You may have noticed two unusual lines of latitude on a map of the world: the Tropic of Cancer in the northern hemisphere at +23.5 degrees latitude and the Tropic of Capricorn in the southern hemisphere at -23.5 degrees latitude. These are the latitudes where the sun shines directly above once a year at noon. The reason for this is simple geometry: since the earth is round, there are two points on its surface that are equally distant from the center of it. These points are called the poles. At the poles, every point on the surface is either up or down, not both. Because of this symmetry, the north pole is always facing upward and the south pole is always facing downward.
As you move away from the poles, more than one point on the surface is below you. In fact, at the equator there are no points that are both above and below you because it is perfectly symmetrical around the equatorial plane. But the earth's rotation is not uniform. It speeds up as you go from the equator to the poles. This means that at any given moment only a small part of the planet is facing toward the sun. The rest is hidden by clouds or darkness. Since sunlight is what gives life to plants, this means that most of the earth is never exposed to direct sunlight.
This is why there are seasons: during the summer months the earth is closest to the sun, so more of it is exposed to light.
In the Cancer Tropics Along the Tropic of Cancer, the sun's rays are straight overhead (the latitude line at 23.5 degrees north, passing through Mexico, Saharan Africa, and India on June 21). The area between the Earth and the Sun is called the Solar Apex.
The Cancer-Capricorn border runs through the solar apex. So this is where you find it! And since the radius vector from center to center of the Earth and Sun is equal about 93 million miles (150 million km), or half way between them, then the sun is directly over head at noon on June 21.
Of course, at other times and in other places over half of the sky is covered by night, so there is no such thing as "directly overhead". But since the horizon is always perpendicular to the local vertical, then when the sun is exactly overhead the observer is looking right down on the equator - part of which may be covered by night, but most of which is not. So even though they can't see all of it, people who are used to looking up at the sky can still tell when the sun is directly overhead.
Have a look at this picture taken in South Australia on January 3, 2015. You can see that it was taken just before noon, so the sun was well up in the sky.
The Capricorn Tropic The sun would be directly overhead (90 degrees from all horizon directions; the zenith) at noon in the southern hemisphere (the winter solstice, December 21) as seen from a latitude of 23.5 degrees south at midday (the winter solstice). It would be noonday locally.
At this time of year, the sun is north of the equator and traveling toward the northern hemisphere's summer solstice. At the northern hemisphere's summer solstice, the sun is directly over the equator at 9 a.m. local time.
At the southern hemisphere's winter solstice, night lasts for nearly half of the planet. A person standing at the South Pole during midnight would see the sky completely dark every day and night for several months while the sun was above the Earth's horizon. After the winter solstice, when night doesn't last half the month, someone standing at the South Pole could never see darkness last for half of the month again.
The North Pole has its own special night tonight: the 24-hour daylight period will end at 3 a.m. Central Time on Dec. 21. From then until mid-January, the sun will not set at the North Pole overnight. During this time, the only thing that moves at the North Pole are the stars that shine through from the Southern Hemisphere.
The South Pole is in a permanent state of darkness.
Outside of the tropics, the sun is never directly overhead, whether to the south or north. The Arctic Circle (circling the North Pole) and the Antarctic Circle are two more notable lines of latitude (around the South Pole). These circles divide the planet into four quadrants, with each quadrant observing a different angle of ascent or descent of the sun.
At the Arctic Circle, the sun is lower in the sky than it is at the South Pole. At the Antarctic Circle, the sun is higher in the sky than it is at the North Pole.
Between these two circles is the temperate zone, where the sun is higher in the sky during the summer months than it is in the winter.
At the North Pole, the sun is highest in the sky during the summer months, but it's still below the horizon for half of the year. At the South Pole, the sun is lowest in the sky during the winter, but it's still above the horizon all year round.
Geographically, the term "pole" refers to a region of high elevation surrounding a central core area that is much deeper than it is wide. Geologists say this happened after a giant meteorite hit Earth at the North Pole about 565 million years ago. The impact created a crater that is still visible today as North America.
The solstices are the times when the subsolar point reaches its farthest northern and southern latitudes. During the June solstice, the sun's vertical rays touch the Tropic of Cancer, 23.5 degrees north of the Equator. At this time of year in Alaska, the sun never sets; it remains continuously over the horizon. The July solstice marks the same point on the opposite side of the Earth relative to the sun. It is called the "solstice" because during this period the length of day and night are at their maximum distance from each other.
In December, the January solstice marks the furthest north where the sun's rays can still strike directly overhead. In Antarctica, this point is located near the South Pole, where the angle between the Sun and the horizontal plane is 90 degrees.
The reason why the northernmost point where the sun's rays can be seen directly overhead changes throughout the year is due to the fact that the Earth experiences seasons too. As we know, plants need light for photosynthesis, so they tend to grow toward the sun. Therefore, the northernmost point where the sun's rays can be seen directly overhead is where the majority of plants are located around the world. This is why during the summer, this point is very close to the North Pole.
However, not all parts of the world experience these temperatures constantly.