hot parts of India the east and especially the west facades are the problem, because the sun is low there at sunrise and sunset and drives almost horizontally into glass - hard to shade and arriving when the day is already hot. The south gets high sun that a horizontal overhang handles easily, and the north gets gentle, glare-free light. So a simple, powerful move is to keep the long facades and main glazing facing north-south and to minimise and protect west-facing glass. In cold climates the logic flips: you want the low winter sun, so you open up to the south.

Shading follows the geometry. A horizontal overhang suits south facades, where the sun is high; vertical fins suit east and west facades, where the sun is low and sweeps sideways; and the right depth of each is read off the sun angles you need to block. Because latitude sets those angles, the same building needs different shading in Chennai and in Delhi - and a design copied from a magazine at another latitude will simply not shade correctly. The discipline is always the same: read the sun path for this latitude, decide which sun to admit and which to reject, then size the device to the angles. Simulation then confirms the hours it actually delivers.

Everything above is set by one number: your latitude. It fixes how high the sun climbs, how the seasons swing, and therefore how you must orient and shade - which is why solar geometry, more than almost any other topic in this course, refuses to travel between places.

Near the equator (0 degrees) the sun is high overhead most of the year and passes nearly straight up at noon, so both solstices are steep and shading is dominated by short, horizontal devices on nearly every face; there is barely a 'winter sun' to court. In the mid-latitudes (London at 51 degrees, much of Europe and North America) the sun is markedly lower even in summer and skims very low across the southern sky in winter, so south-facing solar gain becomes a prize worth designing for and overhangs must be tuned carefully to admit that low winter sun. Toward the poles the sun barely rises in winter and circles the sky in summer, and orientation logic changes again. India spans a revealing range - from about 8 degrees N at Kanyakumari to 34 degrees N in Kashmir - so the summer sun is close to overhead across most of the country (making horizontal shades highly effective) while the northern hill states get a lower, more oblique sun that rewards southern exposure in their cold winters.

The designer's takeaway is a habit, not a formula: always establish your latitude first, then read that latitude's sun path, and never lift a shading strategy from a project at a different one. A brise-soleil that performs beautifully in Chandigarh will over- or under-shade in Kochi, because the sun it was cut to block simply is not in the same place. Latitude is the silent variable behind every orientation and shading decision - name it before you draw the louvre.