The sun's vertical rays are directly over the Tropic of Capricorn, which is located at 23.5 degrees South latitude. The point on Earth's surface directly under the sun's center is called the Sun's Center.
The angle between the horizon and the true north pole is 90 degrees. Therefore, the sun appears in the sky directly overhead at all points on the equator. At the poles, it never sets nor rises again until spring. All around the world, there are two directions that are equivalent with respect to the sun: east and west.
At any given moment during the day, areas within each hemisphere are receiving direct sunlight, while others are in darkness. During a whole year, these areas change as we move across the orbit of the earth around the sun. In January, when New York City is at its most northerly position, parts of Canada and Alaska are in darkness every night; in July, when Los Angeles is at its most southerly position, so too are parts of Australia and Antarctica.
In addition to this annual variation, there is also a daily variation as well. During a single day, areas within each hemisphere are experiencing daylight, while others are in darkness.
The Capricorn Tropic 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. Beneath each tropic is a different kind of latitude, one pointing up toward the north celestial pole and the other down toward the south celestial pole.
On average, the sun is below the horizon for half of the time at mid-northern latitudes and below the horizon for half of the time at mid-southern latitudes. But there are two seasons when the sun is always above the horizon: summer and winter. So at mid-northern latitudes, there are two times of year when the sun is not shining: in the middle of winter and in the middle of summer. At mid-southern latitudes, there are only two times of year when the sun is not shining: in the middle of winter and in the middle of summer.
These four times of the year represent the four quadrants of the sky.
You must be between 23.5° North Latitude (Tropic of Cancer) and 23.5° South Latitude for the sun to be at its zenith (Tropic of Capricorn). At these locations, the sun is always directly over the equator, regardless of how high or low it is in the sky.
The distance that you would have to travel to reach these locations is about 15,000 miles (24,000 km).
The tropics are areas of Earth where the sun appears to rise and set due to day-night cycles caused by changes in the angle of the horizon. There are two tropical regions: the Northern Hemisphere's Atlantic Ocean and Southern Ocean; and the Pacific Ocean's Central America and Caribbean Islands.
In fact, all locations within 300 miles (500 km) of the equator are considered tropical. But only degrees N and S of the poles are truly arctic; those names come from the French word for "ice", because there is no land anywhere near these places to allow for snowfall or ice to accumulate.
Instead, there is constant daylight throughout the year because the rays of the sun are never blocked by any clouds or other objects.
The solar altitude is 90 degrees when the sun is directly overhead. At the equator, this happens during the vernal and autumnal equinoxes. The sun will be 90 degrees above the equator at the Tropics of Cancer and Capricorn on their respective summer solstices. It will be between 69 and 96 degrees above the equator during the rest of the year.
At the poles, the solar altitude is 0 degrees for most of the year. However, on certain days in March and October, the solar altitude is 90 degrees at both the North and South Poles.
In addition to these conditions being met, there must also be no clouds in sight. The sun's rays are reflected back into space by ice crystals that are floating in the atmosphere. Without this reflective surface, sunlight would pass through the cloud layer and into the darkness below. During a clear day with an unobstructed view of the sky, sunlight travels about 5 miles before it reaches the earth's surface. The distance that it travels before disappearing over the horizon is called its altitude.
In conclusion, when the sun is on the horizon and has an altitude of 90 degrees, it is perfectly straight up in the sky and no shadows will fall from it.