Equinox Sun Exposure in Architectural Planning for Mexico

In Mexican architecture between latitude 15 and 25 degrees north, the equinox is the most useful design calibration date. At the solstices, the sun's behavior is extreme — nearly vertical in summer, moderately high in winter. The equinox is the mid-point: the moment of balanced east-west solar travel and the baseline from which seasonal deviations are measured. In MÉTODO, equinox sun exposure analysis is the first site drawing for every project in Mexico.

Why the Equinox Is the Baseline, Not the Exception

In temperate zone architecture (latitudes 35-55 degrees north), the solstices drive design: winter solstice for passive solar heating potential, summer solstice for shading adequacy. The equinox is the transition — not the controlling condition.

At Mexican latitudes (15-25 degrees north), the hierarchy reverses. The summer solstice sun is nearly vertical — the most difficult condition to shade with any fixed element because it comes from nearly overhead. The winter solstice sun is still reasonably high (47-54 degrees altitude at noon). Neither solstice is particularly useful as a shading design baseline because both represent conditions that diverge from the typical.

The equinox represents the annual average solar behavior at Mexican latitudes:

• Sun rises due east, sets due west
• Solar noon altitude at latitude 19.4 degrees: 70.6 degrees — high enough to require deep shading, low enough that standard horizontal overhangs are effective
• Equal 12-hour day/night balance
• Both east and west facades receive equal, predictable morning and afternoon exposure

The equinox gives the designer the balanced, reproducible solar geometry that forms the foundation of shading and daylighting calculations. Seasonal deviations — the summer-solstice overhead sun, the winter-solstice lower noon angle — are modeled as adjustments from the equinox baseline.

Equinox Sun Path for Major Mexican Architecture Sites

The equinox sun path follows the same geometric rule at every latitude: the sun rises due east at 0 degrees azimuth, transits at maximum altitude at solar noon, and sets due west. The only variable between sites is the noon altitude angle:

The difference between Mexico City and Monterrey is significant: Monterrey's lower noon altitude means horizontal overhangs are more effective at blocking summer sun, and south-facing passive solar gain is more accessible in winter. Oaxaca's higher noon altitude (approaching 73 degrees) means south facades are nearly fully shaded by even moderate overhangs at equinox — useful for year-round overheating prevention, but less useful for passive solar heating.

Shading Calculations from Equinox Geometry

For a south-facing wall in Mexico City (latitude 19.4 degrees north), the equinox noon sun altitude of 70.6 degrees is the primary shading calculation point.

To shade a window completely at equinox noon: the required overhang depth above the window head equals the window height divided by the tangent of the solar altitude angle.

For a window 1.8 meters tall with the overhang at the window head:

• Equinox noon shade depth = 1.8 m divided by tan(70.6 degrees) = 1.8 m divided by 2.84 = 0.63 meters

An overhang of 63 cm above a 1.8-meter south window shades the window completely at equinox noon in Mexico City. The same overhang at the winter solstice (noon altitude 47.2 degrees): shade depth penetrates the window approximately 20 cm from the top — the window remains mostly illuminated in December. At summer solstice (noon sun nearly vertical): the overhang extends shadow well past the window, which is already partially shaded by the nearly-overhead sun.

This calculation sequence — equinox as baseline, solstices as seasonal checks — gives a shading element that works across the full annual range.

East and West Facades at Equinox: The Most Consistent Heat Load Period

The equinox is the critical date for east and west facade shading calculations because it represents the most consistent annual condition for these orientations.

At equinox, the sun rises due east and sets due west — the maximum possible angle of incidence on east and west facades. Every day of the year, the sun deviates from due east or west: toward north in summer, toward south in winter. The equinox east-west sunrise/sunset is the anchor point for calculating shading on these facades.

East facade equinox analysis for CDMX:

• Direct sun begins at sunrise (approximately 6 am in March/September)
• Continues with increasing sun altitude until noon
• At 9 am, solar altitude is approximately 35-40 degrees, azimuth 60 degrees east of south
• At 11 am, solar altitude is 60 degrees, azimuth 25 degrees east of south — the sun is nearly overhead and the east facade is partially shaded by the roof overhang

The morning hours from 7-10 am represent the highest direct sun load on east facades at equinox. Vertical fins 40-50 cm deep at 1 meter spacing block 60-70% of this load while maintaining ventilation and partial views.

West facade equinox analysis for CDMX:

• Direct sun begins at noon and increases in intensity through afternoon
• Peak heat stress: 2-4 pm when both sun intensity and ambient air temperature are maximum
• Sun altitude at 3 pm equinox: approximately 40-45 degrees altitude, 45 degrees west of south
• Deep vertical fins (60-70 cm) are required to intercept this angle; horizontal overhangs alone are inadequate

The equinox west facade analysis reveals the correct shading strategy: vertical elements for morning and afternoon low-angle sun, horizontal elements to supplement at midday.

Daylighting Consequences of Equinox Solar Geometry

The high equinox noon sun altitude in Mexico has significant daylighting implications. A south-facing window under a 63 cm overhang at equinox noon:

• The upper 20 cm of the window (above the shading limit) receives direct sun
• The remaining 1.6 meters of window height receives diffuse sky light and reflected ground light
• Direct sun penetration into the room: approximately 0.5 meters from the window at noon

This means that at equinox in CDMX, south-facing rooms are primarily lit by diffuse light — not direct solar penetration — even with moderate shading. Rooms deeper than 3 meters from the south window receive almost entirely indirect, reflected light. For adequate daylighting in deep rooms, supplementary light sources — clerestories, courtyards, light wells — are required.

The equinox analysis thus reveals a common design problem in Mexico City residential work: south-facing rooms that rely on direct solar penetration for daylighting are only well-lit at shallow depths and in winter when the sun is lower. Year-round daylighting design requires either shallow rooms (less than 4 meters deep) or supplementary apertures.

Equinox as the Starting Point, Not the Ending Point

The equinox analysis establishes the design baseline. The seasonal modifiers — summer solstice overhead sun, winter solstice lower angle, morning and afternoon equinox conditions — are then checked against the equinox-calibrated shading and daylighting system.

A correctly designed shading system for a Mexico City facade:

• Blocks full sun at equinox noon (overhang sized to equinox geometry)
• Also blocks the higher summer solstice sun (which is further overhead — easier to shade)
• Admits winter sun partially (winter sun is lower — 47 degrees — the overhang allows it through the lower window zone)
• Controls morning and afternoon equinox sun through vertical fins or adjustable louvers

This is the complete annual performance profile, derived from the equinox baseline and adjusted for seasonal extremes.

Próximos pasos

Equinox sun exposure analysis is the architectural planning baseline for every project in Mexico between latitude 15 and 25 degrees north. It sets the overhang depth, the fin depth, the window placement, and the daylighting supplementary elements needed. Everything else in the passive design follows from it.

For projects across Mexico where climate response begins with the sun path diagram, conoce el método de MÉTODO.