When is the sun directly overhead?

publicThe idea

The subsolar point

At every moment there is one point on Earth where the sun stands exactly overhead, the subsolar point. Over a year it moves north and south between the two tropics.

It reaches the Tropic of Cancer, 23.44° north, at the June solstice and the Tropic of Capricorn, 23.44° south, at the December solstice, and crosses the equator at the equinoxes. Around the equinoxes it moves fast, about 0.4° of latitude or 44 km a day; near the solstices it slows down and turns back. Any place between the tropics is passed twice a year, once on the way north and once on the way south. Places on the tropics are touched once, and places outside them never.

scheduleThe dates

Zenith sun days in sixteen cities

On these days the noon sun stands within a tenth of a degree of the zenith. Dates can shift by a day from one year to the next, and on the days either side the sun is still within about half a degree of overhead, so the effect lasts several days in practice.

wb_sunnyWhat you see

The day without a shadow

At that noon an upright pole casts its shadow straight down onto its own base, and a person’s shadow shrinks to a dark patch under their feet. In India the event is celebrated as Zero Shadow Day, with schools measuring poles; in Hawaii it is called Lahaina Noon. Because the subsolar point moves only slowly near the tropics, places like Havana and Hong Kong see their two passes just a few weeks apart, while near the equator they are six months apart.

Look at the times as well. Havana’s solar noon falls at about half past one on the clock, because the island keeps daylight saving time in summer and lies west of its zone meridian. The article on solar noon explains why.

helpOutside the tropics

Why it never happens in Europe or the USA

North of the Tropic of Cancer the noon sun is highest at the June solstice, at 90° minus your latitude plus 23.44°: 73° in Madrid or New York, 62° in London. However hot the day, it never gets closer to the zenith than that, and an upright pole always has a shadow pointing north at noon. The same holds south of the Tropic of Capricorn in December: Johannesburg comes within three degrees but never reaches it.

In the tropics, by contrast, the noon sun is in the northern sky for part of the year and the southern sky for the rest. North-facing windows get direct sun in the summer months, and a solar panel facing the equator loses part of its advantage, which is why panels in the tropics are laid nearly flat. The solar panel angle calculator takes this into account.

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