As a kid, I was puzzled why sunlight hit the north side of my house in summer. Someone told me it was because the house wasn’t perfectly aligned. I believed this for years, even after learning enough in middle school to know better.
We often assume the sun rises due east, passes overhead at noon, and sets due west. Textbooks rarely challenge this.
Here’s a thought experiment using basic geography:
- Near the equator (e.g., Singapore), on the equinoxes, a perfectly aligned house sees the sun rise due east and set due west. At noon, there’s almost no shadow.
- Near the Tropic of Cancer (e.g., Shantou), on the summer solstice, the sun is overhead at noon. After noon, things get interesting. Visualize this: First, straighten the Earth to its equinox position. Rotate it from noon to evening; your house is on the day-night line. Finally, tilt the Earth back, facing the Tropic of Cancer towards the sun. At step two, a north-south house at dusk on the equinox gets direct sunlight on the west wall. At step three, tilting the Earth exposes the north wall to the sun. On the summer solstice, a house on the Tropic of Cancer gets sun on the north wall, but none on the south.
- Most Chinese live north of the Tropic of Cancer. On the summer solstice, at noon, sunlight mostly hits the roof, with some slanting onto the south wall. Rotate to dusk: straighten the Earth, more sun hits the south wall; rotate to the day-night line, the west wall gets all the light; tilt, and the north wall gets sunlight.
So, even a perfectly straight house will get sunlight on its north wall in summer, even within the Arctic Circle!
From an Earth-centric view, the sun’s summer path is roughly this: rises northeast, moves diagonally, peaks south of overhead at noon, and sets northwest. This happens north and south of the Tropic of Cancer, varying only in duration.
Near the equator, things we take for granted in China are different. Singaporean real estate listings I saw didn’t indicate compass directions. Assuming north is up (a Northern Hemisphere bias), master bedrooms face every direction. We prioritize north-south orientation for sunlight, but near the equator, it’s less crucial. The sun shines on the north wall for half the year and the south wall for the other – you choose which half to keep cooler.